2dc2ffdaab954c2261f319b3e395f61448a9c752
[git/git.git] / Documentation / user-manual.txt
1 Git User Manual
2 ===============
3
4 Git is a fast distributed revision control system.
5
6 This manual is designed to be readable by someone with basic UNIX
7 command-line skills, but no previous knowledge of Git.
8
9 <<repositories-and-branches>> and <<exploring-git-history>> explain how
10 to fetch and study a project using git--read these chapters to learn how
11 to build and test a particular version of a software project, search for
12 regressions, and so on.
13
14 People needing to do actual development will also want to read
15 <<Developing-With-git>> and <<sharing-development>>.
16
17 Further chapters cover more specialized topics.
18
19 Comprehensive reference documentation is available through the man
20 pages, or linkgit:git-help[1] command. For example, for the command
21 `git clone <repo>`, you can either use:
22
23 ------------------------------------------------
24 $ man git-clone
25 ------------------------------------------------
26
27 or:
28
29 ------------------------------------------------
30 $ git help clone
31 ------------------------------------------------
32
33 With the latter, you can use the manual viewer of your choice; see
34 linkgit:git-help[1] for more information.
35
36 See also <<git-quick-start>> for a brief overview of Git commands,
37 without any explanation.
38
39 Finally, see <<todo>> for ways that you can help make this manual more
40 complete.
41
42
43 [[repositories-and-branches]]
44 Repositories and Branches
45 =========================
46
47 [[how-to-get-a-git-repository]]
48 How to get a Git repository
49 ---------------------------
50
51 It will be useful to have a Git repository to experiment with as you
52 read this manual.
53
54 The best way to get one is by using the linkgit:git-clone[1] command to
55 download a copy of an existing repository. If you don't already have a
56 project in mind, here are some interesting examples:
57
58 ------------------------------------------------
59 # Git itself (approx. 40MB download):
60 $ git clone git://git.kernel.org/pub/scm/git/git.git
61 # the Linux kernel (approx. 640MB download):
62 $ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
63 ------------------------------------------------
64
65 The initial clone may be time-consuming for a large project, but you
66 will only need to clone once.
67
68 The clone command creates a new directory named after the project
69 (`git` or `linux` in the examples above). After you cd into this
70 directory, you will see that it contains a copy of the project files,
71 called the <<def_working_tree,working tree>>, together with a special
72 top-level directory named `.git`, which contains all the information
73 about the history of the project.
74
75 [[how-to-check-out]]
76 How to check out a different version of a project
77 -------------------------------------------------
78
79 Git is best thought of as a tool for storing the history of a collection
80 of files. It stores the history as a compressed collection of
81 interrelated snapshots of the project's contents. In Git each such
82 version is called a <<def_commit,commit>>.
83
84 Those snapshots aren't necessarily all arranged in a single line from
85 oldest to newest; instead, work may simultaneously proceed along
86 parallel lines of development, called <<def_branch,branches>>, which may
87 merge and diverge.
88
89 A single Git repository can track development on multiple branches. It
90 does this by keeping a list of <<def_head,heads>> which reference the
91 latest commit on each branch; the linkgit:git-branch[1] command shows
92 you the list of branch heads:
93
94 ------------------------------------------------
95 $ git branch
96 * master
97 ------------------------------------------------
98
99 A freshly cloned repository contains a single branch head, by default
100 named "master", with the working directory initialized to the state of
101 the project referred to by that branch head.
102
103 Most projects also use <<def_tag,tags>>. Tags, like heads, are
104 references into the project's history, and can be listed using the
105 linkgit:git-tag[1] command:
106
107 ------------------------------------------------
108 $ git tag -l
109 v2.6.11
110 v2.6.11-tree
111 v2.6.12
112 v2.6.12-rc2
113 v2.6.12-rc3
114 v2.6.12-rc4
115 v2.6.12-rc5
116 v2.6.12-rc6
117 v2.6.13
118 ...
119 ------------------------------------------------
120
121 Tags are expected to always point at the same version of a project,
122 while heads are expected to advance as development progresses.
123
124 Create a new branch head pointing to one of these versions and check it
125 out using linkgit:git-switch[1]:
126
127 ------------------------------------------------
128 $ git switch -c new v2.6.13
129 ------------------------------------------------
130
131 The working directory then reflects the contents that the project had
132 when it was tagged v2.6.13, and linkgit:git-branch[1] shows two
133 branches, with an asterisk marking the currently checked-out branch:
134
135 ------------------------------------------------
136 $ git branch
137 master
138 * new
139 ------------------------------------------------
140
141 If you decide that you'd rather see version 2.6.17, you can modify
142 the current branch to point at v2.6.17 instead, with
143
144 ------------------------------------------------
145 $ git reset --hard v2.6.17
146 ------------------------------------------------
147
148 Note that if the current branch head was your only reference to a
149 particular point in history, then resetting that branch may leave you
150 with no way to find the history it used to point to; so use this command
151 carefully.
152
153 [[understanding-commits]]
154 Understanding History: Commits
155 ------------------------------
156
157 Every change in the history of a project is represented by a commit.
158 The linkgit:git-show[1] command shows the most recent commit on the
159 current branch:
160
161 ------------------------------------------------
162 $ git show
163 commit 17cf781661e6d38f737f15f53ab552f1e95960d7
164 Author: Linus Torvalds <torvalds@ppc970.osdl.org.(none)>
165 Date: Tue Apr 19 14:11:06 2005 -0700
166
167 Remove duplicate getenv(DB_ENVIRONMENT) call
168
169 Noted by Tony Luck.
170
171 diff --git a/init-db.c b/init-db.c
172 index 65898fa..b002dc6 100644
173 --- a/init-db.c
174 +++ b/init-db.c
175 @@ -7,7 +7,7 @@
176
177 int main(int argc, char **argv)
178 {
179 - char *sha1_dir = getenv(DB_ENVIRONMENT), *path;
180 + char *sha1_dir, *path;
181 int len, i;
182
183 if (mkdir(".git", 0755) < 0) {
184 ------------------------------------------------
185
186 As you can see, a commit shows who made the latest change, what they
187 did, and why.
188
189 Every commit has a 40-hexdigit id, sometimes called the "object name" or the
190 "SHA-1 id", shown on the first line of the `git show` output. You can usually
191 refer to a commit by a shorter name, such as a tag or a branch name, but this
192 longer name can also be useful. Most importantly, it is a globally unique
193 name for this commit: so if you tell somebody else the object name (for
194 example in email), then you are guaranteed that name will refer to the same
195 commit in their repository that it does in yours (assuming their repository
196 has that commit at all). Since the object name is computed as a hash over the
197 contents of the commit, you are guaranteed that the commit can never change
198 without its name also changing.
199
200 In fact, in <<git-concepts>> we shall see that everything stored in Git
201 history, including file data and directory contents, is stored in an object
202 with a name that is a hash of its contents.
203
204 [[understanding-reachability]]
205 Understanding history: commits, parents, and reachability
206 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
207
208 Every commit (except the very first commit in a project) also has a
209 parent commit which shows what happened before this commit.
210 Following the chain of parents will eventually take you back to the
211 beginning of the project.
212
213 However, the commits do not form a simple list; Git allows lines of
214 development to diverge and then reconverge, and the point where two
215 lines of development reconverge is called a "merge". The commit
216 representing a merge can therefore have more than one parent, with
217 each parent representing the most recent commit on one of the lines
218 of development leading to that point.
219
220 The best way to see how this works is using the linkgit:gitk[1]
221 command; running gitk now on a Git repository and looking for merge
222 commits will help understand how Git organizes history.
223
224 In the following, we say that commit X is "reachable" from commit Y
225 if commit X is an ancestor of commit Y. Equivalently, you could say
226 that Y is a descendant of X, or that there is a chain of parents
227 leading from commit Y to commit X.
228
229 [[history-diagrams]]
230 Understanding history: History diagrams
231 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
232
233 We will sometimes represent Git history using diagrams like the one
234 below. Commits are shown as "o", and the links between them with
235 lines drawn with - / and \. Time goes left to right:
236
237
238 ................................................
239 o--o--o <-- Branch A
240 /
241 o--o--o <-- master
242 \
243 o--o--o <-- Branch B
244 ................................................
245
246 If we need to talk about a particular commit, the character "o" may
247 be replaced with another letter or number.
248
249 [[what-is-a-branch]]
250 Understanding history: What is a branch?
251 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
252
253 When we need to be precise, we will use the word "branch" to mean a line
254 of development, and "branch head" (or just "head") to mean a reference
255 to the most recent commit on a branch. In the example above, the branch
256 head named "A" is a pointer to one particular commit, but we refer to
257 the line of three commits leading up to that point as all being part of
258 "branch A".
259
260 However, when no confusion will result, we often just use the term
261 "branch" both for branches and for branch heads.
262
263 [[manipulating-branches]]
264 Manipulating branches
265 ---------------------
266
267 Creating, deleting, and modifying branches is quick and easy; here's
268 a summary of the commands:
269
270 `git branch`::
271 list all branches.
272 `git branch <branch>`::
273 create a new branch named `<branch>`, referencing the same
274 point in history as the current branch.
275 `git branch <branch> <start-point>`::
276 create a new branch named `<branch>`, referencing
277 `<start-point>`, which may be specified any way you like,
278 including using a branch name or a tag name.
279 `git branch -d <branch>`::
280 delete the branch `<branch>`; if the branch is not fully
281 merged in its upstream branch or contained in the current branch,
282 this command will fail with a warning.
283 `git branch -D <branch>`::
284 delete the branch `<branch>` irrespective of its merged status.
285 `git switch <branch>`::
286 make the current branch `<branch>`, updating the working
287 directory to reflect the version referenced by `<branch>`.
288 `git switch -c <new> <start-point>`::
289 create a new branch `<new>` referencing `<start-point>`, and
290 check it out.
291
292 The special symbol "HEAD" can always be used to refer to the current
293 branch. In fact, Git uses a file named `HEAD` in the `.git` directory
294 to remember which branch is current:
295
296 ------------------------------------------------
297 $ cat .git/HEAD
298 ref: refs/heads/master
299 ------------------------------------------------
300
301 [[detached-head]]
302 Examining an old version without creating a new branch
303 ------------------------------------------------------
304
305 The `git switch` command normally expects a branch head, but will also
306 accept an arbitrary commit when invoked with --detach; for example,
307 you can check out the commit referenced by a tag:
308
309 ------------------------------------------------
310 $ git switch --detach v2.6.17
311 Note: checking out 'v2.6.17'.
312
313 You are in 'detached HEAD' state. You can look around, make experimental
314 changes and commit them, and you can discard any commits you make in this
315 state without impacting any branches by performing another switch.
316
317 If you want to create a new branch to retain commits you create, you may
318 do so (now or later) by using -c with the switch command again. Example:
319
320 git switch -c new_branch_name
321
322 HEAD is now at 427abfa Linux v2.6.17
323 ------------------------------------------------
324
325 The HEAD then refers to the SHA-1 of the commit instead of to a branch,
326 and git branch shows that you are no longer on a branch:
327
328 ------------------------------------------------
329 $ cat .git/HEAD
330 427abfa28afedffadfca9dd8b067eb6d36bac53f
331 $ git branch
332 * (detached from v2.6.17)
333 master
334 ------------------------------------------------
335
336 In this case we say that the HEAD is "detached".
337
338 This is an easy way to check out a particular version without having to
339 make up a name for the new branch. You can still create a new branch
340 (or tag) for this version later if you decide to.
341
342 [[examining-remote-branches]]
343 Examining branches from a remote repository
344 -------------------------------------------
345
346 The "master" branch that was created at the time you cloned is a copy
347 of the HEAD in the repository that you cloned from. That repository
348 may also have had other branches, though, and your local repository
349 keeps branches which track each of those remote branches, called
350 remote-tracking branches, which you
351 can view using the `-r` option to linkgit:git-branch[1]:
352
353 ------------------------------------------------
354 $ git branch -r
355 origin/HEAD
356 origin/html
357 origin/maint
358 origin/man
359 origin/master
360 origin/next
361 origin/pu
362 origin/todo
363 ------------------------------------------------
364
365 In this example, "origin" is called a remote repository, or "remote"
366 for short. The branches of this repository are called "remote
367 branches" from our point of view. The remote-tracking branches listed
368 above were created based on the remote branches at clone time and will
369 be updated by `git fetch` (hence `git pull`) and `git push`. See
370 <<Updating-a-repository-With-git-fetch>> for details.
371
372 You might want to build on one of these remote-tracking branches
373 on a branch of your own, just as you would for a tag:
374
375 ------------------------------------------------
376 $ git switch -c my-todo-copy origin/todo
377 ------------------------------------------------
378
379 You can also check out `origin/todo` directly to examine it or
380 write a one-off patch. See <<detached-head,detached head>>.
381
382 Note that the name "origin" is just the name that Git uses by default
383 to refer to the repository that you cloned from.
384
385 [[how-git-stores-references]]
386 Naming branches, tags, and other references
387 -------------------------------------------
388
389 Branches, remote-tracking branches, and tags are all references to
390 commits. All references are named with a slash-separated path name
391 starting with `refs`; the names we've been using so far are actually
392 shorthand:
393
394 - The branch `test` is short for `refs/heads/test`.
395 - The tag `v2.6.18` is short for `refs/tags/v2.6.18`.
396 - `origin/master` is short for `refs/remotes/origin/master`.
397
398 The full name is occasionally useful if, for example, there ever
399 exists a tag and a branch with the same name.
400
401 (Newly created refs are actually stored in the `.git/refs` directory,
402 under the path given by their name. However, for efficiency reasons
403 they may also be packed together in a single file; see
404 linkgit:git-pack-refs[1]).
405
406 As another useful shortcut, the "HEAD" of a repository can be referred
407 to just using the name of that repository. So, for example, "origin"
408 is usually a shortcut for the HEAD branch in the repository "origin".
409
410 For the complete list of paths which Git checks for references, and
411 the order it uses to decide which to choose when there are multiple
412 references with the same shorthand name, see the "SPECIFYING
413 REVISIONS" section of linkgit:gitrevisions[7].
414
415 [[Updating-a-repository-With-git-fetch]]
416 Updating a repository with git fetch
417 ------------------------------------
418
419 After you clone a repository and commit a few changes of your own, you
420 may wish to check the original repository for updates.
421
422 The `git-fetch` command, with no arguments, will update all of the
423 remote-tracking branches to the latest version found in the original
424 repository. It will not touch any of your own branches--not even the
425 "master" branch that was created for you on clone.
426
427 [[fetching-branches]]
428 Fetching branches from other repositories
429 -----------------------------------------
430
431 You can also track branches from repositories other than the one you
432 cloned from, using linkgit:git-remote[1]:
433
434 -------------------------------------------------
435 $ git remote add staging git://git.kernel.org/.../gregkh/staging.git
436 $ git fetch staging
437 ...
438 From git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging
439 * [new branch] master -> staging/master
440 * [new branch] staging-linus -> staging/staging-linus
441 * [new branch] staging-next -> staging/staging-next
442 -------------------------------------------------
443
444 New remote-tracking branches will be stored under the shorthand name
445 that you gave `git remote add`, in this case `staging`:
446
447 -------------------------------------------------
448 $ git branch -r
449 origin/HEAD -> origin/master
450 origin/master
451 staging/master
452 staging/staging-linus
453 staging/staging-next
454 -------------------------------------------------
455
456 If you run `git fetch <remote>` later, the remote-tracking branches
457 for the named `<remote>` will be updated.
458
459 If you examine the file `.git/config`, you will see that Git has added
460 a new stanza:
461
462 -------------------------------------------------
463 $ cat .git/config
464 ...
465 [remote "staging"]
466 url = git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git
467 fetch = +refs/heads/*:refs/remotes/staging/*
468 ...
469 -------------------------------------------------
470
471 This is what causes Git to track the remote's branches; you may modify
472 or delete these configuration options by editing `.git/config` with a
473 text editor. (See the "CONFIGURATION FILE" section of
474 linkgit:git-config[1] for details.)
475
476 [[exploring-git-history]]
477 Exploring Git history
478 =====================
479
480 Git is best thought of as a tool for storing the history of a
481 collection of files. It does this by storing compressed snapshots of
482 the contents of a file hierarchy, together with "commits" which show
483 the relationships between these snapshots.
484
485 Git provides extremely flexible and fast tools for exploring the
486 history of a project.
487
488 We start with one specialized tool that is useful for finding the
489 commit that introduced a bug into a project.
490
491 [[using-bisect]]
492 How to use bisect to find a regression
493 --------------------------------------
494
495 Suppose version 2.6.18 of your project worked, but the version at
496 "master" crashes. Sometimes the best way to find the cause of such a
497 regression is to perform a brute-force search through the project's
498 history to find the particular commit that caused the problem. The
499 linkgit:git-bisect[1] command can help you do this:
500
501 -------------------------------------------------
502 $ git bisect start
503 $ git bisect good v2.6.18
504 $ git bisect bad master
505 Bisecting: 3537 revisions left to test after this
506 [65934a9a028b88e83e2b0f8b36618fe503349f8e] BLOCK: Make USB storage depend on SCSI rather than selecting it [try #6]
507 -------------------------------------------------
508
509 If you run `git branch` at this point, you'll see that Git has
510 temporarily moved you in "(no branch)". HEAD is now detached from any
511 branch and points directly to a commit (with commit id 65934) that
512 is reachable from "master" but not from v2.6.18. Compile and test it,
513 and see whether it crashes. Assume it does crash. Then:
514
515 -------------------------------------------------
516 $ git bisect bad
517 Bisecting: 1769 revisions left to test after this
518 [7eff82c8b1511017ae605f0c99ac275a7e21b867] i2c-core: Drop useless bitmaskings
519 -------------------------------------------------
520
521 checks out an older version. Continue like this, telling Git at each
522 stage whether the version it gives you is good or bad, and notice
523 that the number of revisions left to test is cut approximately in
524 half each time.
525
526 After about 13 tests (in this case), it will output the commit id of
527 the guilty commit. You can then examine the commit with
528 linkgit:git-show[1], find out who wrote it, and mail them your bug
529 report with the commit id. Finally, run
530
531 -------------------------------------------------
532 $ git bisect reset
533 -------------------------------------------------
534
535 to return you to the branch you were on before.
536
537 Note that the version which `git bisect` checks out for you at each
538 point is just a suggestion, and you're free to try a different
539 version if you think it would be a good idea. For example,
540 occasionally you may land on a commit that broke something unrelated;
541 run
542
543 -------------------------------------------------
544 $ git bisect visualize
545 -------------------------------------------------
546
547 which will run gitk and label the commit it chose with a marker that
548 says "bisect". Choose a safe-looking commit nearby, note its commit
549 id, and check it out with:
550
551 -------------------------------------------------
552 $ git reset --hard fb47ddb2db
553 -------------------------------------------------
554
555 then test, run `bisect good` or `bisect bad` as appropriate, and
556 continue.
557
558 Instead of `git bisect visualize` and then `git reset --hard
559 fb47ddb2db`, you might just want to tell Git that you want to skip
560 the current commit:
561
562 -------------------------------------------------
563 $ git bisect skip
564 -------------------------------------------------
565
566 In this case, though, Git may not eventually be able to tell the first
567 bad one between some first skipped commits and a later bad commit.
568
569 There are also ways to automate the bisecting process if you have a
570 test script that can tell a good from a bad commit. See
571 linkgit:git-bisect[1] for more information about this and other `git
572 bisect` features.
573
574 [[naming-commits]]
575 Naming commits
576 --------------
577
578 We have seen several ways of naming commits already:
579
580 - 40-hexdigit object name
581 - branch name: refers to the commit at the head of the given
582 branch
583 - tag name: refers to the commit pointed to by the given tag
584 (we've seen branches and tags are special cases of
585 <<how-git-stores-references,references>>).
586 - HEAD: refers to the head of the current branch
587
588 There are many more; see the "SPECIFYING REVISIONS" section of the
589 linkgit:gitrevisions[7] man page for the complete list of ways to
590 name revisions. Some examples:
591
592 -------------------------------------------------
593 $ git show fb47ddb2 # the first few characters of the object name
594 # are usually enough to specify it uniquely
595 $ git show HEAD^ # the parent of the HEAD commit
596 $ git show HEAD^^ # the grandparent
597 $ git show HEAD~4 # the great-great-grandparent
598 -------------------------------------------------
599
600 Recall that merge commits may have more than one parent; by default,
601 `^` and `~` follow the first parent listed in the commit, but you can
602 also choose:
603
604 -------------------------------------------------
605 $ git show HEAD^1 # show the first parent of HEAD
606 $ git show HEAD^2 # show the second parent of HEAD
607 -------------------------------------------------
608
609 In addition to HEAD, there are several other special names for
610 commits:
611
612 Merges (to be discussed later), as well as operations such as
613 `git reset`, which change the currently checked-out commit, generally
614 set ORIG_HEAD to the value HEAD had before the current operation.
