919c214b71a64a8d555227ff12a9eb4babdb184e
[git/git.git] / Documentation / user-manual.txt
1 = Git User Manual
2
3 Git is a fast distributed revision control system.
4
5 This manual is designed to be readable by someone with basic UNIX
6 command-line skills, but no previous knowledge of Git.
7
8 <<repositories-and-branches>> and <<exploring-git-history>> explain how
9 to fetch and study a project using git--read these chapters to learn how
10 to build and test a particular version of a software project, search for
11 regressions, and so on.
12
13 People needing to do actual development will also want to read
14 <<Developing-With-git>> and <<sharing-development>>.
15
16 Further chapters cover more specialized topics.
17
18 Comprehensive reference documentation is available through the man
19 pages, or linkgit:git-help[1] command. For example, for the command
20 `git clone <repo>`, you can either use:
21
22 ------------------------------------------------
23 $ man git-clone
24 ------------------------------------------------
25
26 or:
27
28 ------------------------------------------------
29 $ git help clone
30 ------------------------------------------------
31
32 With the latter, you can use the manual viewer of your choice; see
33 linkgit:git-help[1] for more information.
34
35 See also <<git-quick-start>> for a brief overview of Git commands,
36 without any explanation.
37
38 Finally, see <<todo>> for ways that you can help make this manual more
39 complete.
40
41
42 [[repositories-and-branches]]
43 == Repositories and Branches
44
45 [[how-to-get-a-git-repository]]
46 === How to get a Git repository
47
48 It will be useful to have a Git repository to experiment with as you
49 read this manual.
50
51 The best way to get one is by using the linkgit:git-clone[1] command to
52 download a copy of an existing repository. If you don't already have a
53 project in mind, here are some interesting examples:
54
55 ------------------------------------------------
56 # Git itself (approx. 40MB download):
57 $ git clone git://git.kernel.org/pub/scm/git/git.git
58 # the Linux kernel (approx. 640MB download):
59 $ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
60 ------------------------------------------------
61
62 The initial clone may be time-consuming for a large project, but you
63 will only need to clone once.
64
65 The clone command creates a new directory named after the project
66 (`git` or `linux` in the examples above). After you cd into this
67 directory, you will see that it contains a copy of the project files,
68 called the <<def_working_tree,working tree>>, together with a special
69 top-level directory named `.git`, which contains all the information
70 about the history of the project.
71
72 [[how-to-check-out]]
73 === How to check out a different version of a project
74
75 Git is best thought of as a tool for storing the history of a collection
76 of files. It stores the history as a compressed collection of
77 interrelated snapshots of the project's contents. In Git each such
78 version is called a <<def_commit,commit>>.
79
80 Those snapshots aren't necessarily all arranged in a single line from
81 oldest to newest; instead, work may simultaneously proceed along
82 parallel lines of development, called <<def_branch,branches>>, which may
83 merge and diverge.
84
85 A single Git repository can track development on multiple branches. It
86 does this by keeping a list of <<def_head,heads>> which reference the
87 latest commit on each branch; the linkgit:git-branch[1] command shows
88 you the list of branch heads:
89
90 ------------------------------------------------
91 $ git branch
92 * master
93 ------------------------------------------------
94
95 A freshly cloned repository contains a single branch head, by default
96 named "master", with the working directory initialized to the state of
97 the project referred to by that branch head.
98
99 Most projects also use <<def_tag,tags>>. Tags, like heads, are
100 references into the project's history, and can be listed using the
101 linkgit:git-tag[1] command:
102
103 ------------------------------------------------
104 $ git tag -l
105 v2.6.11
106 v2.6.11-tree
107 v2.6.12
108 v2.6.12-rc2
109 v2.6.12-rc3
110 v2.6.12-rc4
111 v2.6.12-rc5
112 v2.6.12-rc6
113 v2.6.13
114 ...
115 ------------------------------------------------
116
117 Tags are expected to always point at the same version of a project,
118 while heads are expected to advance as development progresses.
119
120 Create a new branch head pointing to one of these versions and check it
121 out using linkgit:git-switch[1]:
122
123 ------------------------------------------------
124 $ git switch -c new v2.6.13
125 ------------------------------------------------
126
127 The working directory then reflects the contents that the project had
128 when it was tagged v2.6.13, and linkgit:git-branch[1] shows two
129 branches, with an asterisk marking the currently checked-out branch:
130
131 ------------------------------------------------
132 $ git branch
133 master
134 * new
135 ------------------------------------------------
136
137 If you decide that you'd rather see version 2.6.17, you can modify
138 the current branch to point at v2.6.17 instead, with
139
140 ------------------------------------------------
141 $ git reset --hard v2.6.17
142 ------------------------------------------------
143
144 Note that if the current branch head was your only reference to a
145 particular point in history, then resetting that branch may leave you
146 with no way to find the history it used to point to; so use this command
147 carefully.
148
149 [[understanding-commits]]
150 === Understanding History: Commits
151
152 Every change in the history of a project is represented by a commit.
153 The linkgit:git-show[1] command shows the most recent commit on the
154 current branch:
155
156 ------------------------------------------------
157 $ git show
158 commit 17cf781661e6d38f737f15f53ab552f1e95960d7
159 Author: Linus Torvalds <torvalds@ppc970.osdl.org.(none)>
160 Date: Tue Apr 19 14:11:06 2005 -0700
161
162 Remove duplicate getenv(DB_ENVIRONMENT) call
163
164 Noted by Tony Luck.
165
166 diff --git a/init-db.c b/init-db.c
167 index 65898fa..b002dc6 100644
168 --- a/init-db.c
169 +++ b/init-db.c
170 @@ -7,7 +7,7 @@
171
172 int main(int argc, char **argv)
173 {
174 - char *sha1_dir = getenv(DB_ENVIRONMENT), *path;
175 + char *sha1_dir, *path;
176 int len, i;
177
178 if (mkdir(".git", 0755) < 0) {
179 ------------------------------------------------
180
181 As you can see, a commit shows who made the latest change, what they
182 did, and why.
183
184 Every commit has a 40-hexdigit id, sometimes called the "object name" or the
185 "SHA-1 id", shown on the first line of the `git show` output. You can usually
186 refer to a commit by a shorter name, such as a tag or a branch name, but this
187 longer name can also be useful. Most importantly, it is a globally unique
188 name for this commit: so if you tell somebody else the object name (for
189 example in email), then you are guaranteed that name will refer to the same
190 commit in their repository that it does in yours (assuming their repository
191 has that commit at all). Since the object name is computed as a hash over the
192 contents of the commit, you are guaranteed that the commit can never change
193 without its name also changing.