615
616 The `git fetch` operation always stores the head of the last fetched
617 branch in FETCH_HEAD. For example, if you run `git fetch` without
618 specifying a local branch as the target of the operation
619
620 -------------------------------------------------
621 $ git fetch git://example.com/proj.git theirbranch
622 -------------------------------------------------
623
624 the fetched commits will still be available from FETCH_HEAD.
625
626 When we discuss merges we'll also see the special name MERGE_HEAD,
627 which refers to the other branch that we're merging in to the current
628 branch.
629
630 The linkgit:git-rev-parse[1] command is a low-level command that is
631 occasionally useful for translating some name for a commit to the object
632 name for that commit:
633
634 -------------------------------------------------
635 $ git rev-parse origin
636 e05db0fd4f31dde7005f075a84f96b360d05984b
637 -------------------------------------------------
638
639 [[creating-tags]]
640 Creating tags
641 -------------
642
643 We can also create a tag to refer to a particular commit; after
644 running
645
646 -------------------------------------------------
647 $ git tag stable-1 1b2e1d63ff
648 -------------------------------------------------
649
650 You can use `stable-1` to refer to the commit 1b2e1d63ff.
651
652 This creates a "lightweight" tag. If you would also like to include a
653 comment with the tag, and possibly sign it cryptographically, then you
654 should create a tag object instead; see the linkgit:git-tag[1] man page
655 for details.
656
657 [[browsing-revisions]]
658 Browsing revisions
659 ------------------
660
661 The linkgit:git-log[1] command can show lists of commits. On its
662 own, it shows all commits reachable from the parent commit; but you
663 can also make more specific requests:
664
665 -------------------------------------------------
666 $ git log v2.5.. # commits since (not reachable from) v2.5
667 $ git log test..master # commits reachable from master but not test
668 $ git log master..test # ...reachable from test but not master
669 $ git log master...test # ...reachable from either test or master,
670 # but not both
671 $ git log --since="2 weeks ago" # commits from the last 2 weeks
672 $ git log Makefile # commits which modify Makefile
673 $ git log fs/ # ... which modify any file under fs/
674 $ git log -S'foo()' # commits which add or remove any file data
675 # matching the string 'foo()'
676 -------------------------------------------------
677
678 And of course you can combine all of these; the following finds
679 commits since v2.5 which touch the `Makefile` or any file under `fs`:
680
681 -------------------------------------------------
682 $ git log v2.5.. Makefile fs/
683 -------------------------------------------------
684
685 You can also ask git log to show patches:
686
687 -------------------------------------------------
688 $ git log -p
689 -------------------------------------------------
690
691 See the `--pretty` option in the linkgit:git-log[1] man page for more
692 display options.
693
694 Note that git log starts with the most recent commit and works
695 backwards through the parents; however, since Git history can contain
696 multiple independent lines of development, the particular order that
697 commits are listed in may be somewhat arbitrary.
698
699 [[generating-diffs]]
700 Generating diffs
701 ----------------
702
703 You can generate diffs between any two versions using
704 linkgit:git-diff[1]:
705
706 -------------------------------------------------
707 $ git diff master..test
708 -------------------------------------------------
709
710 That will produce the diff between the tips of the two branches. If
711 you'd prefer to find the diff from their common ancestor to test, you
712 can use three dots instead of two:
713
714 -------------------------------------------------
715 $ git diff master...test
716 -------------------------------------------------
717
718 Sometimes what you want instead is a set of patches; for this you can
719 use linkgit:git-format-patch[1]:
720
721 -------------------------------------------------
722 $ git format-patch master..test
723 -------------------------------------------------
724
725 will generate a file with a patch for each commit reachable from test
726 but not from master.
727
728 [[viewing-old-file-versions]]
729 Viewing old file versions
730 -------------------------
731
732 You can always view an old version of a file by just checking out the
733 correct revision first. But sometimes it is more convenient to be
734 able to view an old version of a single file without checking
735 anything out; this command does that:
736
737 -------------------------------------------------
738 $ git show v2.5:fs/locks.c
739 -------------------------------------------------
740
741 Before the colon may be anything that names a commit, and after it
742 may be any path to a file tracked by Git.
743
744 [[history-examples]]
745 Examples
746 --------
747
748 [[counting-commits-on-a-branch]]
749 Counting the number of commits on a branch
750 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
751
752 Suppose you want to know how many commits you've made on `mybranch`
753 since it diverged from `origin`:
754
755 -------------------------------------------------
756 $ git log --pretty=oneline origin..mybranch | wc -l
757 -------------------------------------------------
758
759 Alternatively, you may often see this sort of thing done with the
760 lower-level command linkgit:git-rev-list[1], which just lists the SHA-1's
761 of all the given commits:
762
763 -------------------------------------------------
764 $ git rev-list origin..mybranch | wc -l
765 -------------------------------------------------
766
767 [[checking-for-equal-branches]]
768 Check whether two branches point at the same history
769 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
770
771 Suppose you want to check whether two branches point at the same point
772 in history.
773
774 -------------------------------------------------
775 $ git diff origin..master
776 -------------------------------------------------
777
778 will tell you whether the contents of the project are the same at the
779 two branches; in theory, however, it's possible that the same project
780 contents could have been arrived at by two different historical
781 routes. You could compare the object names:
782
783 -------------------------------------------------
784 $ git rev-list origin
785 e05db0fd4f31dde7005f075a84f96b360d05984b
786 $ git rev-list master
787 e05db0fd4f31dde7005f075a84f96b360d05984b
788 -------------------------------------------------
789
790 Or you could recall that the `...` operator selects all commits
791 reachable from either one reference or the other but not
792 both; so
793
794 -------------------------------------------------
795 $ git log origin...master
796 -------------------------------------------------
797
798 will return no commits when the two branches are equal.
799
800 [[finding-tagged-descendants]]
801 Find first tagged version including a given fix
802 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
803
804 Suppose you know that the commit e05db0fd fixed a certain problem.
805 You'd like to find the earliest tagged release that contains that
806 fix.
807
808 Of course, there may be more than one answer--if the history branched
809 after commit e05db0fd, then there could be multiple "earliest" tagged
810 releases.
811
812 You could just visually inspect the commits since e05db0fd:
813
814 -------------------------------------------------
815 $ gitk e05db0fd..
816 -------------------------------------------------
817
818 or you can use linkgit:git-name-rev[1], which will give the commit a
819 name based on any tag it finds pointing to one of the commit's
820 descendants:
821
822 -------------------------------------------------
823 $ git name-rev --tags e05db0fd
824 e05db0fd tags/v1.5.0-rc1^0~23
825 -------------------------------------------------
826
827 The linkgit:git-describe[1] command does the opposite, naming the
828 revision using a tag on which the given commit is based:
829
830 -------------------------------------------------
831 $ git describe e05db0fd
832 v1.5.0-rc0-260-ge05db0f
833 -------------------------------------------------
834
835 but that may sometimes help you guess which tags might come after the
836 given commit.
837
838 If you just want to verify whether a given tagged version contains a
839 given commit, you could use linkgit:git-merge-base[1]:
840
841 -------------------------------------------------
842 $ git merge-base e05db0fd v1.5.0-rc1
843 e05db0fd4f31dde7005f075a84f96b360d05984b
844 -------------------------------------------------
845
846 The merge-base command finds a common ancestor of the given commits,
847 and always returns one or the other in the case where one is a
848 descendant of the other; so the above output shows that e05db0fd
849 actually is an ancestor of v1.5.0-rc1.
850
851 Alternatively, note that
852
853 -------------------------------------------------
854 $ git log v1.5.0-rc1..e05db0fd
855 -------------------------------------------------
856
857 will produce empty output if and only if v1.5.0-rc1 includes e05db0fd,
858 because it outputs only commits that are not reachable from v1.5.0-rc1.
859
860 As yet another alternative, the linkgit:git-show-branch[1] command lists
861 the commits reachable from its arguments with a display on the left-hand
862 side that indicates which arguments that commit is reachable from.
863 So, if you run something like
864
865 -------------------------------------------------
866 $ git show-branch e05db0fd v1.5.0-rc0 v1.5.0-rc1 v1.5.0-rc2
867 ! [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
868 available
869 ! [v1.5.0-rc0] GIT v1.5.0 preview
870 ! [v1.5.0-rc1] GIT v1.5.0-rc1
871 ! [v1.5.0-rc2] GIT v1.5.0-rc2
872 ...
873 -------------------------------------------------
874
875 then a line like
876
877 -------------------------------------------------
878 + ++ [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
879 available
880 -------------------------------------------------
881
882 shows that e05db0fd is reachable from itself, from v1.5.0-rc1,
883 and from v1.5.0-rc2, and not from v1.5.0-rc0.
884
885 [[showing-commits-unique-to-a-branch]]
886 Showing commits unique to a given branch
887 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
888
889 Suppose you would like to see all the commits reachable from the branch
890 head named `master` but not from any other head in your repository.
891
892 We can list all the heads in this repository with
893 linkgit:git-show-ref[1]:
894
895 -------------------------------------------------
896 $ git show-ref --heads
897 bf62196b5e363d73353a9dcf094c59595f3153b7 refs/heads/core-tutorial
898 db768d5504c1bb46f63ee9d6e1772bd047e05bf9 refs/heads/maint
899 a07157ac624b2524a059a3414e99f6f44bebc1e7 refs/heads/master
900 24dbc180ea14dc1aebe09f14c8ecf32010690627 refs/heads/tutorial-2
901 1e87486ae06626c2f31eaa63d26fc0fd646c8af2 refs/heads/tutorial-fixes
902 -------------------------------------------------
903
904 We can get just the branch-head names, and remove `master`, with
905 the help of the standard utilities cut and grep:
906
907 -------------------------------------------------
908 $ git show-ref --heads | cut -d' ' -f2 | grep -v '^refs/heads/master'
909 refs/heads/core-tutorial
910 refs/heads/maint
911 refs/heads/tutorial-2
912 refs/heads/tutorial-fixes
913 -------------------------------------------------
914
915 And then we can ask to see all the commits reachable from master
916 but not from these other heads:
917
918 -------------------------------------------------
919 $ gitk master --not $( git show-ref --heads | cut -d' ' -f2 |
920 grep -v '^refs/heads/master' )
921 -------------------------------------------------
922
923 Obviously, endless variations are possible; for example, to see all
924 commits reachable from some head but not from any tag in the repository:
925
926 -------------------------------------------------
927 $ gitk $( git show-ref --heads ) --not $( git show-ref --tags )
928 -------------------------------------------------
929
930 (See linkgit:gitrevisions[7] for explanations of commit-selecting
931 syntax such as `--not`.)
932
933 [[making-a-release]]
934 Creating a changelog and tarball for a software release
935 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
936
937 The linkgit:git-archive[1] command can create a tar or zip archive from
938 any version of a project; for example:
939
940 -------------------------------------------------
941 $ git archive -o latest.tar.gz --prefix=project/ HEAD
942 -------------------------------------------------
943
944 will use HEAD to produce a gzipped tar archive in which each filename
945 is preceded by `project/`. The output file format is inferred from
946 the output file extension if possible, see linkgit:git-archive[1] for
947 details.
948
949 Versions of Git older than 1.7.7 don't know about the `tar.gz` format,
950 you'll need to use gzip explicitly:
951
952 -------------------------------------------------
953 $ git archive --format=tar --prefix=project/ HEAD | gzip >latest.tar.gz
954 -------------------------------------------------
955
956 If you're releasing a new version of a software project, you may want
957 to simultaneously make a changelog to include in the release
958 announcement.
959
960 Linus Torvalds, for example, makes new kernel releases by tagging them,
961 then running:
962
963 -------------------------------------------------
964 $ release-script 2.6.12 2.6.13-rc6 2.6.13-rc7
965 -------------------------------------------------
966
967 where release-script is a shell script that looks like:
968
969 -------------------------------------------------
970 #!/bin/sh
971 stable="$1"
972 last="$2"
973 new="$3"
974 echo "# git tag v$new"
975 echo "git archive --prefix=linux-$new/ v$new | gzip -9 > ../linux-$new.tar.gz"
976 echo "git diff v$stable v$new | gzip -9 > ../patch-$new.gz"
977 echo "git log --no-merges v$new ^v$last > ../ChangeLog-$new"
978 echo "git shortlog --no-merges v$new ^v$last > ../ShortLog"
979 echo "git diff --stat --summary -M v$last v$new > ../diffstat-$new"
980 -------------------------------------------------
981
982 and then he just cut-and-pastes the output commands after verifying that
983 they look OK.
984
985 [[Finding-commits-With-given-Content]]
986 Finding commits referencing a file with given content
987 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
988
989 Somebody hands you a copy of a file, and asks which commits modified a
990 file such that it contained the given content either before or after the
991 commit. You can find out with this:
992
993 -------------------------------------------------
994 $ git log --raw --abbrev=40 --pretty=oneline |
995 grep -B 1 `git hash-object filename`
996 -------------------------------------------------
997
998 Figuring out why this works is left as an exercise to the (advanced)
999 student. The linkgit:git-log[1], linkgit:git-diff-tree[1], and
1000 linkgit:git-hash-object[1] man pages may prove helpful.
1001
1002 [[Developing-With-git]]
1003 Developing with Git
1004 ===================
1005
1006 [[telling-git-your-name]]
1007 Telling Git your name
1008 ---------------------
1009
1010 Before creating any commits, you should introduce yourself to Git.
1011 The easiest way to do so is to use linkgit:git-config[1]:
1012
1013 ------------------------------------------------
1014 $ git config --global user.name 'Your Name Comes Here'
1015 $ git config --global user.email 'you@yourdomain.example.com'
1016 ------------------------------------------------
1017
1018 Which will add the following to a file named `.gitconfig` in your
1019 home directory:
1020
1021 ------------------------------------------------
1022 [user]
1023 name = Your Name Comes Here
1024 email = you@yourdomain.example.com
1025 ------------------------------------------------
1026
1027 See the "CONFIGURATION FILE" section of linkgit:git-config[1] for
1028 details on the configuration file. The file is plain text, so you can
1029 also edit it with your favorite editor.
1030
1031
1032 [[creating-a-new-repository]]
1033 Creating a new repository
1034 -------------------------
1035
1036 Creating a new repository from scratch is very easy:
1037
1038 -------------------------------------------------
1039 $ mkdir project
1040 $ cd project
1041 $ git init
1042 -------------------------------------------------
1043
1044 If you have some initial content (say, a tarball):
1045
1046 -------------------------------------------------
1047 $ tar xzvf project.tar.gz
1048 $ cd project
1049 $ git init
1050 $ git add . # include everything below ./ in the first commit:
1051 $ git commit
1052 -------------------------------------------------
1053
1054 [[how-to-make-a-commit]]
1055 How to make a commit
1056 --------------------
1057
1058 Creating a new commit takes three steps:
1059
1060 1. Making some changes to the working directory using your
1061 favorite editor.
1062 2. Telling Git about your changes.
1063 3. Creating the commit using the content you told Git about
1064 in step 2.
1065
1066 In practice, you can interleave and repeat steps 1 and 2 as many
1067 times as you want: in order to keep track of what you want committed
1068 at step 3, Git maintains a snapshot of the tree's contents in a
1069 special staging area called "the index."
1070
1071 At the beginning, the content of the index will be identical to
1072 that of the HEAD. The command `git diff --cached`, which shows
1073 the difference between the HEAD and the index, should therefore
1074 produce no output at that point.
1075
1076 Modifying the index is easy:
1077
1078 To update the index with the contents of a new or modified file, use
1079
1080 -------------------------------------------------
1081 $ git add path/to/file
1082 -------------------------------------------------
1083
1084 To remove a file from the index and from the working tree, use
1085
1086 -------------------------------------------------
1087 $ git rm path/to/file
1088 -------------------------------------------------
1089
1090 After each step you can verify that
1091
1092 -------------------------------------------------
1093 $ git diff --cached
1094 -------------------------------------------------
1095
1096 always shows the difference between the HEAD and the index file--this
1097 is what you'd commit if you created the commit now--and that
1098
1099 -------------------------------------------------
1100 $ git diff
1101 -------------------------------------------------
1102
1103 shows the difference between the working tree and the index file.
1104
1105 Note that `git add` always adds just the current contents of a file
1106 to the index; further changes to the same file will be ignored unless
1107 you run `git add` on the file again.
1108
1109 When you're ready, just run
1110
1111 -------------------------------------------------
1112 $ git commit
1113 -------------------------------------------------
1114
1115 and Git will prompt you for a commit message and then create the new
1116 commit. Check to make sure it looks like what you expected with
1117
1118 -------------------------------------------------
1119 $ git show
1120 -------------------------------------------------
1121
1122 As a special shortcut,
1123
1124 -------------------------------------------------
1125 $ git commit -a
1126 -------------------------------------------------
1127
1128 will update the index with any files that you've modified or removed
1129 and create a commit, all in one step.
1130
1131 A number of commands are useful for keeping track of what you're
1132 about to commit:
1133
1134 -------------------------------------------------
1135 $ git diff --cached # difference between HEAD and the index; what
1136 # would be committed if you ran "commit" now.
1137 $ git diff # difference between the index file and your
1138 # working directory; changes that would not
1139 # be included if you ran "commit" now.
1140 $ git diff HEAD # difference between HEAD and working tree; what
1141 # would be committed if you ran "commit -a" now.
1142 $ git status # a brief per-file summary of the above.
1143 -------------------------------------------------
1144
1145 You can also use linkgit:git-gui[1] to create commits, view changes in
1146 the index and the working tree files, and individually select diff hunks
1147 for inclusion in the index (by right-clicking on the diff hunk and
1148 choosing "Stage Hunk For Commit").
1149
1150 [[creating-good-commit-messages]]
1151 Creating good commit messages
1152 -----------------------------
1153
1154 Though not required, it's a good idea to begin the commit message
1155 with a single short (less than 50 character) line summarizing the
1156 change, followed by a blank line and then a more thorough
1157 description. The text up to the first blank line in a commit
1158 message is treated as the commit title, and that title is used
1159 throughout Git. For example, linkgit:git-format-patch[1] turns a
1160 commit into email, and it uses the title on the Subject line and the
1161 rest of the commit in the body.
1162
1163
1164 [[ignoring-files]]
1165 Ignoring files
1166 --------------
1167
1168 A project will often generate files that you do 'not' want to track with Git.
1169 This typically includes files generated by a build process or temporary
1170 backup files made by your editor. Of course, 'not' tracking files with Git
1171 is just a matter of 'not' calling `git add` on them. But it quickly becomes
1172 annoying to have these untracked files lying around; e.g. they make
1173 `git add .` practically useless, and they keep showing up in the output of
1174 `git status`.
1175
1176 You can tell Git to ignore certain files by creating a file called
1177 `.gitignore` in the top level of your working directory, with contents
1178 such as:
1179
1180 -------------------------------------------------
1181 # Lines starting with '#' are considered comments.
1182 # Ignore any file named foo.txt.
1183 foo.txt
1184 # Ignore (generated) html files,
1185 *.html
1186 # except foo.html which is maintained by hand.
1187 !foo.html
1188 # Ignore objects and archives.
1189 *.[oa]
1190 -------------------------------------------------
1191
1192 See linkgit:gitignore[5] for a detailed explanation of the syntax. You can
1193 also place .gitignore files in other directories in your working tree, and they
1194 will apply to those directories and their subdirectories. The `.gitignore`
1195 files can be added to your repository like any other files (just run `git add
1196 .gitignore` and `git commit`, as usual), which is convenient when the exclude
1197 patterns (such as patterns matching build output files) would also make sense
1198 for other users who clone your repository.
1199
1200 If you wish the exclude patterns to affect only certain repositories
1201 (instead of every repository for a given project), you may instead put
1202 them in a file in your repository named `.git/info/exclude`, or in any
1203 file specified by the `core.excludesFile` configuration variable.
1204 Some Git commands can also take exclude patterns directly on the
1205 command line. See linkgit:gitignore[5] for the details.
1206
1207 [[how-to-merge]]
1208 How to merge
1209 ------------
1210
1211 You can rejoin two diverging branches of development using
1212 linkgit:git-merge[1]:
1213
1214 -------------------------------------------------
1215 $ git merge branchname
1216 -------------------------------------------------
1217
1218 merges the development in the branch `branchname` into the current
1219 branch.
1220
1221 A merge is made by combining the changes made in `branchname` and the
1222 changes made up to the latest commit in your current branch since
1223 their histories forked. The work tree is overwritten by the result of
1224 the merge when this combining is done cleanly, or overwritten by a
1225 half-merged results when this combining results in conflicts.
1226 Therefore, if you have uncommitted changes touching the same files as
1227 the ones impacted by the merge, Git will refuse to proceed. Most of
1228 the time, you will want to commit your changes before you can merge,
1229 and if you don't, then linkgit:git-stash[1] can take these changes
1230 away while you're doing the merge, and reapply them afterwards.