194
195 In fact, in <<git-concepts>> we shall see that everything stored in Git
196 history, including file data and directory contents, is stored in an object
197 with a name that is a hash of its contents.
198
199 [[understanding-reachability]]
200 ==== Understanding history: commits, parents, and reachability
201
202 Every commit (except the very first commit in a project) also has a
203 parent commit which shows what happened before this commit.
204 Following the chain of parents will eventually take you back to the
205 beginning of the project.
206
207 However, the commits do not form a simple list; Git allows lines of
208 development to diverge and then reconverge, and the point where two
209 lines of development reconverge is called a "merge". The commit
210 representing a merge can therefore have more than one parent, with
211 each parent representing the most recent commit on one of the lines
212 of development leading to that point.
213
214 The best way to see how this works is using the linkgit:gitk[1]
215 command; running gitk now on a Git repository and looking for merge
216 commits will help understand how Git organizes history.
217
218 In the following, we say that commit X is "reachable" from commit Y
219 if commit X is an ancestor of commit Y. Equivalently, you could say
220 that Y is a descendant of X, or that there is a chain of parents
221 leading from commit Y to commit X.
222
223 [[history-diagrams]]
224 ==== Understanding history: History diagrams
225
226 We will sometimes represent Git history using diagrams like the one
227 below. Commits are shown as "o", and the links between them with
228 lines drawn with - / and \. Time goes left to right:
229
230
231 ................................................
232 o--o--o <-- Branch A
233 /
234 o--o--o <-- master
235 \
236 o--o--o <-- Branch B
237 ................................................
238
239 If we need to talk about a particular commit, the character "o" may
240 be replaced with another letter or number.
241
242 [[what-is-a-branch]]
243 ==== Understanding history: What is a branch?
244
245 When we need to be precise, we will use the word "branch" to mean a line
246 of development, and "branch head" (or just "head") to mean a reference
247 to the most recent commit on a branch. In the example above, the branch
248 head named "A" is a pointer to one particular commit, but we refer to
249 the line of three commits leading up to that point as all being part of
250 "branch A".
251
252 However, when no confusion will result, we often just use the term
253 "branch" both for branches and for branch heads.
254
255 [[manipulating-branches]]
256 === Manipulating branches
257
258 Creating, deleting, and modifying branches is quick and easy; here's
259 a summary of the commands:
260
261 `git branch`::
262 list all branches.
263 `git branch <branch>`::
264 create a new branch named `<branch>`, referencing the same
265 point in history as the current branch.
266 `git branch <branch> <start-point>`::
267 create a new branch named `<branch>`, referencing
268 `<start-point>`, which may be specified any way you like,
269 including using a branch name or a tag name.
270 `git branch -d <branch>`::
271 delete the branch `<branch>`; if the branch is not fully
272 merged in its upstream branch or contained in the current branch,
273 this command will fail with a warning.
274 `git branch -D <branch>`::
275 delete the branch `<branch>` irrespective of its merged status.
276 `git switch <branch>`::
277 make the current branch `<branch>`, updating the working
278 directory to reflect the version referenced by `<branch>`.
279 `git switch -c <new> <start-point>`::
280 create a new branch `<new>` referencing `<start-point>`, and
281 check it out.
282
283 The special symbol "HEAD" can always be used to refer to the current
284 branch. In fact, Git uses a file named `HEAD` in the `.git` directory
285 to remember which branch is current:
286
287 ------------------------------------------------
288 $ cat .git/HEAD
289 ref: refs/heads/master
290 ------------------------------------------------
291
292 [[detached-head]]
293 === Examining an old version without creating a new branch
294
295 The `git switch` command normally expects a branch head, but will also
296 accept an arbitrary commit when invoked with --detach; for example,
297 you can check out the commit referenced by a tag:
298
299 ------------------------------------------------
300 $ git switch --detach v2.6.17
301 Note: checking out 'v2.6.17'.
302
303 You are in 'detached HEAD' state. You can look around, make experimental
304 changes and commit them, and you can discard any commits you make in this
305 state without impacting any branches by performing another switch.
306
307 If you want to create a new branch to retain commits you create, you may
308 do so (now or later) by using -c with the switch command again. Example:
309
310 git switch -c new_branch_name
311
312 HEAD is now at 427abfa Linux v2.6.17
313 ------------------------------------------------
314
315 The HEAD then refers to the SHA-1 of the commit instead of to a branch,
316 and git branch shows that you are no longer on a branch:
317
318 ------------------------------------------------
319 $ cat .git/HEAD
320 427abfa28afedffadfca9dd8b067eb6d36bac53f
321 $ git branch
322 * (detached from v2.6.17)
323 master
324 ------------------------------------------------
325
326 In this case we say that the HEAD is "detached".
327
328 This is an easy way to check out a particular version without having to
329 make up a name for the new branch. You can still create a new branch
330 (or tag) for this version later if you decide to.
331
332 [[examining-remote-branches]]
333 === Examining branches from a remote repository
334
335 The "master" branch that was created at the time you cloned is a copy
336 of the HEAD in the repository that you cloned from. That repository
337 may also have had other branches, though, and your local repository
338 keeps branches which track each of those remote branches, called
339 remote-tracking branches, which you
340 can view using the `-r` option to linkgit:git-branch[1]:
341
342 ------------------------------------------------
343 $ git branch -r
344 origin/HEAD
345 origin/html
346 origin/maint
347 origin/man
348 origin/master
349 origin/next
350 origin/pu
351 origin/todo
352 ------------------------------------------------
353
354 In this example, "origin" is called a remote repository, or "remote"
355 for short. The branches of this repository are called "remote
356 branches" from our point of view. The remote-tracking branches listed
357 above were created based on the remote branches at clone time and will
358 be updated by `git fetch` (hence `git pull`) and `git push`. See
359 <<Updating-a-repository-With-git-fetch>> for details.
360
361 You might want to build on one of these remote-tracking branches
362 on a branch of your own, just as you would for a tag:
363
364 ------------------------------------------------
365 $ git switch -c my-todo-copy origin/todo
366 ------------------------------------------------
367
368 You can also check out `origin/todo` directly to examine it or
369 write a one-off patch. See <<detached-head,detached head>>.
370
371 Note that the name "origin" is just the name that Git uses by default
372 to refer to the repository that you cloned from.
373
374 [[how-git-stores-references]]
375 === Naming branches, tags, and other references
376
377 Branches, remote-tracking branches, and tags are all references to
378 commits. All references are named with a slash-separated path name
379 starting with `refs`; the names we've been using so far are actually
380 shorthand:
381
382 - The branch `test` is short for `refs/heads/test`.