1231
1232 If the changes are independent enough, Git will automatically complete
1233 the merge and commit the result (or reuse an existing commit in case
1234 of <<fast-forwards,fast-forward>>, see below). On the other hand,
1235 if there are conflicts--for example, if the same file is
1236 modified in two different ways in the remote branch and the local
1237 branch--then you are warned; the output may look something like this:
1238
1239 -------------------------------------------------
1240 $ git merge next
1241 100% (4/4) done
1242 Auto-merged file.txt
1243 CONFLICT (content): Merge conflict in file.txt
1244 Automatic merge failed; fix conflicts and then commit the result.
1245 -------------------------------------------------
1246
1247 Conflict markers are left in the problematic files, and after
1248 you resolve the conflicts manually, you can update the index
1249 with the contents and run Git commit, as you normally would when
1250 creating a new file.
1251
1252 If you examine the resulting commit using gitk, you will see that it
1253 has two parents, one pointing to the top of the current branch, and
1254 one to the top of the other branch.
1255
1256 [[resolving-a-merge]]
1257 Resolving a merge
1258 -----------------
1259
1260 When a merge isn't resolved automatically, Git leaves the index and
1261 the working tree in a special state that gives you all the
1262 information you need to help resolve the merge.
1263
1264 Files with conflicts are marked specially in the index, so until you
1265 resolve the problem and update the index, linkgit:git-commit[1] will
1266 fail:
1267
1268 -------------------------------------------------
1269 $ git commit
1270 file.txt: needs merge
1271 -------------------------------------------------
1272
1273 Also, linkgit:git-status[1] will list those files as "unmerged", and the
1274 files with conflicts will have conflict markers added, like this:
1275
1276 -------------------------------------------------
1277 <<<<<<< HEAD:file.txt
1278 Hello world
1279 =======
1280 Goodbye
1281 >>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
1282 -------------------------------------------------
1283
1284 All you need to do is edit the files to resolve the conflicts, and then
1285
1286 -------------------------------------------------
1287 $ git add file.txt
1288 $ git commit
1289 -------------------------------------------------
1290
1291 Note that the commit message will already be filled in for you with
1292 some information about the merge. Normally you can just use this
1293 default message unchanged, but you may add additional commentary of
1294 your own if desired.
1295
1296 The above is all you need to know to resolve a simple merge. But Git
1297 also provides more information to help resolve conflicts:
1298
1299 [[conflict-resolution]]
1300 Getting conflict-resolution help during a merge
1301 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1302
1303 All of the changes that Git was able to merge automatically are
1304 already added to the index file, so linkgit:git-diff[1] shows only
1305 the conflicts. It uses an unusual syntax:
1306
1307 -------------------------------------------------
1308 $ git diff
1309 diff --cc file.txt
1310 index 802992c,2b60207..0000000
1311 --- a/file.txt
1312 +++ b/file.txt
1313 @@@ -1,1 -1,1 +1,5 @@@
1314 ++<<<<<<< HEAD:file.txt
1315 +Hello world
1316 ++=======
1317 + Goodbye
1318 ++>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
1319 -------------------------------------------------
1320
1321 Recall that the commit which will be committed after we resolve this
1322 conflict will have two parents instead of the usual one: one parent
1323 will be HEAD, the tip of the current branch; the other will be the
1324 tip of the other branch, which is stored temporarily in MERGE_HEAD.
1325
1326 During the merge, the index holds three versions of each file. Each of
1327 these three "file stages" represents a different version of the file:
1328
1329 -------------------------------------------------
1330 $ git show :1:file.txt # the file in a common ancestor of both branches
1331 $ git show :2:file.txt # the version from HEAD.
1332 $ git show :3:file.txt # the version from MERGE_HEAD.
1333 -------------------------------------------------
1334
1335 When you ask linkgit:git-diff[1] to show the conflicts, it runs a
1336 three-way diff between the conflicted merge results in the work tree with
1337 stages 2 and 3 to show only hunks whose contents come from both sides,
1338 mixed (in other words, when a hunk's merge results come only from stage 2,
1339 that part is not conflicting and is not shown. Same for stage 3).
1340
1341 The diff above shows the differences between the working-tree version of
1342 file.txt and the stage 2 and stage 3 versions. So instead of preceding
1343 each line by a single `+` or `-`, it now uses two columns: the first
1344 column is used for differences between the first parent and the working
1345 directory copy, and the second for differences between the second parent
1346 and the working directory copy. (See the "COMBINED DIFF FORMAT" section
1347 of linkgit:git-diff-files[1] for a details of the format.)
1348
1349 After resolving the conflict in the obvious way (but before updating the
1350 index), the diff will look like:
1351
1352 -------------------------------------------------
1353 $ git diff
1354 diff --cc file.txt
1355 index 802992c,2b60207..0000000
1356 --- a/file.txt
1357 +++ b/file.txt
1358 @@@ -1,1 -1,1 +1,1 @@@
1359 - Hello world
1360 -Goodbye
1361 ++Goodbye world
1362 -------------------------------------------------
1363
1364 This shows that our resolved version deleted "Hello world" from the
1365 first parent, deleted "Goodbye" from the second parent, and added
1366 "Goodbye world", which was previously absent from both.
1367
1368 Some special diff options allow diffing the working directory against
1369 any of these stages:
1370
1371 -------------------------------------------------
1372 $ git diff -1 file.txt # diff against stage 1
1373 $ git diff --base file.txt # same as the above
1374 $ git diff -2 file.txt # diff against stage 2
1375 $ git diff --ours file.txt # same as the above
1376 $ git diff -3 file.txt # diff against stage 3
1377 $ git diff --theirs file.txt # same as the above.
1378 -------------------------------------------------
1379
1380 The linkgit:git-log[1] and linkgit:gitk[1] commands also provide special help
1381 for merges:
1382
1383 -------------------------------------------------
1384 $ git log --merge
1385 $ gitk --merge
1386 -------------------------------------------------
1387
1388 These will display all commits which exist only on HEAD or on
1389 MERGE_HEAD, and which touch an unmerged file.
1390
1391 You may also use linkgit:git-mergetool[1], which lets you merge the
1392 unmerged files using external tools such as Emacs or kdiff3.
1393
1394 Each time you resolve the conflicts in a file and update the index:
1395
1396 -------------------------------------------------
1397 $ git add file.txt
1398 -------------------------------------------------
1399
1400 the different stages of that file will be "collapsed", after which
1401 `git diff` will (by default) no longer show diffs for that file.
1402
1403 [[undoing-a-merge]]
1404 Undoing a merge
1405 ---------------
1406
1407 If you get stuck and decide to just give up and throw the whole mess
1408 away, you can always return to the pre-merge state with
1409
1410 -------------------------------------------------
1411 $ git merge --abort
1412 -------------------------------------------------
1413
1414 Or, if you've already committed the merge that you want to throw away,
1415
1416 -------------------------------------------------
1417 $ git reset --hard ORIG_HEAD
1418 -------------------------------------------------
1419
1420 However, this last command can be dangerous in some cases--never
1421 throw away a commit you have already committed if that commit may
1422 itself have been merged into another branch, as doing so may confuse
1423 further merges.
1424
1425 [[fast-forwards]]
1426 Fast-forward merges
1427 -------------------
1428
1429 There is one special case not mentioned above, which is treated
1430 differently. Normally, a merge results in a merge commit, with two
1431 parents, one pointing at each of the two lines of development that
1432 were merged.
1433
1434 However, if the current branch is an ancestor of the other--so every commit
1435 present in the current branch is already contained in the other branch--then Git
1436 just performs a "fast-forward"; the head of the current branch is moved forward
1437 to point at the head of the merged-in branch, without any new commits being
1438 created.
1439
1440 [[fixing-mistakes]]
1441 Fixing mistakes
1442 ---------------
1443
1444 If you've messed up the working tree, but haven't yet committed your
1445 mistake, you can return the entire working tree to the last committed
1446 state with
1447
1448 -------------------------------------------------
1449 $ git restore --staged --worktree :/
1450 -------------------------------------------------
1451
1452 If you make a commit that you later wish you hadn't, there are two
1453 fundamentally different ways to fix the problem:
1454
1455 1. You can create a new commit that undoes whatever was done
1456 by the old commit. This is the correct thing if your
1457 mistake has already been made public.
1458
1459 2. You can go back and modify the old commit. You should
1460 never do this if you have already made the history public;
1461 Git does not normally expect the "history" of a project to
1462 change, and cannot correctly perform repeated merges from
1463 a branch that has had its history changed.
1464
1465 [[reverting-a-commit]]
1466 Fixing a mistake with a new commit
1467 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1468
1469 Creating a new commit that reverts an earlier change is very easy;
1470 just pass the linkgit:git-revert[1] command a reference to the bad
1471 commit; for example, to revert the most recent commit:
1472
1473 -------------------------------------------------
1474 $ git revert HEAD
1475 -------------------------------------------------
1476
1477 This will create a new commit which undoes the change in HEAD. You
1478 will be given a chance to edit the commit message for the new commit.
1479
1480 You can also revert an earlier change, for example, the next-to-last:
1481
1482 -------------------------------------------------
1483 $ git revert HEAD^
1484 -------------------------------------------------
1485
1486 In this case Git will attempt to undo the old change while leaving
1487 intact any changes made since then. If more recent changes overlap
1488 with the changes to be reverted, then you will be asked to fix
1489 conflicts manually, just as in the case of <<resolving-a-merge,
1490 resolving a merge>>.
1491
1492 [[fixing-a-mistake-by-rewriting-history]]
1493 Fixing a mistake by rewriting history
1494 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1495
1496 If the problematic commit is the most recent commit, and you have not
1497 yet made that commit public, then you may just
1498 <<undoing-a-merge,destroy it using `git reset`>>.
1499
1500 Alternatively, you
1501 can edit the working directory and update the index to fix your
1502 mistake, just as if you were going to <<how-to-make-a-commit,create a
1503 new commit>>, then run
1504
1505 -------------------------------------------------
1506 $ git commit --amend
1507 -------------------------------------------------
1508
1509 which will replace the old commit by a new commit incorporating your
1510 changes, giving you a chance to edit the old commit message first.
1511
1512 Again, you should never do this to a commit that may already have
1513 been merged into another branch; use linkgit:git-revert[1] instead in
1514 that case.
1515
1516 It is also possible to replace commits further back in the history, but
1517 this is an advanced topic to be left for
1518 <<cleaning-up-history,another chapter>>.
1519
1520 [[checkout-of-path]]
1521 Checking out an old version of a file
1522 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1523
1524 In the process of undoing a previous bad change, you may find it
1525 useful to check out an older version of a particular file using
1526 linkgit:git-restore[1]. The command
1527
1528 -------------------------------------------------
1529 $ git restore --source=HEAD^ path/to/file
1530 -------------------------------------------------
1531
1532 replaces path/to/file by the contents it had in the commit HEAD^, and
1533 also updates the index to match. It does not change branches.
1534
1535 If you just want to look at an old version of the file, without
1536 modifying the working directory, you can do that with
1537 linkgit:git-show[1]:
1538
1539 -------------------------------------------------
1540 $ git show HEAD^:path/to/file
1541 -------------------------------------------------
1542
1543 which will display the given version of the file.
1544
1545 [[interrupted-work]]
1546 Temporarily setting aside work in progress
1547 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1548
1549 While you are in the middle of working on something complicated, you
1550 find an unrelated but obvious and trivial bug. You would like to fix it
1551 before continuing. You can use linkgit:git-stash[1] to save the current
1552 state of your work, and after fixing the bug (or, optionally after doing
1553 so on a different branch and then coming back), unstash the
1554 work-in-progress changes.
1555
1556 ------------------------------------------------
1557 $ git stash push -m "work in progress for foo feature"
1558 ------------------------------------------------
1559
1560 This command will save your changes away to the `stash`, and
1561 reset your working tree and the index to match the tip of your
1562 current branch. Then you can make your fix as usual.
1563
1564 ------------------------------------------------
1565 ... edit and test ...
1566 $ git commit -a -m "blorpl: typofix"
1567 ------------------------------------------------
1568
1569 After that, you can go back to what you were working on with
1570 `git stash pop`:
1571
1572 ------------------------------------------------
1573 $ git stash pop
1574 ------------------------------------------------
1575
1576
1577 [[ensuring-good-performance]]
1578 Ensuring good performance
1579 -------------------------
1580
1581 On large repositories, Git depends on compression to keep the history
1582 information from taking up too much space on disk or in memory. Some
1583 Git commands may automatically run linkgit:git-gc[1], so you don't
1584 have to worry about running it manually. However, compressing a large
1585 repository may take a while, so you may want to call `gc` explicitly
1586 to avoid automatic compression kicking in when it is not convenient.
1587
1588
1589 [[ensuring-reliability]]
1590 Ensuring reliability
1591 --------------------
1592
1593 [[checking-for-corruption]]
1594 Checking the repository for corruption
1595 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1596
1597 The linkgit:git-fsck[1] command runs a number of self-consistency checks
1598 on the repository, and reports on any problems. This may take some
1599 time.
1600
1601 -------------------------------------------------
1602 $ git fsck
1603 dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
1604 dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
1605 dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
1606 dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb
1607 dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f
1608 dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e
1609 dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085
1610 dangling tree b24c2473f1fd3d91352a624795be026d64c8841f
1611 ...
1612 -------------------------------------------------
1613
1614 You will see informational messages on dangling objects. They are objects
1615 that still exist in the repository but are no longer referenced by any of
1616 your branches, and can (and will) be removed after a while with `gc`.
1617 You can run `git fsck --no-dangling` to suppress these messages, and still
1618 view real errors.
1619
1620 [[recovering-lost-changes]]
1621 Recovering lost changes
1622 ~~~~~~~~~~~~~~~~~~~~~~~
1623
1624 [[reflogs]]
1625 Reflogs
1626 ^^^^^^^
1627
1628 Say you modify a branch with <<fixing-mistakes,`git reset --hard`>>,
1629 and then realize that the branch was the only reference you had to
1630 that point in history.
1631
1632 Fortunately, Git also keeps a log, called a "reflog", of all the
1633 previous values of each branch. So in this case you can still find the
1634 old history using, for example,
1635
1636 -------------------------------------------------
1637 $ git log master@{1}
1638 -------------------------------------------------
1639
1640 This lists the commits reachable from the previous version of the
1641 `master` branch head. This syntax can be used with any Git command
1642 that accepts a commit, not just with `git log`. Some other examples:
1643
1644 -------------------------------------------------
1645 $ git show master@{2} # See where the branch pointed 2,
1646 $ git show master@{3} # 3, ... changes ago.
1647 $ gitk master@{yesterday} # See where it pointed yesterday,
1648 $ gitk master@{"1 week ago"} # ... or last week
1649 $ git log --walk-reflogs master # show reflog entries for master
1650 -------------------------------------------------
1651
1652 A separate reflog is kept for the HEAD, so
1653
1654 -------------------------------------------------
1655 $ git show HEAD@{"1 week ago"}
1656 -------------------------------------------------
1657
1658 will show what HEAD pointed to one week ago, not what the current branch
1659 pointed to one week ago. This allows you to see the history of what
1660 you've checked out.
1661
1662 The reflogs are kept by default for 30 days, after which they may be
1663 pruned. See linkgit:git-reflog[1] and linkgit:git-gc[1] to learn
1664 how to control this pruning, and see the "SPECIFYING REVISIONS"
1665 section of linkgit:gitrevisions[7] for details.
1666
1667 Note that the reflog history is very different from normal Git history.
1668 While normal history is shared by every repository that works on the
1669 same project, the reflog history is not shared: it tells you only about
1670 how the branches in your local repository have changed over time.
1671
1672 [[dangling-object-recovery]]
1673 Examining dangling objects
1674 ^^^^^^^^^^^^^^^^^^^^^^^^^^
1675
1676 In some situations the reflog may not be able to save you. For example,
1677 suppose you delete a branch, then realize you need the history it
1678 contained. The reflog is also deleted; however, if you have not yet
1679 pruned the repository, then you may still be able to find the lost
1680 commits in the dangling objects that `git fsck` reports. See
1681 <<dangling-objects>> for the details.
1682
1683 -------------------------------------------------
1684 $ git fsck
1685 dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
1686 dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
1687 dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
1688 ...
1689 -------------------------------------------------
1690
1691 You can examine
1692 one of those dangling commits with, for example,
1693
1694 ------------------------------------------------
1695 $ gitk 7281251ddd --not --all
1696 ------------------------------------------------
1697
1698 which does what it sounds like: it says that you want to see the commit
1699 history that is described by the dangling commit(s), but not the
1700 history that is described by all your existing branches and tags. Thus
1701 you get exactly the history reachable from that commit that is lost.
1702 (And notice that it might not be just one commit: we only report the
1703 "tip of the line" as being dangling, but there might be a whole deep
1704 and complex commit history that was dropped.)
1705
1706 If you decide you want the history back, you can always create a new
1707 reference pointing to it, for example, a new branch:
1708
1709 ------------------------------------------------
1710 $ git branch recovered-branch 7281251ddd
1711 ------------------------------------------------
1712
1713 Other types of dangling objects (blobs and trees) are also possible, and
1714 dangling objects can arise in other situations.
1715
1716
1717 [[sharing-development]]
1718 Sharing development with others
1719 ===============================
1720
1721 [[getting-updates-With-git-pull]]
1722 Getting updates with git pull
1723 -----------------------------
1724
1725 After you clone a repository and commit a few changes of your own, you
1726 may wish to check the original repository for updates and merge them
1727 into your own work.
1728
1729 We have already seen <<Updating-a-repository-With-git-fetch,how to
1730 keep remote-tracking branches up to date>> with linkgit:git-fetch[1],
1731 and how to merge two branches. So you can merge in changes from the
1732 original repository's master branch with:
1733
1734 -------------------------------------------------
1735 $ git fetch
1736 $ git merge origin/master
1737 -------------------------------------------------
1738
1739 However, the linkgit:git-pull[1] command provides a way to do this in
1740 one step:
1741
1742 -------------------------------------------------
1743 $ git pull origin master
1744 -------------------------------------------------
1745
1746 In fact, if you have `master` checked out, then this branch has been
1747 configured by `git clone` to get changes from the HEAD branch of the
1748 origin repository. So often you can
1749 accomplish the above with just a simple
1750
1751 -------------------------------------------------
1752 $ git pull
1753 -------------------------------------------------
1754
1755 This command will fetch changes from the remote branches to your
1756 remote-tracking branches `origin/*`, and merge the default branch into
1757 the current branch.
1758
1759 More generally, a branch that is created from a remote-tracking branch
1760 will pull
1761 by default from that branch. See the descriptions of the
1762 `branch.<name>.remote` and `branch.<name>.merge` options in
1763 linkgit:git-config[1], and the discussion of the `--track` option in
1764 linkgit:git-checkout[1], to learn how to control these defaults.
1765
1766 In addition to saving you keystrokes, `git pull` also helps you by
1767 producing a default commit message documenting the branch and
1768 repository that you pulled from.
1769
1770 (But note that no such commit will be created in the case of a
1771 <<fast-forwards,fast-forward>>; instead, your branch will just be
1772 updated to point to the latest commit from the upstream branch.)
1773
1774 The `git pull` command can also be given `.` as the "remote" repository,
1775 in which case it just merges in a branch from the current repository; so
1776 the commands
1777
1778 -------------------------------------------------
1779 $ git pull . branch
1780 $ git merge branch
1781 -------------------------------------------------
1782
1783 are roughly equivalent.
1784
1785 [[submitting-patches]]
1786 Submitting patches to a project
1787 -------------------------------
1788
1789 If you just have a few changes, the simplest way to submit them may
1790 just be to send them as patches in email:
1791
1792 First, use linkgit:git-format-patch[1]; for example:
1793
1794 -------------------------------------------------
1795 $ git format-patch origin
1796 -------------------------------------------------
1797
1798 will produce a numbered series of files in the current directory, one
1799 for each patch in the current branch but not in `origin/HEAD`.
1800
1801 `git format-patch` can include an initial "cover letter". You can insert
1802 commentary on individual patches after the three dash line which
1803 `format-patch` places after the commit message but before the patch
1804 itself. If you use `git notes` to track your cover letter material,
1805 `git format-patch --notes` will include the commit's notes in a similar
1806 manner.
1807
1808 You can then import these into your mail client and send them by
1809 hand. However, if you have a lot to send at once, you may prefer to
1810 use the linkgit:git-send-email[1] script to automate the process.
1811 Consult the mailing list for your project first to determine
1812 their requirements for submitting patches.
1813
1814 [[importing-patches]]
1815 Importing patches to a project
1816 ------------------------------
1817
1818 Git also provides a tool called linkgit:git-am[1] (am stands for
1819 "apply mailbox"), for importing such an emailed series of patches.