383 - The tag `v2.6.18` is short for `refs/tags/v2.6.18`.
384 - `origin/master` is short for `refs/remotes/origin/master`.
385
386 The full name is occasionally useful if, for example, there ever
387 exists a tag and a branch with the same name.
388
389 (Newly created refs are actually stored in the `.git/refs` directory,
390 under the path given by their name. However, for efficiency reasons
391 they may also be packed together in a single file; see
392 linkgit:git-pack-refs[1]).
393
394 As another useful shortcut, the "HEAD" of a repository can be referred
395 to just using the name of that repository. So, for example, "origin"
396 is usually a shortcut for the HEAD branch in the repository "origin".
397
398 For the complete list of paths which Git checks for references, and
399 the order it uses to decide which to choose when there are multiple
400 references with the same shorthand name, see the "SPECIFYING
401 REVISIONS" section of linkgit:gitrevisions[7].
402
403 [[Updating-a-repository-With-git-fetch]]
404 === Updating a repository with git fetch
405
406 After you clone a repository and commit a few changes of your own, you
407 may wish to check the original repository for updates.
408
409 The `git-fetch` command, with no arguments, will update all of the
410 remote-tracking branches to the latest version found in the original
411 repository. It will not touch any of your own branches--not even the
412 "master" branch that was created for you on clone.
413
414 [[fetching-branches]]
415 === Fetching branches from other repositories
416
417 You can also track branches from repositories other than the one you
418 cloned from, using linkgit:git-remote[1]:
419
420 -------------------------------------------------
421 $ git remote add staging git://git.kernel.org/.../gregkh/staging.git
422 $ git fetch staging
423 ...
424 From git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging
425 * [new branch] master -> staging/master
426 * [new branch] staging-linus -> staging/staging-linus
427 * [new branch] staging-next -> staging/staging-next
428 -------------------------------------------------
429
430 New remote-tracking branches will be stored under the shorthand name
431 that you gave `git remote add`, in this case `staging`:
432
433 -------------------------------------------------
434 $ git branch -r
435 origin/HEAD -> origin/master
436 origin/master
437 staging/master
438 staging/staging-linus
439 staging/staging-next
440 -------------------------------------------------
441
442 If you run `git fetch <remote>` later, the remote-tracking branches
443 for the named `<remote>` will be updated.
444
445 If you examine the file `.git/config`, you will see that Git has added
446 a new stanza:
447
448 -------------------------------------------------
449 $ cat .git/config
450 ...
451 [remote "staging"]
452 url = git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git
453 fetch = +refs/heads/*:refs/remotes/staging/*
454 ...
455 -------------------------------------------------
456
457 This is what causes Git to track the remote's branches; you may modify
458 or delete these configuration options by editing `.git/config` with a
459 text editor. (See the "CONFIGURATION FILE" section of
460 linkgit:git-config[1] for details.)
461
462 [[exploring-git-history]]
463 == Exploring Git history
464
465 Git is best thought of as a tool for storing the history of a
466 collection of files. It does this by storing compressed snapshots of
467 the contents of a file hierarchy, together with "commits" which show
468 the relationships between these snapshots.
469
470 Git provides extremely flexible and fast tools for exploring the
471 history of a project.
472
473 We start with one specialized tool that is useful for finding the
474 commit that introduced a bug into a project.
475
476 [[using-bisect]]
477 === How to use bisect to find a regression
478
479 Suppose version 2.6.18 of your project worked, but the version at
480 "master" crashes. Sometimes the best way to find the cause of such a
481 regression is to perform a brute-force search through the project's
482 history to find the particular commit that caused the problem. The
483 linkgit:git-bisect[1] command can help you do this:
484
485 -------------------------------------------------
486 $ git bisect start
487 $ git bisect good v2.6.18
488 $ git bisect bad master
489 Bisecting: 3537 revisions left to test after this
490 [65934a9a028b88e83e2b0f8b36618fe503349f8e] BLOCK: Make USB storage depend on SCSI rather than selecting it [try #6]
491 -------------------------------------------------
492
493 If you run `git branch` at this point, you'll see that Git has
494 temporarily moved you in "(no branch)". HEAD is now detached from any
495 branch and points directly to a commit (with commit id 65934) that
496 is reachable from "master" but not from v2.6.18. Compile and test it,
497 and see whether it crashes. Assume it does crash. Then:
498
499 -------------------------------------------------
500 $ git bisect bad
501 Bisecting: 1769 revisions left to test after this
502 [7eff82c8b1511017ae605f0c99ac275a7e21b867] i2c-core: Drop useless bitmaskings
503 -------------------------------------------------
504
505 checks out an older version. Continue like this, telling Git at each
506 stage whether the version it gives you is good or bad, and notice
507 that the number of revisions left to test is cut approximately in
508 half each time.
509
510 After about 13 tests (in this case), it will output the commit id of
511 the guilty commit. You can then examine the commit with
512 linkgit:git-show[1], find out who wrote it, and mail them your bug
513 report with the commit id. Finally, run
514
515 -------------------------------------------------
516 $ git bisect reset
517 -------------------------------------------------
518
519 to return you to the branch you were on before.
520
521 Note that the version which `git bisect` checks out for you at each
522 point is just a suggestion, and you're free to try a different
523 version if you think it would be a good idea. For example,
524 occasionally you may land on a commit that broke something unrelated;
525 run
526
527 -------------------------------------------------
528 $ git bisect visualize
529 -------------------------------------------------
530
531 which will run gitk and label the commit it chose with a marker that
532 says "bisect". Choose a safe-looking commit nearby, note its commit
533 id, and check it out with:
534
535 -------------------------------------------------
536 $ git reset --hard fb47ddb2db
537 -------------------------------------------------
538
539 then test, run `bisect good` or `bisect bad` as appropriate, and
540 continue.
541
542 Instead of `git bisect visualize` and then `git reset --hard
543 fb47ddb2db`, you might just want to tell Git that you want to skip
544 the current commit:
545
546 -------------------------------------------------
547 $ git bisect skip
548 -------------------------------------------------
549
550 In this case, though, Git may not eventually be able to tell the first
551 bad one between some first skipped commits and a later bad commit.
552
553 There are also ways to automate the bisecting process if you have a
554 test script that can tell a good from a bad commit. See
555 linkgit:git-bisect[1] for more information about this and other `git
556 bisect` features.
557
558 [[naming-commits]]
559 === Naming commits
560
561 We have seen several ways of naming commits already:
562
563 - 40-hexdigit object name
564 - branch name: refers to the commit at the head of the given
565 branch
566 - tag name: refers to the commit pointed to by the given tag
567 (we've seen branches and tags are special cases of
568 <<how-git-stores-references,references>>).
569 - HEAD: refers to the head of the current branch
570
571 There are many more; see the "SPECIFYING REVISIONS" section of the
572 linkgit:gitrevisions[7] man page for the complete list of ways to
573 name revisions. Some examples:
574
575 -------------------------------------------------
576 $ git show fb47ddb2 # the first few characters of the object name
577 # are usually enough to specify it uniquely
578 $ git show HEAD^ # the parent of the HEAD commit
579 $ git show HEAD^^ # the grandparent
580 $ git show HEAD~4 # the great-great-grandparent
581 -------------------------------------------------
582
583 Recall that merge commits may have more than one parent; by default,
584 `^` and `~` follow the first parent listed in the commit, but you can
585 also choose:
586
587 -------------------------------------------------
588 $ git show HEAD^1 # show the first parent of HEAD
589 $ git show HEAD^2 # show the second parent of HEAD
590 -------------------------------------------------
591
592 In addition to HEAD, there are several other special names for
593 commits:
594
595 Merges (to be discussed later), as well as operations such as
596 `git reset`, which change the currently checked-out commit, generally
597 set ORIG_HEAD to the value HEAD had before the current operation.
598
599 The `git fetch` operation always stores the head of the last fetched
600 branch in FETCH_HEAD. For example, if you run `git fetch` without
601 specifying a local branch as the target of the operation
602
603 -------------------------------------------------
604 $ git fetch git://example.com/proj.git theirbranch
605 -------------------------------------------------
606
607 the fetched commits will still be available from FETCH_HEAD.
608
609 When we discuss merges we'll also see the special name MERGE_HEAD,
610 which refers to the other branch that we're merging in to the current
611 branch.
612
613 The linkgit:git-rev-parse[1] command is a low-level command that is
614 occasionally useful for translating some name for a commit to the object
615 name for that commit:
616
617 -------------------------------------------------
618 $ git rev-parse origin
619 e05db0fd4f31dde7005f075a84f96b360d05984b
620 -------------------------------------------------
621
622 [[creating-tags]]
623 === Creating tags
624
625 We can also create a tag to refer to a particular commit; after
626 running
627
628 -------------------------------------------------
629 $ git tag stable-1 1b2e1d63ff
630 -------------------------------------------------
631
632 You can use `stable-1` to refer to the commit 1b2e1d63ff.
633
634 This creates a "lightweight" tag. If you would also like to include a
635 comment with the tag, and possibly sign it cryptographically, then you
636 should create a tag object instead; see the linkgit:git-tag[1] man page
637 for details.
638
639 [[browsing-revisions]]
640 === Browsing revisions
641
642 The linkgit:git-log[1] command can show lists of commits. On its
643 own, it shows all commits reachable from the parent commit; but you
644 can also make more specific requests:
645
646 -------------------------------------------------
647 $ git log v2.5.. # commits since (not reachable from) v2.5
648 $ git log test..master # commits reachable from master but not test
649 $ git log master..test # ...reachable from test but not master
650 $ git log master...test # ...reachable from either test or master,
651 # but not both
652 $ git log --since="2 weeks ago" # commits from the last 2 weeks
653 $ git log Makefile # commits which modify Makefile
654 $ git log fs/ # ... which modify any file under fs/
655 $ git log -S'foo()' # commits which add or remove any file data
656 # matching the string 'foo()'
657 -------------------------------------------------
658
659 And of course you can combine all of these; the following finds
660 commits since v2.5 which touch the `Makefile` or any file under `fs`:
661
662 -------------------------------------------------
663 $ git log v2.5.. Makefile fs/
664 -------------------------------------------------
665
666 You can also ask git log to show patches:
667
668 -------------------------------------------------
669 $ git log -p
670 -------------------------------------------------
671
672 See the `--pretty` option in the linkgit:git-log[1] man page for more
673 display options.
674
675 Note that git log starts with the most recent commit and works
676 backwards through the parents; however, since Git history can contain
677 multiple independent lines of development, the particular order that
678 commits are listed in may be somewhat arbitrary.
679
680 [[generating-diffs]]
681 === Generating diffs
682
683 You can generate diffs between any two versions using
684 linkgit:git-diff[1]:
685
686 -------------------------------------------------
687 $ git diff master..test
688 -------------------------------------------------
689
690 That will produce the diff between the tips of the two branches. If
691 you'd prefer to find the diff from their common ancestor to test, you
692 can use three dots instead of two:
693
694 -------------------------------------------------
695 $ git diff master...test
696 -------------------------------------------------
697
698 Sometimes what you want instead is a set of patches; for this you can
699 use linkgit:git-format-patch[1]:
700
701 -------------------------------------------------
702 $ git format-patch master..test
703 -------------------------------------------------
704
705 will generate a file with a patch for each commit reachable from test
706 but not from master.
707
708 [[viewing-old-file-versions]]
709 === Viewing old file versions
710
711 You can always view an old version of a file by just checking out the
712 correct revision first. But sometimes it is more convenient to be
713 able to view an old version of a single file without checking
714 anything out; this command does that:
715
716 -------------------------------------------------
717 $ git show v2.5:fs/locks.c
718 -------------------------------------------------
719
720 Before the colon may be anything that names a commit, and after it
721 may be any path to a file tracked by Git.
722
723 [[history-examples]]
724 === Examples
725
726 [[counting-commits-on-a-branch]]
727 ==== Counting the number of commits on a branch
728
729 Suppose you want to know how many commits you've made on `mybranch`
730 since it diverged from `origin`:
731
732 -------------------------------------------------
733 $ git log --pretty=oneline origin..mybranch | wc -l
734 -------------------------------------------------
735
736 Alternatively, you may often see this sort of thing done with the
737 lower-level command linkgit:git-rev-list[1], which just lists the SHA-1's
738 of all the given commits:
739
740 -------------------------------------------------
741 $ git rev-list origin..mybranch | wc -l
742 -------------------------------------------------
743
744 [[checking-for-equal-branches]]
745 ==== Check whether two branches point at the same history
746
747 Suppose you want to check whether two branches point at the same point
748 in history.
749
750 -------------------------------------------------
751 $ git diff origin..master
752 -------------------------------------------------
753
754 will tell you whether the contents of the project are the same at the
755 two branches; in theory, however, it's possible that the same project
756 contents could have been arrived at by two different historical
757 routes. You could compare the object names:
758
759 -------------------------------------------------
760 $ git rev-list origin
761 e05db0fd4f31dde7005f075a84f96b360d05984b
762 $ git rev-list master
763 e05db0fd4f31dde7005f075a84f96b360d05984b
764 -------------------------------------------------
765
766 Or you could recall that the `...` operator selects all commits
767 reachable from either one reference or the other but not
768 both; so
769
770 -------------------------------------------------
771 $ git log origin...master
772 -------------------------------------------------
773
774 will return no commits when the two branches are equal.