1820 Just save all of the patch-containing messages, in order, into a
1821 single mailbox file, say `patches.mbox`, then run
1822
1823 -------------------------------------------------
1824 $ git am -3 patches.mbox
1825 -------------------------------------------------
1826
1827 Git will apply each patch in order; if any conflicts are found, it
1828 will stop, and you can fix the conflicts as described in
1829 "<<resolving-a-merge,Resolving a merge>>". (The `-3` option tells
1830 Git to perform a merge; if you would prefer it just to abort and
1831 leave your tree and index untouched, you may omit that option.)
1832
1833 Once the index is updated with the results of the conflict
1834 resolution, instead of creating a new commit, just run
1835
1836 -------------------------------------------------
1837 $ git am --continue
1838 -------------------------------------------------
1839
1840 and Git will create the commit for you and continue applying the
1841 remaining patches from the mailbox.
1842
1843 The final result will be a series of commits, one for each patch in
1844 the original mailbox, with authorship and commit log message each
1845 taken from the message containing each patch.
1846
1847 [[public-repositories]]
1848 Public Git repositories
1849 -----------------------
1850
1851 Another way to submit changes to a project is to tell the maintainer
1852 of that project to pull the changes from your repository using
1853 linkgit:git-pull[1]. In the section "<<getting-updates-With-git-pull,
1854 Getting updates with `git pull`>>" we described this as a way to get
1855 updates from the "main" repository, but it works just as well in the
1856 other direction.
1857
1858 If you and the maintainer both have accounts on the same machine, then
1859 you can just pull changes from each other's repositories directly;
1860 commands that accept repository URLs as arguments will also accept a
1861 local directory name:
1862
1863 -------------------------------------------------
1864 $ git clone /path/to/repository
1865 $ git pull /path/to/other/repository
1866 -------------------------------------------------
1867
1868 or an ssh URL:
1869
1870 -------------------------------------------------
1871 $ git clone ssh://yourhost/~you/repository
1872 -------------------------------------------------
1873
1874 For projects with few developers, or for synchronizing a few private
1875 repositories, this may be all you need.
1876
1877 However, the more common way to do this is to maintain a separate public
1878 repository (usually on a different host) for others to pull changes
1879 from. This is usually more convenient, and allows you to cleanly
1880 separate private work in progress from publicly visible work.
1881
1882 You will continue to do your day-to-day work in your personal
1883 repository, but periodically "push" changes from your personal
1884 repository into your public repository, allowing other developers to
1885 pull from that repository. So the flow of changes, in a situation
1886 where there is one other developer with a public repository, looks
1887 like this:
1888
1889 you push
1890 your personal repo ------------------> your public repo
1891 ^ |
1892 | |
1893 | you pull | they pull
1894 | |
1895 | |
1896 | they push V
1897 their public repo <------------------- their repo
1898
1899 We explain how to do this in the following sections.
1900
1901 [[setting-up-a-public-repository]]
1902 Setting up a public repository
1903 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1904
1905 Assume your personal repository is in the directory `~/proj`. We
1906 first create a new clone of the repository and tell `git daemon` that it
1907 is meant to be public:
1908
1909 -------------------------------------------------
1910 $ git clone --bare ~/proj proj.git
1911 $ touch proj.git/git-daemon-export-ok
1912 -------------------------------------------------
1913
1914 The resulting directory proj.git contains a "bare" git repository--it is
1915 just the contents of the `.git` directory, without any files checked out
1916 around it.
1917
1918 Next, copy `proj.git` to the server where you plan to host the
1919 public repository. You can use scp, rsync, or whatever is most
1920 convenient.
1921
1922 [[exporting-via-git]]
1923 Exporting a Git repository via the Git protocol
1924 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1925
1926 This is the preferred method.
1927
1928 If someone else administers the server, they should tell you what
1929 directory to put the repository in, and what `git://` URL it will
1930 appear at. You can then skip to the section
1931 "<<pushing-changes-to-a-public-repository,Pushing changes to a public
1932 repository>>", below.
1933
1934 Otherwise, all you need to do is start linkgit:git-daemon[1]; it will
1935 listen on port 9418. By default, it will allow access to any directory
1936 that looks like a Git directory and contains the magic file
1937 git-daemon-export-ok. Passing some directory paths as `git daemon`
1938 arguments will further restrict the exports to those paths.
1939
1940 You can also run `git daemon` as an inetd service; see the
1941 linkgit:git-daemon[1] man page for details. (See especially the
1942 examples section.)
1943
1944 [[exporting-via-http]]
1945 Exporting a git repository via HTTP
1946 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1947
1948 The Git protocol gives better performance and reliability, but on a
1949 host with a web server set up, HTTP exports may be simpler to set up.
1950
1951 All you need to do is place the newly created bare Git repository in
1952 a directory that is exported by the web server, and make some
1953 adjustments to give web clients some extra information they need:
1954
1955 -------------------------------------------------
1956 $ mv proj.git /home/you/public_html/proj.git
1957 $ cd proj.git
1958 $ git --bare update-server-info
1959 $ mv hooks/post-update.sample hooks/post-update
1960 -------------------------------------------------
1961
1962 (For an explanation of the last two lines, see
1963 linkgit:git-update-server-info[1] and linkgit:githooks[5].)
1964
1965 Advertise the URL of `proj.git`. Anybody else should then be able to
1966 clone or pull from that URL, for example with a command line like:
1967
1968 -------------------------------------------------
1969 $ git clone http://yourserver.com/~you/proj.git
1970 -------------------------------------------------
1971
1972 (See also
1973 link:howto/setup-git-server-over-http.html[setup-git-server-over-http]
1974 for a slightly more sophisticated setup using WebDAV which also
1975 allows pushing over HTTP.)
1976
1977 [[pushing-changes-to-a-public-repository]]
1978 Pushing changes to a public repository
1979 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1980
1981 Note that the two techniques outlined above (exporting via
1982 <<exporting-via-http,http>> or <<exporting-via-git,git>>) allow other
1983 maintainers to fetch your latest changes, but they do not allow write
1984 access, which you will need to update the public repository with the
1985 latest changes created in your private repository.
1986
1987 The simplest way to do this is using linkgit:git-push[1] and ssh; to
1988 update the remote branch named `master` with the latest state of your
1989 branch named `master`, run
1990
1991 -------------------------------------------------
1992 $ git push ssh://yourserver.com/~you/proj.git master:master
1993 -------------------------------------------------
1994
1995 or just
1996
1997 -------------------------------------------------
1998 $ git push ssh://yourserver.com/~you/proj.git master
1999 -------------------------------------------------
2000
2001 As with `git fetch`, `git push` will complain if this does not result in a
2002 <<fast-forwards,fast-forward>>; see the following section for details on
2003 handling this case.
2004
2005 Note that the target of a `push` is normally a
2006 <<def_bare_repository,bare>> repository. You can also push to a
2007 repository that has a checked-out working tree, but a push to update the
2008 currently checked-out branch is denied by default to prevent confusion.
2009 See the description of the receive.denyCurrentBranch option
2010 in linkgit:git-config[1] for details.
2011
2012 As with `git fetch`, you may also set up configuration options to
2013 save typing; so, for example:
2014
2015 -------------------------------------------------
2016 $ git remote add public-repo ssh://yourserver.com/~you/proj.git
2017 -------------------------------------------------
2018
2019 adds the following to `.git/config`:
2020
2021 -------------------------------------------------
2022 [remote "public-repo"]
2023 url = yourserver.com:proj.git
2024 fetch = +refs/heads/*:refs/remotes/example/*
2025 -------------------------------------------------
2026
2027 which lets you do the same push with just
2028
2029 -------------------------------------------------
2030 $ git push public-repo master
2031 -------------------------------------------------
2032
2033 See the explanations of the `remote.<name>.url`,
2034 `branch.<name>.remote`, and `remote.<name>.push` options in
2035 linkgit:git-config[1] for details.
2036
2037 [[forcing-push]]
2038 What to do when a push fails
2039 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2040
2041 If a push would not result in a <<fast-forwards,fast-forward>> of the
2042 remote branch, then it will fail with an error like:
2043
2044 -------------------------------------------------
2045 ! [rejected] master -> master (non-fast-forward)
2046 error: failed to push some refs to '...'
2047 hint: Updates were rejected because the tip of your current branch is behind
2048 hint: its remote counterpart. Integrate the remote changes (e.g.
2049 hint: 'git pull ...') before pushing again.
2050 hint: See the 'Note about fast-forwards' in 'git push --help' for details.
2051 -------------------------------------------------
2052
2053 This can happen, for example, if you:
2054
2055 - use `git reset --hard` to remove already-published commits, or
2056 - use `git commit --amend` to replace already-published commits
2057 (as in <<fixing-a-mistake-by-rewriting-history>>), or
2058 - use `git rebase` to rebase any already-published commits (as
2059 in <<using-git-rebase>>).
2060
2061 You may force `git push` to perform the update anyway by preceding the
2062 branch name with a plus sign:
2063
2064 -------------------------------------------------
2065 $ git push ssh://yourserver.com/~you/proj.git +master
2066 -------------------------------------------------
2067
2068 Note the addition of the `+` sign. Alternatively, you can use the
2069 `-f` flag to force the remote update, as in:
2070
2071 -------------------------------------------------
2072 $ git push -f ssh://yourserver.com/~you/proj.git master
2073 -------------------------------------------------
2074
2075 Normally whenever a branch head in a public repository is modified, it
2076 is modified to point to a descendant of the commit that it pointed to
2077 before. By forcing a push in this situation, you break that convention.
2078 (See <<problems-With-rewriting-history>>.)
2079
2080 Nevertheless, this is a common practice for people that need a simple
2081 way to publish a work-in-progress patch series, and it is an acceptable
2082 compromise as long as you warn other developers that this is how you
2083 intend to manage the branch.
2084
2085 It's also possible for a push to fail in this way when other people have
2086 the right to push to the same repository. In that case, the correct
2087 solution is to retry the push after first updating your work: either by a
2088 pull, or by a fetch followed by a rebase; see the
2089 <<setting-up-a-shared-repository,next section>> and
2090 linkgit:gitcvs-migration[7] for more.
2091
2092 [[setting-up-a-shared-repository]]
2093 Setting up a shared repository
2094 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2095
2096 Another way to collaborate is by using a model similar to that
2097 commonly used in CVS, where several developers with special rights
2098 all push to and pull from a single shared repository. See
2099 linkgit:gitcvs-migration[7] for instructions on how to
2100 set this up.
2101
2102 However, while there is nothing wrong with Git's support for shared
2103 repositories, this mode of operation is not generally recommended,
2104 simply because the mode of collaboration that Git supports--by
2105 exchanging patches and pulling from public repositories--has so many
2106 advantages over the central shared repository:
2107
2108 - Git's ability to quickly import and merge patches allows a
2109 single maintainer to process incoming changes even at very
2110 high rates. And when that becomes too much, `git pull` provides
2111 an easy way for that maintainer to delegate this job to other
2112 maintainers while still allowing optional review of incoming
2113 changes.
2114 - Since every developer's repository has the same complete copy
2115 of the project history, no repository is special, and it is
2116 trivial for another developer to take over maintenance of a
2117 project, either by mutual agreement, or because a maintainer
2118 becomes unresponsive or difficult to work with.
2119 - The lack of a central group of "committers" means there is
2120 less need for formal decisions about who is "in" and who is
2121 "out".
2122
2123 [[setting-up-gitweb]]
2124 Allowing web browsing of a repository
2125 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2126
2127 The gitweb cgi script provides users an easy way to browse your
2128 project's revisions, file contents and logs without having to install
2129 Git. Features like RSS/Atom feeds and blame/annotation details may
2130 optionally be enabled.
2131
2132 The linkgit:git-instaweb[1] command provides a simple way to start
2133 browsing the repository using gitweb. The default server when using
2134 instaweb is lighttpd.
2135
2136 See the file gitweb/INSTALL in the Git source tree and
2137 linkgit:gitweb[1] for instructions on details setting up a permanent
2138 installation with a CGI or Perl capable server.
2139
2140 [[how-to-get-a-git-repository-with-minimal-history]]
2141 How to get a Git repository with minimal history
2142 ------------------------------------------------
2143
2144 A <<def_shallow_clone,shallow clone>>, with its truncated
2145 history, is useful when one is interested only in recent history
2146 of a project and getting full history from the upstream is
2147 expensive.
2148
2149 A <<def_shallow_clone,shallow clone>> is created by specifying
2150 the linkgit:git-clone[1] `--depth` switch. The depth can later be
2151 changed with the linkgit:git-fetch[1] `--depth` switch, or full
2152 history restored with `--unshallow`.
2153
2154 Merging inside a <<def_shallow_clone,shallow clone>> will work as long
2155 as a merge base is in the recent history.
2156 Otherwise, it will be like merging unrelated histories and may
2157 have to result in huge conflicts. This limitation may make such
2158 a repository unsuitable to be used in merge based workflows.
2159
2160 [[sharing-development-examples]]
2161 Examples
2162 --------
2163
2164 [[maintaining-topic-branches]]
2165 Maintaining topic branches for a Linux subsystem maintainer
2166 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2167
2168 This describes how Tony Luck uses Git in his role as maintainer of the
2169 IA64 architecture for the Linux kernel.
2170
2171 He uses two public branches:
2172
2173 - A "test" tree into which patches are initially placed so that they
2174 can get some exposure when integrated with other ongoing development.
2175 This tree is available to Andrew for pulling into -mm whenever he
2176 wants.
2177
2178 - A "release" tree into which tested patches are moved for final sanity
2179 checking, and as a vehicle to send them upstream to Linus (by sending
2180 him a "please pull" request.)
2181
2182 He also uses a set of temporary branches ("topic branches"), each
2183 containing a logical grouping of patches.
2184
2185 To set this up, first create your work tree by cloning Linus's public
2186 tree:
2187
2188 -------------------------------------------------
2189 $ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git work
2190 $ cd work
2191 -------------------------------------------------
2192
2193 Linus's tree will be stored in the remote-tracking branch named origin/master,
2194 and can be updated using linkgit:git-fetch[1]; you can track other
2195 public trees using linkgit:git-remote[1] to set up a "remote" and
2196 linkgit:git-fetch[1] to keep them up to date; see
2197 <<repositories-and-branches>>.
2198
2199 Now create the branches in which you are going to work; these start out
2200 at the current tip of origin/master branch, and should be set up (using
2201 the `--track` option to linkgit:git-branch[1]) to merge changes in from
2202 Linus by default.
2203
2204 -------------------------------------------------
2205 $ git branch --track test origin/master
2206 $ git branch --track release origin/master
2207 -------------------------------------------------
2208
2209 These can be easily kept up to date using linkgit:git-pull[1].
2210
2211 -------------------------------------------------
2212 $ git switch test && git pull
2213 $ git switch release && git pull
2214 -------------------------------------------------
2215
2216 Important note! If you have any local changes in these branches, then
2217 this merge will create a commit object in the history (with no local
2218 changes Git will simply do a "fast-forward" merge). Many people dislike
2219 the "noise" that this creates in the Linux history, so you should avoid
2220 doing this capriciously in the `release` branch, as these noisy commits
2221 will become part of the permanent history when you ask Linus to pull
2222 from the release branch.
2223
2224 A few configuration variables (see linkgit:git-config[1]) can
2225 make it easy to push both branches to your public tree. (See
2226 <<setting-up-a-public-repository>>.)
2227
2228 -------------------------------------------------
2229 $ cat >> .git/config <<EOF
2230 [remote "mytree"]
2231 url = master.kernel.org:/pub/scm/linux/kernel/git/aegl/linux.git
2232 push = release
2233 push = test
2234 EOF
2235 -------------------------------------------------
2236
2237 Then you can push both the test and release trees using
2238 linkgit:git-push[1]:
2239
2240 -------------------------------------------------
2241 $ git push mytree
2242 -------------------------------------------------
2243
2244 or push just one of the test and release branches using:
2245
2246 -------------------------------------------------
2247 $ git push mytree test
2248 -------------------------------------------------
2249
2250 or
2251
2252 -------------------------------------------------
2253 $ git push mytree release
2254 -------------------------------------------------
2255
2256 Now to apply some patches from the community. Think of a short
2257 snappy name for a branch to hold this patch (or related group of
2258 patches), and create a new branch from a recent stable tag of
2259 Linus's branch. Picking a stable base for your branch will:
2260 1) help you: by avoiding inclusion of unrelated and perhaps lightly
2261 tested changes
2262 2) help future bug hunters that use `git bisect` to find problems
2263
2264 -------------------------------------------------
2265 $ git switch -c speed-up-spinlocks v2.6.35
2266 -------------------------------------------------
2267
2268 Now you apply the patch(es), run some tests, and commit the change(s). If
2269 the patch is a multi-part series, then you should apply each as a separate
2270 commit to this branch.
2271
2272 -------------------------------------------------
2273 $ ... patch ... test ... commit [ ... patch ... test ... commit ]*
2274 -------------------------------------------------
2275
2276 When you are happy with the state of this change, you can merge it into the
2277 "test" branch in preparation to make it public:
2278
2279 -------------------------------------------------
2280 $ git switch test && git merge speed-up-spinlocks
2281 -------------------------------------------------
2282
2283 It is unlikely that you would have any conflicts here ... but you might if you
2284 spent a while on this step and had also pulled new versions from upstream.
2285
2286 Sometime later when enough time has passed and testing done, you can pull the
2287 same branch into the `release` tree ready to go upstream. This is where you
2288 see the value of keeping each patch (or patch series) in its own branch. It
2289 means that the patches can be moved into the `release` tree in any order.
2290
2291 -------------------------------------------------
2292 $ git switch release && git merge speed-up-spinlocks
2293 -------------------------------------------------
2294
2295 After a while, you will have a number of branches, and despite the
2296 well chosen names you picked for each of them, you may forget what
2297 they are for, or what status they are in. To get a reminder of what
2298 changes are in a specific branch, use:
2299
2300 -------------------------------------------------
2301 $ git log linux..branchname | git shortlog
2302 -------------------------------------------------
2303
2304 To see whether it has already been merged into the test or release branches,
2305 use:
2306
2307 -------------------------------------------------
2308 $ git log test..branchname
2309 -------------------------------------------------
2310
2311 or
2312
2313 -------------------------------------------------
2314 $ git log release..branchname
2315 -------------------------------------------------
2316
2317 (If this branch has not yet been merged, you will see some log entries.
2318 If it has been merged, then there will be no output.)
2319
2320 Once a patch completes the great cycle (moving from test to release,
2321 then pulled by Linus, and finally coming back into your local
2322 `origin/master` branch), the branch for this change is no longer needed.
2323 You detect this when the output from:
2324
2325 -------------------------------------------------
2326 $ git log origin..branchname
2327 -------------------------------------------------
2328
2329 is empty. At this point the branch can be deleted:
2330
2331 -------------------------------------------------
2332 $ git branch -d branchname
2333 -------------------------------------------------
2334
2335 Some changes are so trivial that it is not necessary to create a separate
2336 branch and then merge into each of the test and release branches. For
2337 these changes, just apply directly to the `release` branch, and then
2338 merge that into the `test` branch.
2339
2340 After pushing your work to `mytree`, you can use
2341 linkgit:git-request-pull[1] to prepare a "please pull" request message
2342 to send to Linus:
2343
2344 -------------------------------------------------
2345 $ git push mytree
2346 $ git request-pull origin mytree release
2347 -------------------------------------------------
2348
2349 Here are some of the scripts that simplify all this even further.
2350
2351 -------------------------------------------------
2352 ==== update script ====
2353 # Update a branch in my Git tree. If the branch to be updated
2354 # is origin, then pull from kernel.org. Otherwise merge
2355 # origin/master branch into test|release branch
2356
2357 case "$1" in
2358 test|release)
2359 git checkout $1 && git pull . origin
2360 ;;
2361 origin)
2362 before=$(git rev-parse refs/remotes/origin/master)
2363 git fetch origin
2364 after=$(git rev-parse refs/remotes/origin/master)
2365 if [ $before != $after ]
2366 then
2367 git log $before..$after | git shortlog
2368 fi
2369 ;;
2370 *)
2371 echo "usage: $0 origin|test|release" 1>&2
2372 exit 1
2373 ;;
2374 esac
2375 -------------------------------------------------
2376
2377 -------------------------------------------------
2378 ==== merge script ====
2379 # Merge a branch into either the test or release branch
2380
2381 pname=$0
2382
2383 usage()
2384 {
2385 echo "usage: $pname branch test|release" 1>&2
2386 exit 1
2387 }
2388
2389 git show-ref -q --verify -- refs/heads/"$1" || {
2390 echo "Can't see branch <$1>" 1>&2
2391 usage
2392 }
2393
2394 case "$2" in
2395 test|release)
2396 if [ $(git log $2..$1 | wc -c) -eq 0 ]
2397 then
2398 echo $1 already merged into $2 1>&2
2399 exit 1
2400 fi
2401 git checkout $2 && git pull . $1
2402 ;;
2403 *)
2404 usage
2405 ;;
2406 esac
2407 -------------------------------------------------
2408
2409 -------------------------------------------------
2410 ==== status script ====
2411 # report on status of my ia64 Git tree
2412
2413 gb=$(tput setab 2)
2414 rb=$(tput setab 1)
2415 restore=$(tput setab 9)
2416
2417 if [ `git rev-list test..release | wc -c` -gt 0 ]
2418 then
2419 echo $rb Warning: commits in release that are not in test $restore
2420 git log test..release
2421 fi
2422
2423 for branch in `git show-ref --heads | sed 's|^.*/||'`
2424 do
2425 if [ $branch = test -o $branch = release ]
2426 then
2427 continue
2428 fi
2429
2430 echo -n $gb ======= $branch ====== $restore " "
2431 status=
2432 for ref in test release origin/master
2433 do
2434 if [ `git rev-list $ref..$branch | wc -c` -gt 0 ]
2435 then
2436 status=$status${ref:0:1}
2437 fi
2438 done
2439 case $status in
2440 trl)
2441 echo $rb Need to pull into test $restore
2442 ;;
2443 rl)
2444 echo "In test"
2445 ;;
2446 l)
2447 echo "Waiting for linus"
2448 ;;
2449 "")
2450 echo $rb All done $restore
2451 ;;
2452 *)
2453 echo $rb "<$status>" $restore
2454 ;;
2455 esac
2456 git log origin/master..$branch | git shortlog
2457 done
2458 -------------------------------------------------
2459
2460
2461 [[cleaning-up-history]]
2462 Rewriting history and maintaining patch series
2463 ==============================================
2464
2465 Normally commits are only added to a project, never taken away or
2466 replaced. Git is designed with this assumption, and violating it will
2467 cause Git's merge machinery (for example) to do the wrong thing.