775
776 [[finding-tagged-descendants]]
777 ==== Find first tagged version including a given fix
778
779 Suppose you know that the commit e05db0fd fixed a certain problem.
780 You'd like to find the earliest tagged release that contains that
781 fix.
782
783 Of course, there may be more than one answer--if the history branched
784 after commit e05db0fd, then there could be multiple "earliest" tagged
785 releases.
786
787 You could just visually inspect the commits since e05db0fd:
788
789 -------------------------------------------------
790 $ gitk e05db0fd..
791 -------------------------------------------------
792
793 or you can use linkgit:git-name-rev[1], which will give the commit a
794 name based on any tag it finds pointing to one of the commit's
795 descendants:
796
797 -------------------------------------------------
798 $ git name-rev --tags e05db0fd
799 e05db0fd tags/v1.5.0-rc1^0~23
800 -------------------------------------------------
801
802 The linkgit:git-describe[1] command does the opposite, naming the
803 revision using a tag on which the given commit is based:
804
805 -------------------------------------------------
806 $ git describe e05db0fd
807 v1.5.0-rc0-260-ge05db0f
808 -------------------------------------------------
809
810 but that may sometimes help you guess which tags might come after the
811 given commit.
812
813 If you just want to verify whether a given tagged version contains a
814 given commit, you could use linkgit:git-merge-base[1]:
815
816 -------------------------------------------------
817 $ git merge-base e05db0fd v1.5.0-rc1
818 e05db0fd4f31dde7005f075a84f96b360d05984b
819 -------------------------------------------------
820
821 The merge-base command finds a common ancestor of the given commits,
822 and always returns one or the other in the case where one is a
823 descendant of the other; so the above output shows that e05db0fd
824 actually is an ancestor of v1.5.0-rc1.
825
826 Alternatively, note that
827
828 -------------------------------------------------
829 $ git log v1.5.0-rc1..e05db0fd
830 -------------------------------------------------
831
832 will produce empty output if and only if v1.5.0-rc1 includes e05db0fd,
833 because it outputs only commits that are not reachable from v1.5.0-rc1.
834
835 As yet another alternative, the linkgit:git-show-branch[1] command lists
836 the commits reachable from its arguments with a display on the left-hand
837 side that indicates which arguments that commit is reachable from.
838 So, if you run something like
839
840 -------------------------------------------------
841 $ git show-branch e05db0fd v1.5.0-rc0 v1.5.0-rc1 v1.5.0-rc2
842 ! [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
843 available
844 ! [v1.5.0-rc0] GIT v1.5.0 preview
845 ! [v1.5.0-rc1] GIT v1.5.0-rc1
846 ! [v1.5.0-rc2] GIT v1.5.0-rc2
847 ...
848 -------------------------------------------------
849
850 then a line like
851
852 -------------------------------------------------
853 + ++ [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
854 available
855 -------------------------------------------------
856
857 shows that e05db0fd is reachable from itself, from v1.5.0-rc1,
858 and from v1.5.0-rc2, and not from v1.5.0-rc0.
859
860 [[showing-commits-unique-to-a-branch]]
861 ==== Showing commits unique to a given branch
862
863 Suppose you would like to see all the commits reachable from the branch
864 head named `master` but not from any other head in your repository.
865
866 We can list all the heads in this repository with
867 linkgit:git-show-ref[1]:
868
869 -------------------------------------------------
870 $ git show-ref --heads
871 bf62196b5e363d73353a9dcf094c59595f3153b7 refs/heads/core-tutorial
872 db768d5504c1bb46f63ee9d6e1772bd047e05bf9 refs/heads/maint
873 a07157ac624b2524a059a3414e99f6f44bebc1e7 refs/heads/master
874 24dbc180ea14dc1aebe09f14c8ecf32010690627 refs/heads/tutorial-2
875 1e87486ae06626c2f31eaa63d26fc0fd646c8af2 refs/heads/tutorial-fixes
876 -------------------------------------------------
877
878 We can get just the branch-head names, and remove `master`, with
879 the help of the standard utilities cut and grep:
880
881 -------------------------------------------------
882 $ git show-ref --heads | cut -d' ' -f2 | grep -v '^refs/heads/master'
883 refs/heads/core-tutorial
884 refs/heads/maint
885 refs/heads/tutorial-2
886 refs/heads/tutorial-fixes
887 -------------------------------------------------
888
889 And then we can ask to see all the commits reachable from master
890 but not from these other heads:
891
892 -------------------------------------------------
893 $ gitk master --not $( git show-ref --heads | cut -d' ' -f2 |
894 grep -v '^refs/heads/master' )
895 -------------------------------------------------
896
897 Obviously, endless variations are possible; for example, to see all
898 commits reachable from some head but not from any tag in the repository:
899
900 -------------------------------------------------
901 $ gitk $( git show-ref --heads ) --not $( git show-ref --tags )
902 -------------------------------------------------
903
904 (See linkgit:gitrevisions[7] for explanations of commit-selecting
905 syntax such as `--not`.)
906
907 [[making-a-release]]
908 ==== Creating a changelog and tarball for a software release
909
910 The linkgit:git-archive[1] command can create a tar or zip archive from
911 any version of a project; for example:
912
913 -------------------------------------------------
914 $ git archive -o latest.tar.gz --prefix=project/ HEAD
915 -------------------------------------------------
916
917 will use HEAD to produce a gzipped tar archive in which each filename
918 is preceded by `project/`. The output file format is inferred from
919 the output file extension if possible, see linkgit:git-archive[1] for
920 details.
921
922 Versions of Git older than 1.7.7 don't know about the `tar.gz` format,
923 you'll need to use gzip explicitly:
924
925 -------------------------------------------------
926 $ git archive --format=tar --prefix=project/ HEAD | gzip >latest.tar.gz
927 -------------------------------------------------
928
929 If you're releasing a new version of a software project, you may want
930 to simultaneously make a changelog to include in the release
931 announcement.