2468
2469 However, there is a situation in which it can be useful to violate this
2470 assumption.
2471
2472 [[patch-series]]
2473 Creating the perfect patch series
2474 ---------------------------------
2475
2476 Suppose you are a contributor to a large project, and you want to add a
2477 complicated feature, and to present it to the other developers in a way
2478 that makes it easy for them to read your changes, verify that they are
2479 correct, and understand why you made each change.
2480
2481 If you present all of your changes as a single patch (or commit), they
2482 may find that it is too much to digest all at once.
2483
2484 If you present them with the entire history of your work, complete with
2485 mistakes, corrections, and dead ends, they may be overwhelmed.
2486
2487 So the ideal is usually to produce a series of patches such that:
2488
2489 1. Each patch can be applied in order.
2490
2491 2. Each patch includes a single logical change, together with a
2492 message explaining the change.
2493
2494 3. No patch introduces a regression: after applying any initial
2495 part of the series, the resulting project still compiles and
2496 works, and has no bugs that it didn't have before.
2497
2498 4. The complete series produces the same end result as your own
2499 (probably much messier!) development process did.
2500
2501 We will introduce some tools that can help you do this, explain how to
2502 use them, and then explain some of the problems that can arise because
2503 you are rewriting history.
2504
2505 [[using-git-rebase]]
2506 Keeping a patch series up to date using git rebase
2507 --------------------------------------------------
2508
2509 Suppose that you create a branch `mywork` on a remote-tracking branch
2510 `origin`, and create some commits on top of it:
2511
2512 -------------------------------------------------
2513 $ git switch -c mywork origin
2514 $ vi file.txt
2515 $ git commit
2516 $ vi otherfile.txt
2517 $ git commit
2518 ...
2519 -------------------------------------------------
2520
2521 You have performed no merges into mywork, so it is just a simple linear
2522 sequence of patches on top of `origin`:
2523
2524 ................................................
2525 o--o--O <-- origin
2526 \
2527 a--b--c <-- mywork
2528 ................................................
2529
2530 Some more interesting work has been done in the upstream project, and
2531 `origin` has advanced:
2532
2533 ................................................
2534 o--o--O--o--o--o <-- origin
2535 \
2536 a--b--c <-- mywork
2537 ................................................
2538
2539 At this point, you could use `pull` to merge your changes back in;
2540 the result would create a new merge commit, like this:
2541
2542 ................................................
2543 o--o--O--o--o--o <-- origin
2544 \ \
2545 a--b--c--m <-- mywork
2546 ................................................
2547
2548 However, if you prefer to keep the history in mywork a simple series of
2549 commits without any merges, you may instead choose to use
2550 linkgit:git-rebase[1]:
2551
2552 -------------------------------------------------
2553 $ git switch mywork
2554 $ git rebase origin
2555 -------------------------------------------------
2556
2557 This will remove each of your commits from mywork, temporarily saving
2558 them as patches (in a directory named `.git/rebase-apply`), update mywork to
2559 point at the latest version of origin, then apply each of the saved
2560 patches to the new mywork. The result will look like:
2561
2562
2563 ................................................
2564 o--o--O--o--o--o <-- origin
2565 \
2566 a'--b'--c' <-- mywork
2567 ................................................
2568
2569 In the process, it may discover conflicts. In that case it will stop
2570 and allow you to fix the conflicts; after fixing conflicts, use `git add`
2571 to update the index with those contents, and then, instead of
2572 running `git commit`, just run
2573
2574 -------------------------------------------------
2575 $ git rebase --continue
2576 -------------------------------------------------
2577
2578 and Git will continue applying the rest of the patches.
2579
2580 At any point you may use the `--abort` option to abort this process and
2581 return mywork to the state it had before you started the rebase:
2582
2583 -------------------------------------------------
2584 $ git rebase --abort
2585 -------------------------------------------------
2586
2587 If you need to reorder or edit a number of commits in a branch, it may
2588 be easier to use `git rebase -i`, which allows you to reorder and
2589 squash commits, as well as marking them for individual editing during
2590 the rebase. See <<interactive-rebase>> for details, and
2591 <<reordering-patch-series>> for alternatives.
2592
2593 [[rewriting-one-commit]]
2594 Rewriting a single commit
2595 -------------------------
2596
2597 We saw in <<fixing-a-mistake-by-rewriting-history>> that you can replace the
2598 most recent commit using
2599
2600 -------------------------------------------------
2601 $ git commit --amend
2602 -------------------------------------------------
2603
2604 which will replace the old commit by a new commit incorporating your
2605 changes, giving you a chance to edit the old commit message first.
2606 This is useful for fixing typos in your last commit, or for adjusting
2607 the patch contents of a poorly staged commit.
2608
2609 If you need to amend commits from deeper in your history, you can
2610 use <<interactive-rebase,interactive rebase's `edit` instruction>>.
2611
2612 [[reordering-patch-series]]
2613 Reordering or selecting from a patch series
2614 -------------------------------------------
2615
2616 Sometimes you want to edit a commit deeper in your history. One
2617 approach is to use `git format-patch` to create a series of patches
2618 and then reset the state to before the patches:
2619
2620 -------------------------------------------------
2621 $ git format-patch origin
2622 $ git reset --hard origin
2623 -------------------------------------------------
2624
2625 Then modify, reorder, or eliminate patches as needed before applying
2626 them again with linkgit:git-am[1]:
2627
2628 -------------------------------------------------
2629 $ git am *.patch
2630 -------------------------------------------------
2631
2632 [[interactive-rebase]]
2633 Using interactive rebases
2634 -------------------------
2635
2636 You can also edit a patch series with an interactive rebase. This is
2637 the same as <<reordering-patch-series,reordering a patch series using
2638 `format-patch`>>, so use whichever interface you like best.
2639
2640 Rebase your current HEAD on the last commit you want to retain as-is.
2641 For example, if you want to reorder the last 5 commits, use:
2642
2643 -------------------------------------------------
2644 $ git rebase -i HEAD~5
2645 -------------------------------------------------
2646
2647 This will open your editor with a list of steps to be taken to perform
2648 your rebase.
2649
2650 -------------------------------------------------
2651 pick deadbee The oneline of this commit
2652 pick fa1afe1 The oneline of the next commit
2653 ...
2654
2655 # Rebase c0ffeee..deadbee onto c0ffeee
2656 #
2657 # Commands:
2658 # p, pick = use commit
2659 # r, reword = use commit, but edit the commit message
2660 # e, edit = use commit, but stop for amending
2661 # s, squash = use commit, but meld into previous commit
2662 # f, fixup = like "squash", but discard this commit's log message
2663 # x, exec = run command (the rest of the line) using shell
2664 #
2665 # These lines can be re-ordered; they are executed from top to bottom.
2666 #
2667 # If you remove a line here THAT COMMIT WILL BE LOST.
2668 #
2669 # However, if you remove everything, the rebase will be aborted.
2670 #
2671 # Note that empty commits are commented out
2672 -------------------------------------------------
2673
2674 As explained in the comments, you can reorder commits, squash them
2675 together, edit commit messages, etc. by editing the list. Once you
2676 are satisfied, save the list and close your editor, and the rebase
2677 will begin.
2678
2679 The rebase will stop where `pick` has been replaced with `edit` or
2680 when a step in the list fails to mechanically resolve conflicts and
2681 needs your help. When you are done editing and/or resolving conflicts
2682 you can continue with `git rebase --continue`. If you decide that
2683 things are getting too hairy, you can always bail out with `git rebase
2684 --abort`. Even after the rebase is complete, you can still recover
2685 the original branch by using the <<reflogs,reflog>>.
2686
2687 For a more detailed discussion of the procedure and additional tips,
2688 see the "INTERACTIVE MODE" section of linkgit:git-rebase[1].
2689
2690 [[patch-series-tools]]
2691 Other tools
2692 -----------
2693
2694 There are numerous other tools, such as StGit, which exist for the
2695 purpose of maintaining a patch series. These are outside of the scope of
2696 this manual.
2697
2698 [[problems-With-rewriting-history]]
2699 Problems with rewriting history
2700 -------------------------------
2701
2702 The primary problem with rewriting the history of a branch has to do
2703 with merging. Suppose somebody fetches your branch and merges it into
2704 their branch, with a result something like this:
2705
2706 ................................................
2707 o--o--O--o--o--o <-- origin
2708 \ \
2709 t--t--t--m <-- their branch:
2710 ................................................
2711
2712 Then suppose you modify the last three commits:
2713
2714 ................................................
2715 o--o--o <-- new head of origin
2716 /
2717 o--o--O--o--o--o <-- old head of origin
2718 ................................................
2719
2720 If we examined all this history together in one repository, it will
2721 look like:
2722
2723 ................................................
2724 o--o--o <-- new head of origin
2725 /
2726 o--o--O--o--o--o <-- old head of origin
2727 \ \
2728 t--t--t--m <-- their branch:
2729 ................................................
2730
2731 Git has no way of knowing that the new head is an updated version of
2732 the old head; it treats this situation exactly the same as it would if
2733 two developers had independently done the work on the old and new heads
2734 in parallel. At this point, if someone attempts to merge the new head
2735 in to their branch, Git will attempt to merge together the two (old and
2736 new) lines of development, instead of trying to replace the old by the
2737 new. The results are likely to be unexpected.
2738
2739 You may still choose to publish branches whose history is rewritten,
2740 and it may be useful for others to be able to fetch those branches in
2741 order to examine or test them, but they should not attempt to pull such
2742 branches into their own work.
2743
2744 For true distributed development that supports proper merging,
2745 published branches should never be rewritten.
2746
2747 [[bisect-merges]]
2748 Why bisecting merge commits can be harder than bisecting linear history
2749 -----------------------------------------------------------------------
2750
2751 The linkgit:git-bisect[1] command correctly handles history that
2752 includes merge commits. However, when the commit that it finds is a
2753 merge commit, the user may need to work harder than usual to figure out
2754 why that commit introduced a problem.
2755
2756 Imagine this history:
2757
2758 ................................................
2759 ---Z---o---X---...---o---A---C---D
2760 \ /
2761 o---o---Y---...---o---B
2762 ................................................
2763
2764 Suppose that on the upper line of development, the meaning of one
2765 of the functions that exists at Z is changed at commit X. The
2766 commits from Z leading to A change both the function's
2767 implementation and all calling sites that exist at Z, as well
2768 as new calling sites they add, to be consistent. There is no
2769 bug at A.
2770
2771 Suppose that in the meantime on the lower line of development somebody
2772 adds a new calling site for that function at commit Y. The
2773 commits from Z leading to B all assume the old semantics of that
2774 function and the callers and the callee are consistent with each
2775 other. There is no bug at B, either.
2776
2777 Suppose further that the two development lines merge cleanly at C,
2778 so no conflict resolution is required.
2779
2780 Nevertheless, the code at C is broken, because the callers added
2781 on the lower line of development have not been converted to the new
2782 semantics introduced on the upper line of development. So if all
2783 you know is that D is bad, that Z is good, and that
2784 linkgit:git-bisect[1] identifies C as the culprit, how will you
2785 figure out that the problem is due to this change in semantics?
2786
2787 When the result of a `git bisect` is a non-merge commit, you should
2788 normally be able to discover the problem by examining just that commit.
2789 Developers can make this easy by breaking their changes into small
2790 self-contained commits. That won't help in the case above, however,
2791 because the problem isn't obvious from examination of any single
2792 commit; instead, a global view of the development is required. To
2793 make matters worse, the change in semantics in the problematic
2794 function may be just one small part of the changes in the upper
2795 line of development.
2796
2797 On the other hand, if instead of merging at C you had rebased the
2798 history between Z to B on top of A, you would have gotten this
2799 linear history:
2800
2801 ................................................................
2802 ---Z---o---X--...---o---A---o---o---Y*--...---o---B*--D*
2803 ................................................................
2804
2805 Bisecting between Z and D* would hit a single culprit commit Y*,
2806 and understanding why Y* was broken would probably be easier.
2807
2808 Partly for this reason, many experienced Git users, even when
2809 working on an otherwise merge-heavy project, keep the history
2810 linear by rebasing against the latest upstream version before
2811 publishing.
2812
2813 [[advanced-branch-management]]
2814 Advanced branch management
2815 ==========================
2816
2817 [[fetching-individual-branches]]
2818 Fetching individual branches
2819 ----------------------------
2820
2821 Instead of using linkgit:git-remote[1], you can also choose just
2822 to update one branch at a time, and to store it locally under an
2823 arbitrary name:
2824
2825 -------------------------------------------------
2826 $ git fetch origin todo:my-todo-work
2827 -------------------------------------------------
2828
2829 The first argument, `origin`, just tells Git to fetch from the
2830 repository you originally cloned from. The second argument tells Git
2831 to fetch the branch named `todo` from the remote repository, and to
2832 store it locally under the name `refs/heads/my-todo-work`.
2833
2834 You can also fetch branches from other repositories; so
2835
2836 -------------------------------------------------
2837 $ git fetch git://example.com/proj.git master:example-master
2838 -------------------------------------------------
2839
2840 will create a new branch named `example-master` and store in it the
2841 branch named `master` from the repository at the given URL. If you
2842 already have a branch named example-master, it will attempt to
2843 <<fast-forwards,fast-forward>> to the commit given by example.com's
2844 master branch. In more detail:
2845
2846 [[fetch-fast-forwards]]
2847 git fetch and fast-forwards
2848 ---------------------------
2849
2850 In the previous example, when updating an existing branch, `git fetch`
2851 checks to make sure that the most recent commit on the remote
2852 branch is a descendant of the most recent commit on your copy of the
2853 branch before updating your copy of the branch to point at the new
2854 commit. Git calls this process a <<fast-forwards,fast-forward>>.
2855
2856 A fast-forward looks something like this:
2857
2858 ................................................
2859 o--o--o--o <-- old head of the branch
2860 \
2861 o--o--o <-- new head of the branch
2862 ................................................
2863
2864
2865 In some cases it is possible that the new head will *not* actually be
2866 a descendant of the old head. For example, the developer may have
2867 realized she made a serious mistake, and decided to backtrack,
2868 resulting in a situation like:
2869
2870 ................................................
2871 o--o--o--o--a--b <-- old head of the branch
2872 \
2873 o--o--o <-- new head of the branch
2874 ................................................
2875
2876 In this case, `git fetch` will fail, and print out a warning.
2877
2878 In that case, you can still force Git to update to the new head, as
2879 described in the following section. However, note that in the
2880 situation above this may mean losing the commits labeled `a` and `b`,
2881 unless you've already created a reference of your own pointing to
2882 them.
2883
2884 [[forcing-fetch]]
2885 Forcing git fetch to do non-fast-forward updates
2886 ------------------------------------------------
2887
2888 If git fetch fails because the new head of a branch is not a
2889 descendant of the old head, you may force the update with:
2890
2891 -------------------------------------------------
2892 $ git fetch git://example.com/proj.git +master:refs/remotes/example/master
2893 -------------------------------------------------
2894
2895 Note the addition of the `+` sign. Alternatively, you can use the `-f`
2896 flag to force updates of all the fetched branches, as in:
2897
2898 -------------------------------------------------
2899 $ git fetch -f origin
2900 -------------------------------------------------
2901
2902 Be aware that commits that the old version of example/master pointed at
2903 may be lost, as we saw in the previous section.
2904
2905 [[remote-branch-configuration]]
2906 Configuring remote-tracking branches
2907 ------------------------------------
2908
2909 We saw above that `origin` is just a shortcut to refer to the
2910 repository that you originally cloned from. This information is
2911 stored in Git configuration variables, which you can see using
2912 linkgit:git-config[1]:
2913
2914 -------------------------------------------------
2915 $ git config -l
2916 core.repositoryformatversion=0
2917 core.filemode=true
2918 core.logallrefupdates=true
2919 remote.origin.url=git://git.kernel.org/pub/scm/git/git.git
2920 remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
2921 branch.master.remote=origin
2922 branch.master.merge=refs/heads/master
2923 -------------------------------------------------
2924
2925 If there are other repositories that you also use frequently, you can
2926 create similar configuration options to save typing; for example,
2927
2928 -------------------------------------------------
2929 $ git remote add example git://example.com/proj.git
2930 -------------------------------------------------
2931
2932 adds the following to `.git/config`:
2933
2934 -------------------------------------------------
2935 [remote "example"]
2936 url = git://example.com/proj.git
2937 fetch = +refs/heads/*:refs/remotes/example/*
2938 -------------------------------------------------
2939
2940 Also note that the above configuration can be performed by directly
2941 editing the file `.git/config` instead of using linkgit:git-remote[1].
2942
2943 After configuring the remote, the following three commands will do the
2944 same thing:
2945
2946 -------------------------------------------------
2947 $ git fetch git://example.com/proj.git +refs/heads/*:refs/remotes/example/*
2948 $ git fetch example +refs/heads/*:refs/remotes/example/*
2949 $ git fetch example
2950 -------------------------------------------------
2951
2952 See linkgit:git-config[1] for more details on the configuration
2953 options mentioned above and linkgit:git-fetch[1] for more details on
2954 the refspec syntax.
2955
2956
2957 [[git-concepts]]
2958 Git concepts
2959 ============
2960
2961 Git is built on a small number of simple but powerful ideas. While it
2962 is possible to get things done without understanding them, you will find
2963 Git much more intuitive if you do.
2964
2965 We start with the most important, the <<def_object_database,object
2966 database>> and the <<def_index,index>>.
2967
2968 [[the-object-database]]
2969 The Object Database
2970 -------------------
2971
2972
2973 We already saw in <<understanding-commits>> that all commits are stored
2974 under a 40-digit "object name". In fact, all the information needed to
2975 represent the history of a project is stored in objects with such names.
2976 In each case the name is calculated by taking the SHA-1 hash of the
2977 contents of the object. The SHA-1 hash is a cryptographic hash function.
2978 What that means to us is that it is impossible to find two different
2979 objects with the same name. This has a number of advantages; among
2980 others:
2981
2982 - Git can quickly determine whether two objects are identical or not,
2983 just by comparing names.
2984 - Since object names are computed the same way in every repository, the
2985 same content stored in two repositories will always be stored under
2986 the same name.
2987 - Git can detect errors when it reads an object, by checking that the
2988 object's name is still the SHA-1 hash of its contents.
2989
2990 (See <<object-details>> for the details of the object formatting and
2991 SHA-1 calculation.)
2992
2993 There are four different types of objects: "blob", "tree", "commit", and
2994 "tag".
2995
2996 - A <<def_blob_object,"blob" object>> is used to store file data.
2997 - A <<def_tree_object,"tree" object>> ties one or more
2998 "blob" objects into a directory structure. In addition, a tree object
2999 can refer to other tree objects, thus creating a directory hierarchy.
3000 - A <<def_commit_object,"commit" object>> ties such directory hierarchies
3001 together into a <<def_DAG,directed acyclic graph>> of revisions--each
3002 commit contains the object name of exactly one tree designating the
3003 directory hierarchy at the time of the commit. In addition, a commit
3004 refers to "parent" commit objects that describe the history of how we
3005 arrived at that directory hierarchy.
3006 - A <<def_tag_object,"tag" object>> symbolically identifies and can be
3007 used to sign other objects. It contains the object name and type of
3008 another object, a symbolic name (of course!) and, optionally, a
3009 signature.