932
933 Linus Torvalds, for example, makes new kernel releases by tagging them,
934 then running:
935
936 -------------------------------------------------
937 $ release-script 2.6.12 2.6.13-rc6 2.6.13-rc7
938 -------------------------------------------------
939
940 where release-script is a shell script that looks like:
941
942 -------------------------------------------------
943 #!/bin/sh
944 stable="$1"
945 last="$2"
946 new="$3"
947 echo "# git tag v$new"
948 echo "git archive --prefix=linux-$new/ v$new | gzip -9 > ../linux-$new.tar.gz"
949 echo "git diff v$stable v$new | gzip -9 > ../patch-$new.gz"
950 echo "git log --no-merges v$new ^v$last > ../ChangeLog-$new"
951 echo "git shortlog --no-merges v$new ^v$last > ../ShortLog"
952 echo "git diff --stat --summary -M v$last v$new > ../diffstat-$new"
953 -------------------------------------------------
954
955 and then he just cut-and-pastes the output commands after verifying that
956 they look OK.
957
958 [[Finding-commits-With-given-Content]]
959 ==== Finding commits referencing a file with given content
960
961 Somebody hands you a copy of a file, and asks which commits modified a
962 file such that it contained the given content either before or after the
963 commit. You can find out with this:
964
965 -------------------------------------------------
966 $ git log --raw --abbrev=40 --pretty=oneline |
967 grep -B 1 `git hash-object filename`
968 -------------------------------------------------
969
970 Figuring out why this works is left as an exercise to the (advanced)
971 student. The linkgit:git-log[1], linkgit:git-diff-tree[1], and
972 linkgit:git-hash-object[1] man pages may prove helpful.
973
974 [[Developing-With-git]]
975 == Developing with Git
976
977 [[telling-git-your-name]]
978 === Telling Git your name
979
980 Before creating any commits, you should introduce yourself to Git.
981 The easiest way to do so is to use linkgit:git-config[1]:
982
983 ------------------------------------------------
984 $ git config --global user.name 'Your Name Comes Here'
985 $ git config --global user.email 'you@yourdomain.example.com'
986 ------------------------------------------------
987
988 Which will add the following to a file named `.gitconfig` in your
989 home directory:
990
991 ------------------------------------------------
992 [user]
993 name = Your Name Comes Here
994 email = you@yourdomain.example.com
995 ------------------------------------------------
996
997 See the "CONFIGURATION FILE" section of linkgit:git-config[1] for
998 details on the configuration file. The file is plain text, so you can
999 also edit it with your favorite editor.
1000
1001
1002 [[creating-a-new-repository]]
1003 === Creating a new repository
1004
1005 Creating a new repository from scratch is very easy:
1006
1007 -------------------------------------------------
1008 $ mkdir project
1009 $ cd project
1010 $ git init
1011 -------------------------------------------------
1012
1013 If you have some initial content (say, a tarball):
1014
1015 -------------------------------------------------
1016 $ tar xzvf project.tar.gz
1017 $ cd project
1018 $ git init
1019 $ git add . # include everything below ./ in the first commit:
1020 $ git commit
1021 -------------------------------------------------
1022
1023 [[how-to-make-a-commit]]
1024 === How to make a commit
1025
1026 Creating a new commit takes three steps:
1027
1028 1. Making some changes to the working directory using your
1029 favorite editor.
1030 2. Telling Git about your changes.
1031 3. Creating the commit using the content you told Git about
1032 in step 2.
1033
1034 In practice, you can interleave and repeat steps 1 and 2 as many
1035 times as you want: in order to keep track of what you want committed
1036 at step 3, Git maintains a snapshot of the tree's contents in a
1037 special staging area called "the index."
1038
1039 At the beginning, the content of the index will be identical to
1040 that of the HEAD. The command `git diff --cached`, which shows
1041 the difference between the HEAD and the index, should therefore
1042 produce no output at that point.
1043
1044 Modifying the index is easy:
1045
1046 To update the index with the contents of a new or modified file, use
1047
1048 -------------------------------------------------
1049 $ git add path/to/file
1050 -------------------------------------------------
1051
1052 To remove a file from the index and from the working tree, use
1053
1054 -------------------------------------------------
1055 $ git rm path/to/file
1056 -------------------------------------------------
1057
1058 After each step you can verify that
1059
1060 -------------------------------------------------
1061 $ git diff --cached
1062 -------------------------------------------------
1063
1064 always shows the difference between the HEAD and the index file--this
1065 is what you'd commit if you created the commit now--and that
1066
1067 -------------------------------------------------
1068 $ git diff
1069 -------------------------------------------------
1070
1071 shows the difference between the working tree and the index file.
1072
1073 Note that `git add` always adds just the current contents of a file
1074 to the index; further changes to the same file will be ignored unless
1075 you run `git add` on the file again.
1076
1077 When you're ready, just run
1078
1079 -------------------------------------------------
1080 $ git commit
1081 -------------------------------------------------
1082
1083 and Git will prompt you for a commit message and then create the new
1084 commit. Check to make sure it looks like what you expected with
1085
1086 -------------------------------------------------
1087 $ git show
1088 -------------------------------------------------
1089
1090 As a special shortcut,
1091
1092 -------------------------------------------------
1093 $ git commit -a
1094 -------------------------------------------------
1095
1096 will update the index with any files that you've modified or removed
1097 and create a commit, all in one step.
1098
1099 A number of commands are useful for keeping track of what you're
1100 about to commit:
1101
1102 -------------------------------------------------
1103 $ git diff --cached # difference between HEAD and the index; what
1104 # would be committed if you ran "commit" now.
1105 $ git diff # difference between the index file and your
1106 # working directory; changes that would not
1107 # be included if you ran "commit" now.
1108 $ git diff HEAD # difference between HEAD and working tree; what
1109 # would be committed if you ran "commit -a" now.
1110 $ git status # a brief per-file summary of the above.
1111 -------------------------------------------------
1112
1113 You can also use linkgit:git-gui[1] to create commits, view changes in
1114 the index and the working tree files, and individually select diff hunks
1115 for inclusion in the index (by right-clicking on the diff hunk and
1116 choosing "Stage Hunk For Commit").
1117
1118 [[creating-good-commit-messages]]
1119 === Creating good commit messages
1120
1121 Though not required, it's a good idea to begin the commit message
1122 with a single short (less than 50 character) line summarizing the
1123 change, followed by a blank line and then a more thorough
1124 description. The text up to the first blank line in a commit
1125 message is treated as the commit title, and that title is used
1126 throughout Git. For example, linkgit:git-format-patch[1] turns a
1127 commit into email, and it uses the title on the Subject line and the
1128 rest of the commit in the body.
1129
1130
1131 [[ignoring-files]]
1132 === Ignoring files
1133
1134 A project will often generate files that you do 'not' want to track with Git.
1135 This typically includes files generated by a build process or temporary
1136 backup files made by your editor. Of course, 'not' tracking files with Git
1137 is just a matter of 'not' calling `git add` on them. But it quickly becomes
1138 annoying to have these untracked files lying around; e.g. they make
1139 `git add .` practically useless, and they keep showing up in the output of
1140 `git status`.