3010
3011 The object types in some more detail:
3012
3013 [[commit-object]]
3014 Commit Object
3015 ~~~~~~~~~~~~~
3016
3017 The "commit" object links a physical state of a tree with a description
3018 of how we got there and why. Use the `--pretty=raw` option to
3019 linkgit:git-show[1] or linkgit:git-log[1] to examine your favorite
3020 commit:
3021
3022 ------------------------------------------------
3023 $ git show -s --pretty=raw 2be7fcb476
3024 commit 2be7fcb4764f2dbcee52635b91fedb1b3dcf7ab4
3025 tree fb3a8bdd0ceddd019615af4d57a53f43d8cee2bf
3026 parent 257a84d9d02e90447b149af58b271c19405edb6a
3027 author Dave Watson <dwatson@mimvista.com> 1187576872 -0400
3028 committer Junio C Hamano <gitster@pobox.com> 1187591163 -0700
3029
3030 Fix misspelling of 'suppress' in docs
3031
3032 Signed-off-by: Junio C Hamano <gitster@pobox.com>
3033 ------------------------------------------------
3034
3035 As you can see, a commit is defined by:
3036
3037 - a tree: The SHA-1 name of a tree object (as defined below), representing
3038 the contents of a directory at a certain point in time.
3039 - parent(s): The SHA-1 name(s) of some number of commits which represent the
3040 immediately previous step(s) in the history of the project. The
3041 example above has one parent; merge commits may have more than
3042 one. A commit with no parents is called a "root" commit, and
3043 represents the initial revision of a project. Each project must have
3044 at least one root. A project can also have multiple roots, though
3045 that isn't common (or necessarily a good idea).
3046 - an author: The name of the person responsible for this change, together
3047 with its date.
3048 - a committer: The name of the person who actually created the commit,
3049 with the date it was done. This may be different from the author, for
3050 example, if the author was someone who wrote a patch and emailed it
3051 to the person who used it to create the commit.
3052 - a comment describing this commit.
3053
3054 Note that a commit does not itself contain any information about what
3055 actually changed; all changes are calculated by comparing the contents
3056 of the tree referred to by this commit with the trees associated with
3057 its parents. In particular, Git does not attempt to record file renames
3058 explicitly, though it can identify cases where the existence of the same
3059 file data at changing paths suggests a rename. (See, for example, the
3060 `-M` option to linkgit:git-diff[1]).
3061
3062 A commit is usually created by linkgit:git-commit[1], which creates a
3063 commit whose parent is normally the current HEAD, and whose tree is
3064 taken from the content currently stored in the index.
3065
3066 [[tree-object]]
3067 Tree Object
3068 ~~~~~~~~~~~
3069
3070 The ever-versatile linkgit:git-show[1] command can also be used to
3071 examine tree objects, but linkgit:git-ls-tree[1] will give you more
3072 details:
3073
3074 ------------------------------------------------
3075 $ git ls-tree fb3a8bdd0ce
3076 100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c .gitignore
3077 100644 blob 5529b198e8d14decbe4ad99db3f7fb632de0439d .mailmap
3078 100644 blob 6ff87c4664981e4397625791c8ea3bbb5f2279a3 COPYING
3079 040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745 Documentation
3080 100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200 GIT-VERSION-GEN
3081 100644 blob 289b046a443c0647624607d471289b2c7dcd470b INSTALL
3082 100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1 Makefile
3083 100644 blob 548142c327a6790ff8821d67c2ee1eff7a656b52 README
3084 ...
3085 ------------------------------------------------
3086
3087 As you can see, a tree object contains a list of entries, each with a
3088 mode, object type, SHA-1 name, and name, sorted by name. It represents
3089 the contents of a single directory tree.
3090
3091 The object type may be a blob, representing the contents of a file, or
3092 another tree, representing the contents of a subdirectory. Since trees
3093 and blobs, like all other objects, are named by the SHA-1 hash of their
3094 contents, two trees have the same SHA-1 name if and only if their
3095 contents (including, recursively, the contents of all subdirectories)
3096 are identical. This allows Git to quickly determine the differences
3097 between two related tree objects, since it can ignore any entries with
3098 identical object names.
3099
3100 (Note: in the presence of submodules, trees may also have commits as
3101 entries. See <<submodules>> for documentation.)
3102
3103 Note that the files all have mode 644 or 755: Git actually only pays
3104 attention to the executable bit.
3105
3106 [[blob-object]]
3107 Blob Object
3108 ~~~~~~~~~~~
3109
3110 You can use linkgit:git-show[1] to examine the contents of a blob; take,
3111 for example, the blob in the entry for `COPYING` from the tree above:
3112
3113 ------------------------------------------------
3114 $ git show 6ff87c4664
3115
3116 Note that the only valid version of the GPL as far as this project
3117 is concerned is _this_ particular version of the license (ie v2, not
3118 v2.2 or v3.x or whatever), unless explicitly otherwise stated.
3119 ...
3120 ------------------------------------------------
3121
3122 A "blob" object is nothing but a binary blob of data. It doesn't refer
3123 to anything else or have attributes of any kind.
3124
3125 Since the blob is entirely defined by its data, if two files in a
3126 directory tree (or in multiple different versions of the repository)
3127 have the same contents, they will share the same blob object. The object
3128 is totally independent of its location in the directory tree, and
3129 renaming a file does not change the object that file is associated with.
3130
3131 Note that any tree or blob object can be examined using
3132 linkgit:git-show[1] with the <revision>:<path> syntax. This can
3133 sometimes be useful for browsing the contents of a tree that is not
3134 currently checked out.
3135
3136 [[trust]]
3137 Trust
3138 ~~~~~
3139
3140 If you receive the SHA-1 name of a blob from one source, and its contents
3141 from another (possibly untrusted) source, you can still trust that those
3142 contents are correct as long as the SHA-1 name agrees. This is because
3143 the SHA-1 is designed so that it is infeasible to find different contents
3144 that produce the same hash.
3145
3146 Similarly, you need only trust the SHA-1 name of a top-level tree object
3147 to trust the contents of the entire directory that it refers to, and if
3148 you receive the SHA-1 name of a commit from a trusted source, then you
3149 can easily verify the entire history of commits reachable through
3150 parents of that commit, and all of those contents of the trees referred
3151 to by those commits.
3152
3153 So to introduce some real trust in the system, the only thing you need
3154 to do is to digitally sign just 'one' special note, which includes the
3155 name of a top-level commit. Your digital signature shows others
3156 that you trust that commit, and the immutability of the history of
3157 commits tells others that they can trust the whole history.
3158
3159 In other words, you can easily validate a whole archive by just
3160 sending out a single email that tells the people the name (SHA-1 hash)
3161 of the top commit, and digitally sign that email using something
3162 like GPG/PGP.
3163
3164 To assist in this, Git also provides the tag object...
3165
3166 [[tag-object]]
3167 Tag Object
3168 ~~~~~~~~~~
3169
3170 A tag object contains an object, object type, tag name, the name of the
3171 person ("tagger") who created the tag, and a message, which may contain
3172 a signature, as can be seen using linkgit:git-cat-file[1]:
3173
3174 ------------------------------------------------
3175 $ git cat-file tag v1.5.0
3176 object 437b1b20df4b356c9342dac8d38849f24ef44f27
3177 type commit
3178 tag v1.5.0
3179 tagger Junio C Hamano <junkio@cox.net> 1171411200 +0000
3180
3181 GIT 1.5.0
3182 -----BEGIN PGP SIGNATURE-----
3183 Version: GnuPG v1.4.6 (GNU/Linux)
3184
3185 iD8DBQBF0lGqwMbZpPMRm5oRAuRiAJ9ohBLd7s2kqjkKlq1qqC57SbnmzQCdG4ui
3186 nLE/L9aUXdWeTFPron96DLA=
3187 =2E+0
3188 -----END PGP SIGNATURE-----
3189 ------------------------------------------------
3190
3191 See the linkgit:git-tag[1] command to learn how to create and verify tag
3192 objects. (Note that linkgit:git-tag[1] can also be used to create
3193 "lightweight tags", which are not tag objects at all, but just simple
3194 references whose names begin with `refs/tags/`).
3195
3196 [[pack-files]]
3197 How Git stores objects efficiently: pack files
3198 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3199
3200 Newly created objects are initially created in a file named after the
3201 object's SHA-1 hash (stored in `.git/objects`).
3202
3203 Unfortunately this system becomes inefficient once a project has a
3204 lot of objects. Try this on an old project:
3205
3206 ------------------------------------------------
3207 $ git count-objects
3208 6930 objects, 47620 kilobytes
3209 ------------------------------------------------
3210
3211 The first number is the number of objects which are kept in
3212 individual files. The second is the amount of space taken up by
3213 those "loose" objects.
3214
3215 You can save space and make Git faster by moving these loose objects in
3216 to a "pack file", which stores a group of objects in an efficient
3217 compressed format; the details of how pack files are formatted can be
3218 found in link:technical/pack-format.html[pack format].
3219
3220 To put the loose objects into a pack, just run git repack:
3221
3222 ------------------------------------------------
3223 $ git repack
3224 Counting objects: 6020, done.
3225 Delta compression using up to 4 threads.
3226 Compressing objects: 100% (6020/6020), done.
3227 Writing objects: 100% (6020/6020), done.
3228 Total 6020 (delta 4070), reused 0 (delta 0)
3229 ------------------------------------------------
3230
3231 This creates a single "pack file" in .git/objects/pack/
3232 containing all currently unpacked objects. You can then run
3233
3234 ------------------------------------------------
3235 $ git prune
3236 ------------------------------------------------
3237
3238 to remove any of the "loose" objects that are now contained in the
3239 pack. This will also remove any unreferenced objects (which may be
3240 created when, for example, you use `git reset` to remove a commit).
3241 You can verify that the loose objects are gone by looking at the
3242 `.git/objects` directory or by running
3243
3244 ------------------------------------------------
3245 $ git count-objects
3246 0 objects, 0 kilobytes
3247 ------------------------------------------------
3248
3249 Although the object files are gone, any commands that refer to those
3250 objects will work exactly as they did before.
3251
3252 The linkgit:git-gc[1] command performs packing, pruning, and more for
3253 you, so is normally the only high-level command you need.
3254
3255 [[dangling-objects]]
3256 Dangling objects
3257 ~~~~~~~~~~~~~~~~
3258
3259 The linkgit:git-fsck[1] command will sometimes complain about dangling
3260 objects. They are not a problem.
3261
3262 The most common cause of dangling objects is that you've rebased a
3263 branch, or you have pulled from somebody else who rebased a branch--see
3264 <<cleaning-up-history>>. In that case, the old head of the original
3265 branch still exists, as does everything it pointed to. The branch
3266 pointer itself just doesn't, since you replaced it with another one.
3267
3268 There are also other situations that cause dangling objects. For
3269 example, a "dangling blob" may arise because you did a `git add` of a
3270 file, but then, before you actually committed it and made it part of the
3271 bigger picture, you changed something else in that file and committed
3272 that *updated* thing--the old state that you added originally ends up
3273 not being pointed to by any commit or tree, so it's now a dangling blob
3274 object.
3275
3276 Similarly, when the "recursive" merge strategy runs, and finds that
3277 there are criss-cross merges and thus more than one merge base (which is
3278 fairly unusual, but it does happen), it will generate one temporary
3279 midway tree (or possibly even more, if you had lots of criss-crossing
3280 merges and more than two merge bases) as a temporary internal merge
3281 base, and again, those are real objects, but the end result will not end
3282 up pointing to them, so they end up "dangling" in your repository.
3283
3284 Generally, dangling objects aren't anything to worry about. They can
3285 even be very useful: if you screw something up, the dangling objects can
3286 be how you recover your old tree (say, you did a rebase, and realized
3287 that you really didn't want to--you can look at what dangling objects
3288 you have, and decide to reset your head to some old dangling state).
3289
3290 For commits, you can just use:
3291
3292 ------------------------------------------------
3293 $ gitk <dangling-commit-sha-goes-here> --not --all
3294 ------------------------------------------------
3295
3296 This asks for all the history reachable from the given commit but not
3297 from any branch, tag, or other reference. If you decide it's something
3298 you want, you can always create a new reference to it, e.g.,
3299
3300 ------------------------------------------------
3301 $ git branch recovered-branch <dangling-commit-sha-goes-here>
3302 ------------------------------------------------
3303
3304 For blobs and trees, you can't do the same, but you can still examine
3305 them. You can just do
3306
3307 ------------------------------------------------
3308 $ git show <dangling-blob/tree-sha-goes-here>
3309 ------------------------------------------------
3310
3311 to show what the contents of the blob were (or, for a tree, basically
3312 what the `ls` for that directory was), and that may give you some idea
3313 of what the operation was that left that dangling object.
3314
3315 Usually, dangling blobs and trees aren't very interesting. They're
3316 almost always the result of either being a half-way mergebase (the blob
3317 will often even have the conflict markers from a merge in it, if you
3318 have had conflicting merges that you fixed up by hand), or simply
3319 because you interrupted a `git fetch` with ^C or something like that,
3320 leaving _some_ of the new objects in the object database, but just
3321 dangling and useless.
3322
3323 Anyway, once you are sure that you're not interested in any dangling
3324 state, you can just prune all unreachable objects:
3325
3326 ------------------------------------------------
3327 $ git prune
3328 ------------------------------------------------
3329
3330 and they'll be gone. (You should only run `git prune` on a quiescent
3331 repository--it's kind of like doing a filesystem fsck recovery: you
3332 don't want to do that while the filesystem is mounted.
3333 `git prune` is designed not to cause any harm in such cases of concurrent
3334 accesses to a repository but you might receive confusing or scary messages.)
3335
3336 [[recovering-from-repository-corruption]]
3337 Recovering from repository corruption
3338 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3339
3340 By design, Git treats data trusted to it with caution. However, even in
3341 the absence of bugs in Git itself, it is still possible that hardware or
3342 operating system errors could corrupt data.
3343
3344 The first defense against such problems is backups. You can back up a
3345 Git directory using clone, or just using cp, tar, or any other backup
3346 mechanism.
3347
3348 As a last resort, you can search for the corrupted objects and attempt
3349 to replace them by hand. Back up your repository before attempting this
3350 in case you corrupt things even more in the process.
3351
3352 We'll assume that the problem is a single missing or corrupted blob,
3353 which is sometimes a solvable problem. (Recovering missing trees and
3354 especially commits is *much* harder).
3355
3356 Before starting, verify that there is corruption, and figure out where
3357 it is with linkgit:git-fsck[1]; this may be time-consuming.
3358
3359 Assume the output looks like this:
3360
3361 ------------------------------------------------
3362 $ git fsck --full --no-dangling
3363 broken link from tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
3364 to blob 4b9458b3786228369c63936db65827de3cc06200
3365 missing blob 4b9458b3786228369c63936db65827de3cc06200
3366 ------------------------------------------------
3367
3368 Now you know that blob 4b9458b3 is missing, and that the tree 2d9263c6
3369 points to it. If you could find just one copy of that missing blob
3370 object, possibly in some other repository, you could move it into
3371 `.git/objects/4b/9458b3...` and be done. Suppose you can't. You can
3372 still examine the tree that pointed to it with linkgit:git-ls-tree[1],
3373 which might output something like:
3374
3375 ------------------------------------------------
3376 $ git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
3377 100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8 .gitignore
3378 100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883 .mailmap
3379 100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c COPYING
3380 ...
3381 100644 blob 4b9458b3786228369c63936db65827de3cc06200 myfile
3382 ...
3383 ------------------------------------------------
3384
3385 So now you know that the missing blob was the data for a file named
3386 `myfile`. And chances are you can also identify the directory--let's
3387 say it's in `somedirectory`. If you're lucky the missing copy might be
3388 the same as the copy you have checked out in your working tree at
3389 `somedirectory/myfile`; you can test whether that's right with
3390 linkgit:git-hash-object[1]:
3391
3392 ------------------------------------------------
3393 $ git hash-object -w somedirectory/myfile
3394 ------------------------------------------------
3395
3396 which will create and store a blob object with the contents of
3397 somedirectory/myfile, and output the SHA-1 of that object. if you're
3398 extremely lucky it might be 4b9458b3786228369c63936db65827de3cc06200, in
3399 which case you've guessed right, and the corruption is fixed!
3400
3401 Otherwise, you need more information. How do you tell which version of
3402 the file has been lost?
3403
3404 The easiest way to do this is with:
3405
3406 ------------------------------------------------
3407 $ git log --raw --all --full-history -- somedirectory/myfile
3408 ------------------------------------------------
3409
3410 Because you're asking for raw output, you'll now get something like
3411
3412 ------------------------------------------------
3413 commit abc
3414 Author:
3415 Date:
3416 ...
3417 :100644 100644 4b9458b newsha M somedirectory/myfile
3418
3419
3420 commit xyz
3421 Author:
3422 Date:
3423
3424 ...
3425 :100644 100644 oldsha 4b9458b M somedirectory/myfile
3426 ------------------------------------------------
3427
3428 This tells you that the immediately following version of the file was
3429 "newsha", and that the immediately preceding version was "oldsha".
3430 You also know the commit messages that went with the change from oldsha
3431 to 4b9458b and with the change from 4b9458b to newsha.
3432
3433 If you've been committing small enough changes, you may now have a good
3434 shot at reconstructing the contents of the in-between state 4b9458b.
3435
3436 If you can do that, you can now recreate the missing object with
3437
3438 ------------------------------------------------
3439 $ git hash-object -w <recreated-file>
3440 ------------------------------------------------
3441
3442 and your repository is good again!
3443
3444 (Btw, you could have ignored the `fsck`, and started with doing a
3445
3446 ------------------------------------------------
3447 $ git log --raw --all
3448 ------------------------------------------------
3449
3450 and just looked for the sha of the missing object (4b9458b) in that
3451 whole thing. It's up to you--Git does *have* a lot of information, it is
3452 just missing one particular blob version.
3453
3454 [[the-index]]
3455 The index
3456 ---------
3457
3458 The index is a binary file (generally kept in `.git/index`) containing a
3459 sorted list of path names, each with permissions and the SHA-1 of a blob
3460 object; linkgit:git-ls-files[1] can show you the contents of the index:
3461
3462 -------------------------------------------------
3463 $ git ls-files --stage
3464 100644 63c918c667fa005ff12ad89437f2fdc80926e21c 0 .gitignore
3465 100644 5529b198e8d14decbe4ad99db3f7fb632de0439d 0 .mailmap
3466 100644 6ff87c4664981e4397625791c8ea3bbb5f2279a3 0 COPYING
3467 100644 a37b2152bd26be2c2289e1f57a292534a51a93c7 0 Documentation/.gitignore
3468 100644 fbefe9a45b00a54b58d94d06eca48b03d40a50e0 0 Documentation/Makefile
3469 ...
3470 100644 2511aef8d89ab52be5ec6a5e46236b4b6bcd07ea 0 xdiff/xtypes.h
3471 100644 2ade97b2574a9f77e7ae4002a4e07a6a38e46d07 0 xdiff/xutils.c
3472 100644 d5de8292e05e7c36c4b68857c1cf9855e3d2f70a 0 xdiff/xutils.h
3473 -------------------------------------------------
3474
3475 Note that in older documentation you may see the index called the
3476 "current directory cache" or just the "cache". It has three important
3477 properties:
3478
3479 1. The index contains all the information necessary to generate a single
3480 (uniquely determined) tree object.
3481 +
3482 For example, running linkgit:git-commit[1] generates this tree object
3483 from the index, stores it in the object database, and uses it as the
3484 tree object associated with the new commit.
3485
3486 2. The index enables fast comparisons between the tree object it defines
3487 and the working tree.
3488 +
3489 It does this by storing some additional data for each entry (such as
3490 the last modified time). This data is not displayed above, and is not
3491 stored in the created tree object, but it can be used to determine
3492 quickly which files in the working directory differ from what was
3493 stored in the index, and thus save Git from having to read all of the
3494 data from such files to look for changes.
3495
3496 3. It can efficiently represent information about merge conflicts
3497 between different tree objects, allowing each pathname to be
3498 associated with sufficient information about the trees involved that
3499 you can create a three-way merge between them.
3500 +
3501 We saw in <<conflict-resolution>> that during a merge the index can
3502 store multiple versions of a single file (called "stages"). The third
3503 column in the linkgit:git-ls-files[1] output above is the stage
3504 number, and will take on values other than 0 for files with merge
3505 conflicts.
3506
3507 The index is thus a sort of temporary staging area, which is filled with
3508 a tree which you are in the process of working on.
3509
3510 If you blow the index away entirely, you generally haven't lost any
3511 information as long as you have the name of the tree that it described.
3512
3513 [[submodules]]
3514 Submodules
3515 ==========
3516
3517 Large projects are often composed of smaller, self-contained modules. For
3518 example, an embedded Linux distribution's source tree would include every
3519 piece of software in the distribution with some local modifications; a movie
3520 player might need to build against a specific, known-working version of a
3521 decompression library; several independent programs might all share the same
3522 build scripts.
3523
3524 With centralized revision control systems this is often accomplished by
3525 including every module in one single repository. Developers can check out
3526 all modules or only the modules they need to work with. They can even modify
3527 files across several modules in a single commit while moving things around
3528 or updating APIs and translations.
3529
3530 Git does not allow partial checkouts, so duplicating this approach in Git
3531 would force developers to keep a local copy of modules they are not
3532 interested in touching. Commits in an enormous checkout would be slower
3533 than you'd expect as Git would have to scan every directory for changes.
3534 If modules have a lot of local history, clones would take forever.
3535
3536 On the plus side, distributed revision control systems can much better
3537 integrate with external sources. In a centralized model, a single arbitrary
3538 snapshot of the external project is exported from its own revision control
3539 and then imported into the local revision control on a vendor branch. All
3540 the history is hidden. With distributed revision control you can clone the
3541 entire external history and much more easily follow development and re-merge
3542 local changes.
3543
3544 Git's submodule support allows a repository to contain, as a subdirectory, a
3545 checkout of an external project. Submodules maintain their own identity;
3546 the submodule support just stores the submodule repository location and
3547 commit ID, so other developers who clone the containing project
3548 ("superproject") can easily clone all the submodules at the same revision.
3549 Partial checkouts of the superproject are possible: you can tell Git to
3550 clone none, some or all of the submodules.
3551
3552 The linkgit:git-submodule[1] command is available since Git 1.5.3. Users
3553 with Git 1.5.2 can look up the submodule commits in the repository and
3554 manually check them out; earlier versions won't recognize the submodules at
3555 all.
3556
3557 To see how submodule support works, create four example
3558 repositories that can be used later as a submodule:
3559
3560 -------------------------------------------------
3561 $ mkdir ~/git
3562 $ cd ~/git
3563 $ for i in a b c d
3564 do
3565 mkdir $i
3566 cd $i
3567 git init
3568 echo "module $i" > $i.txt
3569 git add $i.txt
3570 git commit -m "Initial commit, submodule $i"
3571 cd ..
3572 done
3573 -------------------------------------------------
3574
3575 Now create the superproject and add all the submodules:
3576
3577 -------------------------------------------------
3578 $ mkdir super
3579 $ cd super
3580 $ git init
3581 $ for i in a b c d
3582 do
3583 git submodule add ~/git/$i $i
3584 done
3585 -------------------------------------------------
3586
3587 NOTE: Do not use local URLs here if you plan to publish your superproject!
3588
3589 See what files `git submodule` created:
3590
3591 -------------------------------------------------
3592 $ ls -a
3593 . .. .git .gitmodules a b c d
3594 -------------------------------------------------
3595
3596 The `git submodule add <repo> <path>` command does a couple of things:
3597
3598 - It clones the submodule from `<repo>` to the given `<path>` under the
3599 current directory and by default checks out the master branch.
3600 - It adds the submodule's clone path to the linkgit:gitmodules[5] file and
3601 adds this file to the index, ready to be committed.
3602 - It adds the submodule's current commit ID to the index, ready to be
3603 committed.
3604
3605 Commit the superproject:
3606
3607 -------------------------------------------------
3608 $ git commit -m "Add submodules a, b, c and d."
3609 -------------------------------------------------
3610
3611 Now clone the superproject:
3612
3613 -------------------------------------------------
3614 $ cd ..
3615 $ git clone super cloned
3616 $ cd cloned
3617 -------------------------------------------------
3618
3619 The submodule directories are there, but they're empty:
3620
3621 -------------------------------------------------
3622 $ ls -a a
3623 . ..
3624 $ git submodule status
3625 -d266b9873ad50488163457f025db7cdd9683d88b a
3626 -e81d457da15309b4fef4249aba9b50187999670d b
3627 -c1536a972b9affea0f16e0680ba87332dc059146 c
3628 -d96249ff5d57de5de093e6baff9e0aafa5276a74 d
3629 -------------------------------------------------
3630
3631 NOTE: The commit object names shown above would be different for you, but they
3632 should match the HEAD commit object names of your repositories. You can check
3633 it by running `git ls-remote ../a`.
3634
3635 Pulling down the submodules is a two-step process. First run `git submodule
3636 init` to add the submodule repository URLs to `.git/config`:
3637
3638 -------------------------------------------------
3639 $ git submodule init
3640 -------------------------------------------------
3641
3642 Now use `git submodule update` to clone the repositories and check out the
3643 commits specified in the superproject:
3644
3645 -------------------------------------------------
3646 $ git submodule update
3647 $ cd a
3648 $ ls -a
3649 . .. .git a.txt
3650 -------------------------------------------------
3651
3652 One major difference between `git submodule update` and `git submodule add` is
3653 that `git submodule update` checks out a specific commit, rather than the tip
3654 of a branch. It's like checking out a tag: the head is detached, so you're not
3655 working on a branch.
3656
3657 -------------------------------------------------
3658 $ git branch
3659 * (detached from d266b98)
3660 master
3661 -------------------------------------------------
3662
3663 If you want to make a change within a submodule and you have a detached head,
3664 then you should create or checkout a branch, make your changes, publish the
3665 change within the submodule, and then update the superproject to reference the
3666 new commit:
3667
3668 -------------------------------------------------
3669 $ git switch master
3670 -------------------------------------------------
3671
3672 or
3673
3674 -------------------------------------------------
3675 $ git switch -c fix-up
3676 -------------------------------------------------
3677
3678 then
3679
3680 -------------------------------------------------
3681 $ echo "adding a line again" >> a.txt
3682 $ git commit -a -m "Updated the submodule from within the superproject."
3683 $ git push
3684 $ cd ..
3685 $ git diff
3686 diff --git a/a b/a
3687 index d266b98..261dfac 160000
3688 --- a/a
3689 +++ b/a
3690 @@ -1 +1 @@
3691 -Subproject commit d266b9873ad50488163457f025db7cdd9683d88b
3692 +Subproject commit 261dfac35cb99d380eb966e102c1197139f7fa24
3693 $ git add a
3694 $ git commit -m "Updated submodule a."
3695 $ git push
3696 -------------------------------------------------
3697
3698 You have to run `git submodule update` after `git pull` if you want to update
3699 submodules, too.
3700
3701 [[pitfalls-with-submodules]]
3702 Pitfalls with submodules
3703 ------------------------
3704
3705 Always publish the submodule change before publishing the change to the
3706 superproject that references it. If you forget to publish the submodule change,
3707 others won't be able to clone the repository:
3708
3709 -------------------------------------------------
3710 $ cd ~/git/super/a
3711 $ echo i added another line to this file >> a.txt
3712 $ git commit -a -m "doing it wrong this time"
3713 $ cd ..
3714 $ git add a
3715 $ git commit -m "Updated submodule a again."
3716 $ git push
3717 $ cd ~/git/cloned
3718 $ git pull
3719 $ git submodule update
3720 error: pathspec '261dfac35cb99d380eb966e102c1197139f7fa24' did not match any file(s) known to git.
3721 Did you forget to 'git add'?
3722 Unable to checkout '261dfac35cb99d380eb966e102c1197139f7fa24' in submodule path 'a'
3723 -------------------------------------------------
3724
3725 In older Git versions it could be easily forgotten to commit new or modified
3726 files in a submodule, which silently leads to similar problems as not pushing
3727 the submodule changes. Starting with Git 1.7.0 both `git status` and `git diff`
3728 in the superproject show submodules as modified when they contain new or
3729 modified files to protect against accidentally committing such a state. `git
3730 diff` will also add a `-dirty` to the work tree side when generating patch
3731 output or used with the `--submodule` option:
3732
3733 -------------------------------------------------
3734 $ git diff
3735 diff --git a/sub b/sub
3736 --- a/sub
3737 +++ b/sub
3738 @@ -1 +1 @@
3739 -Subproject commit 3f356705649b5d566d97ff843cf193359229a453
3740 +Subproject commit 3f356705649b5d566d97ff843cf193359229a453-dirty
3741 $ git diff --submodule
3742 Submodule sub 3f35670..3f35670-dirty:
3743 -------------------------------------------------
3744
3745 You also should not rewind branches in a submodule beyond commits that were
3746 ever recorded in any superproject.
3747
3748 It's not safe to run `git submodule update` if you've made and committed
3749 changes within a submodule without checking out a branch first. They will be
3750 silently overwritten:
3751
3752 -------------------------------------------------
3753 $ cat a.txt
3754 module a
3755 $ echo line added from private2 >> a.txt
3756 $ git commit -a -m "line added inside private2"
3757 $ cd ..
3758 $ git submodule update
3759 Submodule path 'a': checked out 'd266b9873ad50488163457f025db7cdd9683d88b'
3760 $ cd a
3761 $ cat a.txt
3762 module a
3763 -------------------------------------------------
3764
3765 NOTE: The changes are still visible in the submodule's reflog.
3766
3767 If you have uncommitted changes in your submodule working tree, `git
3768 submodule update` will not overwrite them. Instead, you get the usual
3769 warning about not being able switch from a dirty branch.
3770
3771 [[low-level-operations]]
3772 Low-level Git operations
3773 ========================
3774
3775 Many of the higher-level commands were originally implemented as shell
3776 scripts using a smaller core of low-level Git commands. These can still
3777 be useful when doing unusual things with Git, or just as a way to
3778 understand its inner workings.
3779
3780 [[object-manipulation]]
3781 Object access and manipulation
3782 ------------------------------
3783
3784 The linkgit:git-cat-file[1] command can show the contents of any object,
3785 though the higher-level linkgit:git-show[1] is usually more useful.
3786
3787 The linkgit:git-commit-tree[1] command allows constructing commits with
3788 arbitrary parents and trees.
3789
3790 A tree can be created with linkgit:git-write-tree[1] and its data can be
3791 accessed by linkgit:git-ls-tree[1]. Two trees can be compared with
3792 linkgit:git-diff-tree[1].
3793
3794 A tag is created with linkgit:git-mktag[1], and the signature can be
3795 verified by linkgit:git-verify-tag[1], though it is normally simpler to
3796 use linkgit:git-tag[1] for both.
3797
3798 [[the-workflow]]
3799 The Workflow
3800 ------------
3801
3802 High-level operations such as linkgit:git-commit[1] and
3803 linkgit:git-restore[1] work by moving data
3804 between the working tree, the index, and the object database. Git
3805 provides low-level operations which perform each of these steps
3806 individually.
3807
3808 Generally, all Git operations work on the index file. Some operations
3809 work *purely* on the index file (showing the current state of the
3810 index), but most operations move data between the index file and either
3811 the database or the working directory. Thus there are four main
3812 combinations:
3813
3814 [[working-directory-to-index]]
3815 working directory -> index
3816 ~~~~~~~~~~~~~~~~~~~~~~~~~~
3817
3818 The linkgit:git-update-index[1] command updates the index with
3819 information from the working directory. You generally update the
3820 index information by just specifying the filename you want to update,
3821 like so:
3822
3823 -------------------------------------------------
3824 $ git update-index filename
3825 -------------------------------------------------
3826
3827 but to avoid common mistakes with filename globbing etc., the command
3828 will not normally add totally new entries or remove old entries,
3829 i.e. it will normally just update existing cache entries.
3830
3831 To tell Git that yes, you really do realize that certain files no
3832 longer exist, or that new files should be added, you
3833 should use the `--remove` and `--add` flags respectively.
3834
3835 NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
3836 necessarily be removed: if the files still exist in your directory
3837 structure, the index will be updated with their new status, not
3838 removed. The only thing `--remove` means is that update-index will be
3839 considering a removed file to be a valid thing, and if the file really
3840 does not exist any more, it will update the index accordingly.
3841
3842 As a special case, you can also do `git update-index --refresh`, which
3843 will refresh the "stat" information of each index to match the current
3844 stat information. It will 'not' update the object status itself, and
3845 it will only update the fields that are used to quickly test whether
3846 an object still matches its old backing store object.
3847
3848 The previously introduced linkgit:git-add[1] is just a wrapper for
3849 linkgit:git-update-index[1].
3850
3851 [[index-to-object-database]]
3852 index -> object database
3853 ~~~~~~~~~~~~~~~~~~~~~~~~
3854
3855 You write your current index file to a "tree" object with the program
3856
3857 -------------------------------------------------
3858 $ git write-tree
3859 -------------------------------------------------
3860
3861 that doesn't come with any options--it will just write out the
3862 current index into the set of tree objects that describe that state,
3863 and it will return the name of the resulting top-level tree. You can
3864 use that tree to re-generate the index at any time by going in the
3865 other direction:
3866
3867 [[object-database-to-index]]
3868 object database -> index
3869 ~~~~~~~~~~~~~~~~~~~~~~~~
3870
3871 You read a "tree" file from the object database, and use that to
3872 populate (and overwrite--don't do this if your index contains any
3873 unsaved state that you might want to restore later!) your current
3874 index. Normal operation is just
3875
3876 -------------------------------------------------
3877 $ git read-tree <SHA-1 of tree>
3878 -------------------------------------------------
3879
3880 and your index file will now be equivalent to the tree that you saved
3881 earlier. However, that is only your 'index' file: your working
3882 directory contents have not been modified.
3883
3884 [[index-to-working-directory]]
3885 index -> working directory
3886 ~~~~~~~~~~~~~~~~~~~~~~~~~~
3887
3888 You update your working directory from the index by "checking out"
3889 files. This is not a very common operation, since normally you'd just
3890 keep your files updated, and rather than write to your working
3891 directory, you'd tell the index files about the changes in your
3892 working directory (i.e. `git update-index`).
3893
3894 However, if you decide to jump to a new version, or check out somebody
3895 else's version, or just restore a previous tree, you'd populate your
3896 index file with read-tree, and then you need to check out the result
3897 with
3898
3899 -------------------------------------------------
3900 $ git checkout-index filename
3901 -------------------------------------------------
3902
3903 or, if you want to check out all of the index, use `-a`.
3904
3905 NOTE! `git checkout-index` normally refuses to overwrite old files, so
3906 if you have an old version of the tree already checked out, you will
3907 need to use the `-f` flag ('before' the `-a` flag or the filename) to
3908 'force' the checkout.
3909
3910
3911 Finally, there are a few odds and ends which are not purely moving
3912 from one representation to the other:
3913
3914 [[tying-it-all-together]]
3915 Tying it all together
3916 ~~~~~~~~~~~~~~~~~~~~~
3917
3918 To commit a tree you have instantiated with `git write-tree`, you'd
3919 create a "commit" object that refers to that tree and the history
3920 behind it--most notably the "parent" commits that preceded it in
3921 history.
3922
3923 Normally a "commit" has one parent: the previous state of the tree
3924 before a certain change was made. However, sometimes it can have two
3925 or more parent commits, in which case we call it a "merge", due to the
3926 fact that such a commit brings together ("merges") two or more
3927 previous states represented by other commits.
3928
3929 In other words, while a "tree" represents a particular directory state
3930 of a working directory, a "commit" represents that state in time,
3931 and explains how we got there.
3932
3933 You create a commit object by giving it the tree that describes the
3934 state at the time of the commit, and a list of parents:
3935
3936 -------------------------------------------------
3937 $ git commit-tree <tree> -p <parent> [(-p <parent2>)...]
3938 -------------------------------------------------
3939
3940 and then giving the reason for the commit on stdin (either through
3941 redirection from a pipe or file, or by just typing it at the tty).
3942
3943 `git commit-tree` will return the name of the object that represents
3944 that commit, and you should save it away for later use. Normally,
3945 you'd commit a new `HEAD` state, and while Git doesn't care where you
3946 save the note about that state, in practice we tend to just write the
3947 result to the file pointed at by `.git/HEAD`, so that we can always see
3948 what the last committed state was.
3949
3950 Here is a picture that illustrates how various pieces fit together:
3951
3952 ------------
3953
3954 commit-tree
3955 commit obj
3956 +----+
3957 | |
3958 | |
3959 V V
3960 +-----------+
3961 | Object DB |
3962 | Backing |
3963 | Store |
3964 +-----------+
3965 ^
3966 write-tree | |
3967 tree obj | |
3968 | | read-tree
3969 | | tree obj
3970 V
3971 +-----------+
3972 | Index |
3973 | "cache" |
3974 +-----------+
3975 update-index ^
3976 blob obj | |
3977 | |
3978 checkout-index -u | | checkout-index
3979 stat | | blob obj
3980 V
3981 +-----------+
3982 | Working |
3983 | Directory |
3984 +-----------+
3985
3986 ------------
3987
3988
3989 [[examining-the-data]]
3990 Examining the data
3991 ------------------
3992
3993 You can examine the data represented in the object database and the
3994 index with various helper tools. For every object, you can use
3995 linkgit:git-cat-file[1] to examine details about the
3996 object:
3997
3998 -------------------------------------------------
3999 $ git cat-file -t <objectname>
4000 -------------------------------------------------
4001
4002 shows the type of the object, and once you have the type (which is
4003 usually implicit in where you find the object), you can use
4004
4005 -------------------------------------------------
4006 $ git cat-file blob|tree|commit|tag <objectname>
4007 -------------------------------------------------
4008
4009 to show its contents. NOTE! Trees have binary content, and as a result
4010 there is a special helper for showing that content, called
4011 `git ls-tree`, which turns the binary content into a more easily
4012 readable form.
4013
4014 It's especially instructive to look at "commit" objects, since those
4015 tend to be small and fairly self-explanatory. In particular, if you
4016 follow the convention of having the top commit name in `.git/HEAD`,
4017 you can do
4018
4019 -------------------------------------------------
4020 $ git cat-file commit HEAD
4021 -------------------------------------------------
4022
4023 to see what the top commit was.
4024
4025 [[merging-multiple-trees]]
4026 Merging multiple trees
4027 ----------------------
4028
4029 Git can help you perform a three-way merge, which can in turn be
4030 used for a many-way merge by repeating the merge procedure several
4031 times. The usual situation is that you only do one three-way merge
4032 (reconciling two lines of history) and commit the result, but if
4033 you like to, you can merge several branches in one go.
4034
4035 To perform a three-way merge, you start with the two commits you
4036 want to merge, find their closest common parent (a third commit),
4037 and compare the trees corresponding to these three commits.
4038
4039 To get the "base" for the merge, look up the common parent of two
4040 commits:
4041
4042 -------------------------------------------------
4043 $ git merge-base <commit1> <commit2>
4044 -------------------------------------------------
4045
4046 This prints the name of a commit they are both based on. You should
4047 now look up the tree objects of those commits, which you can easily
4048 do with
4049
4050 -------------------------------------------------
4051 $ git cat-file commit <commitname> | head -1
4052 -------------------------------------------------
4053
4054 since the tree object information is always the first line in a commit
4055 object.
4056
4057 Once you know the three trees you are going to merge (the one "original"
4058 tree, aka the common tree, and the two "result" trees, aka the branches
4059 you want to merge), you do a "merge" read into the index. This will
4060 complain if it has to throw away your old index contents, so you should
4061 make sure that you've committed those--in fact you would normally
4062 always do a merge against your last commit (which should thus match what
4063 you have in your current index anyway).
4064
4065 To do the merge, do
4066
4067 -------------------------------------------------
4068 $ git read-tree -m -u <origtree> <yourtree> <targettree>
4069 -------------------------------------------------
4070
4071 which will do all trivial merge operations for you directly in the
4072 index file, and you can just write the result out with
4073 `git write-tree`.
4074
4075
4076 [[merging-multiple-trees-2]]
4077 Merging multiple trees, continued
4078 ---------------------------------
4079
4080 Sadly, many merges aren't trivial. If there are files that have
4081 been added, moved or removed, or if both branches have modified the
4082 same file, you will be left with an index tree that contains "merge
4083 entries" in it. Such an index tree can 'NOT' be written out to a tree
4084 object, and you will have to resolve any such merge clashes using
4085 other tools before you can write out the result.
4086
4087 You can examine such index state with `git ls-files --unmerged`
4088 command. An example:
4089
4090 ------------------------------------------------
4091 $ git read-tree -m $orig HEAD $target
4092 $ git ls-files --unmerged
4093 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello.c
4094 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello.c
4095 100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello.c
4096 ------------------------------------------------
4097
4098 Each line of the `git ls-files --unmerged` output begins with
4099 the blob mode bits, blob SHA-1, 'stage number', and the
4100 filename. The 'stage number' is Git's way to say which tree it
4101 came from: stage 1 corresponds to the `$orig` tree, stage 2 to
4102 the `HEAD` tree, and stage 3 to the `$target` tree.
4103
4104 Earlier we said that trivial merges are done inside
4105 `git read-tree -m`. For example, if the file did not change
4106 from `$orig` to `HEAD` or `$target`, or if the file changed
4107 from `$orig` to `HEAD` and `$orig` to `$target` the same way,
4108 obviously the final outcome is what is in `HEAD`. What the
4109 above example shows is that file `hello.c` was changed from
4110 `$orig` to `HEAD` and `$orig` to `$target` in a different way.