1141
1142 You can tell Git to ignore certain files by creating a file called
1143 `.gitignore` in the top level of your working directory, with contents
1144 such as:
1145
1146 -------------------------------------------------
1147 # Lines starting with '#' are considered comments.
1148 # Ignore any file named foo.txt.
1149 foo.txt
1150 # Ignore (generated) html files,
1151 *.html
1152 # except foo.html which is maintained by hand.
1153 !foo.html
1154 # Ignore objects and archives.
1155 *.[oa]
1156 -------------------------------------------------
1157
1158 See linkgit:gitignore[5] for a detailed explanation of the syntax. You can
1159 also place .gitignore files in other directories in your working tree, and they
1160 will apply to those directories and their subdirectories. The `.gitignore`
1161 files can be added to your repository like any other files (just run `git add
1162 .gitignore` and `git commit`, as usual), which is convenient when the exclude
1163 patterns (such as patterns matching build output files) would also make sense
1164 for other users who clone your repository.
1165
1166 If you wish the exclude patterns to affect only certain repositories
1167 (instead of every repository for a given project), you may instead put
1168 them in a file in your repository named `.git/info/exclude`, or in any
1169 file specified by the `core.excludesFile` configuration variable.
1170 Some Git commands can also take exclude patterns directly on the
1171 command line. See linkgit:gitignore[5] for the details.
1172
1173 [[how-to-merge]]
1174 === How to merge
1175
1176 You can rejoin two diverging branches of development using
1177 linkgit:git-merge[1]:
1178
1179 -------------------------------------------------
1180 $ git merge branchname
1181 -------------------------------------------------
1182
1183 merges the development in the branch `branchname` into the current
1184 branch.
1185
1186 A merge is made by combining the changes made in `branchname` and the
1187 changes made up to the latest commit in your current branch since
1188 their histories forked. The work tree is overwritten by the result of
1189 the merge when this combining is done cleanly, or overwritten by a
1190 half-merged results when this combining results in conflicts.
1191 Therefore, if you have uncommitted changes touching the same files as
1192 the ones impacted by the merge, Git will refuse to proceed. Most of
1193 the time, you will want to commit your changes before you can merge,
1194 and if you don't, then linkgit:git-stash[1] can take these changes
1195 away while you're doing the merge, and reapply them afterwards.
1196
1197 If the changes are independent enough, Git will automatically complete
1198 the merge and commit the result (or reuse an existing commit in case
1199 of <<fast-forwards,fast-forward>>, see below). On the other hand,
1200 if there are conflicts--for example, if the same file is
1201 modified in two different ways in the remote branch and the local
1202 branch--then you are warned; the output may look something like this:
1203
1204 -------------------------------------------------
1205 $ git merge next
1206 100% (4/4) done
1207 Auto-merged file.txt
1208 CONFLICT (content): Merge conflict in file.txt
1209 Automatic merge failed; fix conflicts and then commit the result.
1210 -------------------------------------------------
1211
1212 Conflict markers are left in the problematic files, and after
1213 you resolve the conflicts manually, you can update the index
1214 with the contents and run Git commit, as you normally would when
1215 creating a new file.
1216
1217 If you examine the resulting commit using gitk, you will see that it
1218 has two parents, one pointing to the top of the current branch, and
1219 one to the top of the other branch.
1220
1221 [[resolving-a-merge]]
1222 === Resolving a merge
1223
1224 When a merge isn't resolved automatically, Git leaves the index and
1225 the working tree in a special state that gives you all the
1226 information you need to help resolve the merge.
1227
1228 Files with conflicts are marked specially in the index, so until you
1229 resolve the problem and update the index, linkgit:git-commit[1] will
1230 fail:
1231
1232 -------------------------------------------------
1233 $ git commit
1234 file.txt: needs merge
1235 -------------------------------------------------
1236
1237 Also, linkgit:git-status[1] will list those files as "unmerged", and the
1238 files with conflicts will have conflict markers added, like this:
1239
1240 -------------------------------------------------
1241 <<<<<<< HEAD:file.txt
1242 Hello world
1243 =======
1244 Goodbye
1245 >>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
1246 -------------------------------------------------
1247
1248 All you need to do is edit the files to resolve the conflicts, and then
1249
1250 -------------------------------------------------
1251 $ git add file.txt
1252 $ git commit
1253 -------------------------------------------------
1254
1255 Note that the commit message will already be filled in for you with
1256 some information about the merge. Normally you can just use this
1257 default message unchanged, but you may add additional commentary of
1258 your own if desired.
1259
1260 The above is all you need to know to resolve a simple merge. But Git
1261 also provides more information to help resolve conflicts:
1262
1263 [[conflict-resolution]]
1264 ==== Getting conflict-resolution help during a merge
1265
1266 All of the changes that Git was able to merge automatically are
1267 already added to the index file, so linkgit:git-diff[1] shows only
1268 the conflicts. It uses an unusual syntax:
1269
1270 -------------------------------------------------
1271 $ git diff
1272 diff --cc file.txt
1273 index 802992c,2b60207..0000000
1274 --- a/file.txt
1275 +++ b/file.txt
1276 @@@ -1,1 -1,1 +1,5 @@@
1277 ++<<<<<<< HEAD:file.txt
1278 +Hello world
1279 ++=======
1280 + Goodbye
1281 ++>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
1282 -------------------------------------------------
1283
1284 Recall that the commit which will be committed after we resolve this
1285 conflict will have two parents instead of the usual one: one parent
1286 will be HEAD, the tip of the current branch; the other will be the
1287 tip of the other branch, which is stored temporarily in MERGE_HEAD.
1288
1289 During the merge, the index holds three versions of each file. Each of
1290 these three "file stages" represents a different version of the file:
1291
1292 -------------------------------------------------
1293 $ git show :1:file.txt # the file in a common ancestor of both branches
1294 $ git show :2:file.txt # the version from HEAD.
1295 $ git show :3:file.txt # the version from MERGE_HEAD.
1296 -------------------------------------------------
1297
1298 When you ask linkgit:git-diff[1] to show the conflicts, it runs a
1299 three-way diff between the conflicted merge results in the work tree with
1300 stages 2 and 3 to show only hunks whose contents come from both sides,
1301 mixed (in other words, when a hunk's merge results come only from stage 2,
1302 that part is not conflicting and is not shown. Same for stage 3).