4111 You could resolve this by running your favorite 3-way merge
4112 program, e.g. `diff3`, `merge`, or Git's own merge-file, on
4113 the blob objects from these three stages yourself, like this:
4114
4115 ------------------------------------------------
4116 $ git cat-file blob 263414f >hello.c~1
4117 $ git cat-file blob 06fa6a2 >hello.c~2
4118 $ git cat-file blob cc44c73 >hello.c~3
4119 $ git merge-file hello.c~2 hello.c~1 hello.c~3
4120 ------------------------------------------------
4121
4122 This would leave the merge result in `hello.c~2` file, along
4123 with conflict markers if there are conflicts. After verifying
4124 the merge result makes sense, you can tell Git what the final
4125 merge result for this file is by:
4126
4127 -------------------------------------------------
4128 $ mv -f hello.c~2 hello.c
4129 $ git update-index hello.c
4130 -------------------------------------------------
4131
4132 When a path is in the "unmerged" state, running `git update-index` for
4133 that path tells Git to mark the path resolved.
4134
4135 The above is the description of a Git merge at the lowest level,
4136 to help you understand what conceptually happens under the hood.
4137 In practice, nobody, not even Git itself, runs `git cat-file` three times
4138 for this. There is a `git merge-index` program that extracts the
4139 stages to temporary files and calls a "merge" script on it:
4140
4141 -------------------------------------------------
4142 $ git merge-index git-merge-one-file hello.c
4143 -------------------------------------------------
4144
4145 and that is what higher level `git merge -s resolve` is implemented with.
4146
4147 [[hacking-git]]
4148 Hacking Git
4149 ===========
4150
4151 This chapter covers internal details of the Git implementation which
4152 probably only Git developers need to understand.
4153
4154 [[object-details]]
4155 Object storage format
4156 ---------------------
4157
4158 All objects have a statically determined "type" which identifies the
4159 format of the object (i.e. how it is used, and how it can refer to other
4160 objects). There are currently four different object types: "blob",
4161 "tree", "commit", and "tag".
4162
4163 Regardless of object type, all objects share the following
4164 characteristics: they are all deflated with zlib, and have a header
4165 that not only specifies their type, but also provides size information
4166 about the data in the object. It's worth noting that the SHA-1 hash
4167 that is used to name the object is the hash of the original data
4168 plus this header, so `sha1sum` 'file' does not match the object name
4169 for 'file'.
4170
4171 As a result, the general consistency of an object can always be tested
4172 independently of the contents or the type of the object: all objects can
4173 be validated by verifying that (a) their hashes match the content of the
4174 file and (b) the object successfully inflates to a stream of bytes that
4175 forms a sequence of
4176 `<ascii type without space> + <space> + <ascii decimal size> +
4177 <byte\0> + <binary object data>`.
4178
4179 The structured objects can further have their structure and
4180 connectivity to other objects verified. This is generally done with
4181 the `git fsck` program, which generates a full dependency graph
4182 of all objects, and verifies their internal consistency (in addition
4183 to just verifying their superficial consistency through the hash).
4184
4185 [[birdview-on-the-source-code]]
4186 A birds-eye view of Git's source code
4187 -------------------------------------
4188
4189 It is not always easy for new developers to find their way through Git's
4190 source code. This section gives you a little guidance to show where to
4191 start.
4192
4193 A good place to start is with the contents of the initial commit, with:
4194
4195 ----------------------------------------------------
4196 $ git switch --detach e83c5163
4197 ----------------------------------------------------
4198
4199 The initial revision lays the foundation for almost everything Git has
4200 today, but is small enough to read in one sitting.
4201
4202 Note that terminology has changed since that revision. For example, the
4203 README in that revision uses the word "changeset" to describe what we
4204 now call a <<def_commit_object,commit>>.
4205
4206 Also, we do not call it "cache" any more, but rather "index"; however, the
4207 file is still called `cache.h`. Remark: Not much reason to change it now,
4208 especially since there is no good single name for it anyway, because it is
4209 basically _the_ header file which is included by _all_ of Git's C sources.
4210
4211 If you grasp the ideas in that initial commit, you should check out a
4212 more recent version and skim `cache.h`, `object.h` and `commit.h`.
4213
4214 In the early days, Git (in the tradition of UNIX) was a bunch of programs
4215 which were extremely simple, and which you used in scripts, piping the
4216 output of one into another. This turned out to be good for initial
4217 development, since it was easier to test new things. However, recently
4218 many of these parts have become builtins, and some of the core has been
4219 "libified", i.e. put into libgit.a for performance, portability reasons,
4220 and to avoid code duplication.
4221
4222 By now, you know what the index is (and find the corresponding data
4223 structures in `cache.h`), and that there are just a couple of object types
4224 (blobs, trees, commits and tags) which inherit their common structure from
4225 `struct object`, which is their first member (and thus, you can cast e.g.
4226 `(struct object *)commit` to achieve the _same_ as `&commit->object`, i.e.
4227 get at the object name and flags).
4228
4229 Now is a good point to take a break to let this information sink in.
4230
4231 Next step: get familiar with the object naming. Read <<naming-commits>>.
4232 There are quite a few ways to name an object (and not only revisions!).
4233 All of these are handled in `sha1_name.c`. Just have a quick look at
4234 the function `get_sha1()`. A lot of the special handling is done by
4235 functions like `get_sha1_basic()` or the likes.
4236
4237 This is just to get you into the groove for the most libified part of Git:
4238 the revision walker.
4239
4240 Basically, the initial version of `git log` was a shell script:
4241
4242 ----------------------------------------------------------------
4243 $ git-rev-list --pretty $(git-rev-parse --default HEAD "$@") | \
4244 LESS=-S ${PAGER:-less}
4245 ----------------------------------------------------------------
4246
4247 What does this mean?
4248
4249 `git rev-list` is the original version of the revision walker, which
4250 _always_ printed a list of revisions to stdout. It is still functional,
4251 and needs to, since most new Git commands start out as scripts using
4252 `git rev-list`.
4253
4254 `git rev-parse` is not as important any more; it was only used to filter out
4255 options that were relevant for the different plumbing commands that were
4256 called by the script.
4257
4258 Most of what `git rev-list` did is contained in `revision.c` and
4259 `revision.h`. It wraps the options in a struct named `rev_info`, which
4260 controls how and what revisions are walked, and more.
4261
4262 The original job of `git rev-parse` is now taken by the function
4263 `setup_revisions()`, which parses the revisions and the common command-line
4264 options for the revision walker. This information is stored in the struct
4265 `rev_info` for later consumption. You can do your own command-line option
4266 parsing after calling `setup_revisions()`. After that, you have to call
4267 `prepare_revision_walk()` for initialization, and then you can get the
4268 commits one by one with the function `get_revision()`.
4269
4270 If you are interested in more details of the revision walking process,
4271 just have a look at the first implementation of `cmd_log()`; call
4272 `git show v1.3.0~155^2~4` and scroll down to that function (note that you
4273 no longer need to call `setup_pager()` directly).
4274
4275 Nowadays, `git log` is a builtin, which means that it is _contained_ in the
4276 command `git`. The source side of a builtin is
4277
4278 - a function called `cmd_<bla>`, typically defined in `builtin/<bla.c>`
4279 (note that older versions of Git used to have it in `builtin-<bla>.c`
4280 instead), and declared in `builtin.h`.
4281
4282 - an entry in the `commands[]` array in `git.c`, and
4283
4284 - an entry in `BUILTIN_OBJECTS` in the `Makefile`.
4285
4286 Sometimes, more than one builtin is contained in one source file. For
4287 example, `cmd_whatchanged()` and `cmd_log()` both reside in `builtin/log.c`,
4288 since they share quite a bit of code. In that case, the commands which are
4289 _not_ named like the `.c` file in which they live have to be listed in
4290 `BUILT_INS` in the `Makefile`.
4291
4292 `git log` looks more complicated in C than it does in the original script,
4293 but that allows for a much greater flexibility and performance.
4294
4295 Here again it is a good point to take a pause.
4296
4297 Lesson three is: study the code. Really, it is the best way to learn about
4298 the organization of Git (after you know the basic concepts).
4299
4300 So, think about something which you are interested in, say, "how can I
4301 access a blob just knowing the object name of it?". The first step is to
4302 find a Git command with which you can do it. In this example, it is either
4303 `git show` or `git cat-file`.
4304
4305 For the sake of clarity, let's stay with `git cat-file`, because it
4306
4307 - is plumbing, and
4308
4309 - was around even in the initial commit (it literally went only through
4310 some 20 revisions as `cat-file.c`, was renamed to `builtin/cat-file.c`
4311 when made a builtin, and then saw less than 10 versions).
4312
4313 So, look into `builtin/cat-file.c`, search for `cmd_cat_file()` and look what
4314 it does.
4315
4316 ------------------------------------------------------------------
4317 git_config(git_default_config);
4318 if (argc != 3)
4319 usage("git cat-file [-t|-s|-e|-p|<type>] <sha1>");
4320 if (get_sha1(argv[2], sha1))
4321 die("Not a valid object name %s", argv[2]);
4322 ------------------------------------------------------------------
4323
4324 Let's skip over the obvious details; the only really interesting part
4325 here is the call to `get_sha1()`. It tries to interpret `argv[2]` as an
4326 object name, and if it refers to an object which is present in the current
4327 repository, it writes the resulting SHA-1 into the variable `sha1`.
4328
4329 Two things are interesting here:
4330
4331 - `get_sha1()` returns 0 on _success_. This might surprise some new
4332 Git hackers, but there is a long tradition in UNIX to return different
4333 negative numbers in case of different errors--and 0 on success.
4334
4335 - the variable `sha1` in the function signature of `get_sha1()` is `unsigned
4336 char *`, but is actually expected to be a pointer to `unsigned
4337 char[20]`. This variable will contain the 160-bit SHA-1 of the given
4338 commit. Note that whenever a SHA-1 is passed as `unsigned char *`, it
4339 is the binary representation, as opposed to the ASCII representation in
4340 hex characters, which is passed as `char *`.
4341
4342 You will see both of these things throughout the code.
4343
4344 Now, for the meat:
4345
4346 -----------------------------------------------------------------------------
4347 case 0:
4348 buf = read_object_with_reference(sha1, argv[1], &size, NULL);
4349 -----------------------------------------------------------------------------
4350
4351 This is how you read a blob (actually, not only a blob, but any type of
4352 object). To know how the function `read_object_with_reference()` actually
4353 works, find the source code for it (something like `git grep
4354 read_object_with | grep ":[a-z]"` in the Git repository), and read
4355 the source.
4356
4357 To find out how the result can be used, just read on in `cmd_cat_file()`:
4358
4359 -----------------------------------
4360 write_or_die(1, buf, size);
4361 -----------------------------------
4362
4363 Sometimes, you do not know where to look for a feature. In many such cases,
4364 it helps to search through the output of `git log`, and then `git show` the
4365 corresponding commit.
4366
4367 Example: If you know that there was some test case for `git bundle`, but
4368 do not remember where it was (yes, you _could_ `git grep bundle t/`, but that
4369 does not illustrate the point!):
4370
4371 ------------------------
4372 $ git log --no-merges t/
4373 ------------------------
4374
4375 In the pager (`less`), just search for "bundle", go a few lines back,
4376 and see that it is in commit 18449ab0. Now just copy this object name,
4377 and paste it into the command line
4378
4379 -------------------
4380 $ git show 18449ab0
4381 -------------------
4382
4383 Voila.
4384
4385 Another example: Find out what to do in order to make some script a
4386 builtin:
4387
4388 -------------------------------------------------
4389 $ git log --no-merges --diff-filter=A builtin/*.c
4390 -------------------------------------------------
4391
4392 You see, Git is actually the best tool to find out about the source of Git
4393 itself!
4394
4395 [[glossary]]
4396 Git Glossary
4397 ============
4398
4399 [[git-explained]]
4400 Git explained
4401 -------------
4402
4403 include::glossary-content.txt[]
4404
4405 [[git-quick-start]]
4406 Appendix A: Git Quick Reference
4407 ===============================
4408
4409 This is a quick summary of the major commands; the previous chapters
4410 explain how these work in more detail.
4411
4412 [[quick-creating-a-new-repository]]
4413 Creating a new repository
4414 -------------------------
4415
4416 From a tarball:
4417
4418 -----------------------------------------------
4419 $ tar xzf project.tar.gz
4420 $ cd project
4421 $ git init
4422 Initialized empty Git repository in .git/
4423 $ git add .
4424 $ git commit
4425 -----------------------------------------------
4426
4427 From a remote repository:
4428
4429 -----------------------------------------------
4430 $ git clone git://example.com/pub/project.git
4431 $ cd project
4432 -----------------------------------------------
4433
4434 [[managing-branches]]
4435 Managing branches
4436 -----------------
4437
4438 -----------------------------------------------
4439 $ git branch # list all local branches in this repo
4440 $ git switch test # switch working directory to branch "test"
4441 $ git branch new # create branch "new" starting at current HEAD
4442 $ git branch -d new # delete branch "new"
4443 -----------------------------------------------
4444
4445 Instead of basing a new branch on current HEAD (the default), use:
4446
4447 -----------------------------------------------
4448 $ git branch new test # branch named "test"
4449 $ git branch new v2.6.15 # tag named v2.6.15
4450 $ git branch new HEAD^ # commit before the most recent
4451 $ git branch new HEAD^^ # commit before that
4452 $ git branch new test~10 # ten commits before tip of branch "test"
4453 -----------------------------------------------
4454
4455 Create and switch to a new branch at the same time:
4456
4457 -----------------------------------------------
4458 $ git switch -c new v2.6.15
4459 -----------------------------------------------
4460
4461 Update and examine branches from the repository you cloned from:
4462
4463 -----------------------------------------------
4464 $ git fetch # update
4465 $ git branch -r # list
4466 origin/master
4467 origin/next
4468 ...
4469 $ git switch -c masterwork origin/master
4470 -----------------------------------------------
4471
4472 Fetch a branch from a different repository, and give it a new
4473 name in your repository:
4474
4475 -----------------------------------------------
4476 $ git fetch git://example.com/project.git theirbranch:mybranch
4477 $ git fetch git://example.com/project.git v2.6.15:mybranch
4478 -----------------------------------------------
4479
4480 Keep a list of repositories you work with regularly:
4481
4482 -----------------------------------------------
4483 $ git remote add example git://example.com/project.git
4484 $ git remote # list remote repositories
4485 example
4486 origin
4487 $ git remote show example # get details
4488 * remote example
4489 URL: git://example.com/project.git
4490 Tracked remote branches
4491 master
4492 next
4493 ...
4494 $ git fetch example # update branches from example
4495 $ git branch -r # list all remote branches
4496 -----------------------------------------------
4497
4498
4499 [[exploring-history]]
4500 Exploring history
4501 -----------------
4502
4503 -----------------------------------------------
4504 $ gitk # visualize and browse history
4505 $ git log # list all commits
4506 $ git log src/ # ...modifying src/
4507 $ git log v2.6.15..v2.6.16 # ...in v2.6.16, not in v2.6.15
4508 $ git log master..test # ...in branch test, not in branch master
4509 $ git log test..master # ...in branch master, but not in test
4510 $ git log test...master # ...in one branch, not in both
4511 $ git log -S'foo()' # ...where difference contain "foo()"
4512 $ git log --since="2 weeks ago"
4513 $ git log -p # show patches as well
4514 $ git show # most recent commit
4515 $ git diff v2.6.15..v2.6.16 # diff between two tagged versions
4516 $ git diff v2.6.15..HEAD # diff with current head
4517 $ git grep "foo()" # search working directory for "foo()"
4518 $ git grep v2.6.15 "foo()" # search old tree for "foo()"
4519 $ git show v2.6.15:a.txt # look at old version of a.txt
4520 -----------------------------------------------
4521
4522 Search for regressions:
4523
4524 -----------------------------------------------
4525 $ git bisect start
4526 $ git bisect bad # current version is bad
4527 $ git bisect good v2.6.13-rc2 # last known good revision
4528 Bisecting: 675 revisions left to test after this
4529 # test here, then:
4530 $ git bisect good # if this revision is good, or
4531 $ git bisect bad # if this revision is bad.
4532 # repeat until done.
4533 -----------------------------------------------
4534
4535 [[making-changes]]
4536 Making changes
4537 --------------
4538
4539 Make sure Git knows who to blame:
4540
4541 ------------------------------------------------
4542 $ cat >>~/.gitconfig <<\EOF
4543 [user]
4544 name = Your Name Comes Here
4545 email = you@yourdomain.example.com
4546 EOF
4547 ------------------------------------------------
4548
4549 Select file contents to include in the next commit, then make the
4550 commit:
4551
4552 -----------------------------------------------
4553 $ git add a.txt # updated file
4554 $ git add b.txt # new file
4555 $ git rm c.txt # old file
4556 $ git commit
4557 -----------------------------------------------
4558
4559 Or, prepare and create the commit in one step:
4560
4561 -----------------------------------------------
4562 $ git commit d.txt # use latest content only of d.txt
4563 $ git commit -a # use latest content of all tracked files
4564 -----------------------------------------------
4565
4566 [[merging]]
4567 Merging
4568 -------
4569
4570 -----------------------------------------------
4571 $ git merge test # merge branch "test" into the current branch
4572 $ git pull git://example.com/project.git master
4573 # fetch and merge in remote branch
4574 $ git pull . test # equivalent to git merge test
4575 -----------------------------------------------
4576
4577 [[sharing-your-changes]]
4578 Sharing your changes
4579 --------------------
4580
4581 Importing or exporting patches:
4582
4583 -----------------------------------------------
4584 $ git format-patch origin..HEAD # format a patch for each commit
4585 # in HEAD but not in origin
4586 $ git am mbox # import patches from the mailbox "mbox"
4587 -----------------------------------------------
4588
4589 Fetch a branch in a different Git repository, then merge into the
4590 current branch:
4591
4592 -----------------------------------------------
4593 $ git pull git://example.com/project.git theirbranch
4594 -----------------------------------------------
4595
4596 Store the fetched branch into a local branch before merging into the
4597 current branch:
4598
4599 -----------------------------------------------
4600 $ git pull git://example.com/project.git theirbranch:mybranch
4601 -----------------------------------------------
4602
4603 After creating commits on a local branch, update the remote
4604 branch with your commits:
4605
4606 -----------------------------------------------
4607 $ git push ssh://example.com/project.git mybranch:theirbranch
4608 -----------------------------------------------
4609
4610 When remote and local branch are both named "test":
4611
4612 -----------------------------------------------
4613 $ git push ssh://example.com/project.git test
4614 -----------------------------------------------
4615
4616 Shortcut version for a frequently used remote repository:
4617
4618 -----------------------------------------------
4619 $ git remote add example ssh://example.com/project.git
4620 $ git push example test
4621 -----------------------------------------------
4622
4623 [[repository-maintenance]]
4624 Repository maintenance
4625 ----------------------
4626
4627 Check for corruption:
4628
4629 -----------------------------------------------
4630 $ git fsck
4631 -----------------------------------------------
4632
4633 Recompress, remove unused cruft:
4634
4635 -----------------------------------------------
4636 $ git gc
4637 -----------------------------------------------
4638
4639
4640 [[todo]]
4641 Appendix B: Notes and todo list for this manual
4642 ===============================================
4643
4644 [[todo-list]]
4645 Todo list
4646 ---------
4647
4648 This is a work in progress.
4649
4650 The basic requirements:
4651
4652 - It must be readable in order, from beginning to end, by someone
4653 intelligent with a basic grasp of the UNIX command line, but without
4654 any special knowledge of Git. If necessary, any other prerequisites
4655 should be specifically mentioned as they arise.
4656 - Whenever possible, section headings should clearly describe the task
4657 they explain how to do, in language that requires no more knowledge
4658 than necessary: for example, "importing patches into a project" rather
4659 than "the `git am` command"
4660
4661 Think about how to create a clear chapter dependency graph that will
4662 allow people to get to important topics without necessarily reading
4663 everything in between.
4664
4665 Scan `Documentation/` for other stuff left out; in particular:
4666
4667 - howto's
4668 - some of `technical/`?
4669 - hooks
4670 - list of commands in linkgit:git[1]
4671
4672 Scan email archives for other stuff left out
4673
4674 Scan man pages to see if any assume more background than this manual
4675 provides.
4676
4677 Add more good examples. Entire sections of just cookbook examples
4678 might be a good idea; maybe make an "advanced examples" section a
4679 standard end-of-chapter section?
4680
4681 Include cross-references to the glossary, where appropriate.
4682
4683 Add a section on working with other version control systems, including
4684 CVS, Subversion, and just imports of series of release tarballs.
4685
4686 Write a chapter on using plumbing and writing scripts.
4687
4688 Alternates, clone -reference, etc.
4689
4690 More on recovery from repository corruption. See:
4691 http://marc.info/?l=git&m=117263864820799&w=2
4692 http://marc.info/?l=git&m=117147855503798&w=2