1303
1304 The diff above shows the differences between the working-tree version of
1305 file.txt and the stage 2 and stage 3 versions. So instead of preceding
1306 each line by a single `+` or `-`, it now uses two columns: the first
1307 column is used for differences between the first parent and the working
1308 directory copy, and the second for differences between the second parent
1309 and the working directory copy. (See the "COMBINED DIFF FORMAT" section
1310 of linkgit:git-diff-files[1] for a details of the format.)
1311
1312 After resolving the conflict in the obvious way (but before updating the
1313 index), the diff will look like:
1314
1315 -------------------------------------------------
1316 $ git diff
1317 diff --cc file.txt
1318 index 802992c,2b60207..0000000
1319 --- a/file.txt
1320 +++ b/file.txt
1321 @@@ -1,1 -1,1 +1,1 @@@
1322 - Hello world
1323 -Goodbye
1324 ++Goodbye world
1325 -------------------------------------------------
1326
1327 This shows that our resolved version deleted "Hello world" from the
1328 first parent, deleted "Goodbye" from the second parent, and added
1329 "Goodbye world", which was previously absent from both.
1330
1331 Some special diff options allow diffing the working directory against
1332 any of these stages:
1333
1334 -------------------------------------------------
1335 $ git diff -1 file.txt # diff against stage 1
1336 $ git diff --base file.txt # same as the above
1337 $ git diff -2 file.txt # diff against stage 2
1338 $ git diff --ours file.txt # same as the above
1339 $ git diff -3 file.txt # diff against stage 3
1340 $ git diff --theirs file.txt # same as the above.
1341 -------------------------------------------------
1342
1343 The linkgit:git-log[1] and linkgit:gitk[1] commands also provide special help
1344 for merges:
1345
1346 -------------------------------------------------
1347 $ git log --merge
1348 $ gitk --merge
1349 -------------------------------------------------
1350
1351 These will display all commits which exist only on HEAD or on
1352 MERGE_HEAD, and which touch an unmerged file.
1353
1354 You may also use linkgit:git-mergetool[1], which lets you merge the
1355 unmerged files using external tools such as Emacs or kdiff3.
1356
1357 Each time you resolve the conflicts in a file and update the index:
1358
1359 -------------------------------------------------
1360 $ git add file.txt
1361 -------------------------------------------------
1362
1363 the different stages of that file will be "collapsed", after which
1364 `git diff` will (by default) no longer show diffs for that file.
1365
1366 [[undoing-a-merge]]
1367 === Undoing a merge
1368
1369 If you get stuck and decide to just give up and throw the whole mess
1370 away, you can always return to the pre-merge state with
1371
1372 -------------------------------------------------
1373 $ git merge --abort
1374 -------------------------------------------------
1375
1376 Or, if you've already committed the merge that you want to throw away,
1377
1378 -------------------------------------------------
1379 $ git reset --hard ORIG_HEAD
1380 -------------------------------------------------
1381
1382 However, this last command can be dangerous in some cases--never
1383 throw away a commit you have already committed if that commit may
1384 itself have been merged into another branch, as doing so may confuse
1385 further merges.
1386
1387 [[fast-forwards]]
1388 === Fast-forward merges
1389
1390 There is one special case not mentioned above, which is treated
1391 differently. Normally, a merge results in a merge commit, with two
1392 parents, one pointing at each of the two lines of development that
1393 were merged.
1394
1395 However, if the current branch is an ancestor of the other--so every commit
1396 present in the current branch is already contained in the other branch--then Git
1397 just performs a "fast-forward"; the head of the current branch is moved forward
1398 to point at the head of the merged-in branch, without any new commits being
1399 created.
1400
1401 [[fixing-mistakes]]
1402 === Fixing mistakes
1403
1404 If you've messed up the working tree, but haven't yet committed your
1405 mistake, you can return the entire working tree to the last committed
1406 state with
1407
1408 -------------------------------------------------
1409 $ git restore --staged --worktree :/
1410 -------------------------------------------------
1411
1412 If you make a commit that you later wish you hadn't, there are two
1413 fundamentally different ways to fix the problem:
1414
1415 1. You can create a new commit that undoes whatever was done
1416 by the old commit. This is the correct thing if your
1417 mistake has already been made public.
1418
1419 2. You can go back and modify the old commit. You should
1420 never do this if you have already made the history public;
1421 Git does not normally expect the "history" of a project to
1422 change, and cannot correctly perform repeated merges from
1423 a branch that has had its history changed.
1424
1425 [[reverting-a-commit]]
1426 ==== Fixing a mistake with a new commit
1427
1428 Creating a new commit that reverts an earlier change is very easy;
1429 just pass the linkgit:git-revert[1] command a reference to the bad
1430 commit; for example, to revert the most recent commit:
1431
1432 -------------------------------------------------
1433 $ git revert HEAD
1434 -------------------------------------------------
1435
1436 This will create a new commit which undoes the change in HEAD. You
1437 will be given a chance to edit the commit message for the new commit.
1438
1439 You can also revert an earlier change, for example, the next-to-last:
1440
1441 -------------------------------------------------
1442 $ git revert HEAD^
1443 -------------------------------------------------
1444
1445 In this case Git will attempt to undo the old change while leaving
1446 intact any changes made since then. If more recent changes overlap
1447 with the changes to be reverted, then you will be asked to fix
1448 conflicts manually, just as in the case of <<resolving-a-merge,
1449 resolving a merge>>.
1450
1451 [[fixing-a-mistake-by-rewriting-history]]
1452 ==== Fixing a mistake by rewriting history
1453
1454 If the problematic commit is the most recent commit, and you have not
1455 yet made that commit public, then you may just
1456 <<undoing-a-merge,destroy it using `git reset`>>.
1457
1458 Alternatively, you
1459 can edit the working directory and update the index to fix your
1460 mistake, just as if you were going to <<how-to-make-a-commit,create a
1461 new commit>>, then run
1462
1463 -------------------------------------------------
1464 $ git commit --amend
1465 -------------------------------------------------
1466
1467 which will replace the old commit by a new commit incorporating your
1468 changes, giving you a chance to edit the old commit message first.
1469
1470 Again, you should never do this to a commit that may already have
1471 been merged into another branch; use linkgit:git-revert[1] instead in
1472 that case.
1473
1474 It is also possible to replace commits further back in the history, but
1475 this is an advanced topic to be left for
1476 <<cleaning-up-history,another chapter>>.
1477
1478 [[checkout-of-path]]
1479 ==== Checking out an old version of a file
1480
1481 In the process of undoing a previous bad change, you may find it
1482 useful to check out an older version of a particular file using
1483 linkgit:git-restore[1]. The command
1484
1485 -------------------------------------------------
1486 $ git restore --source=HEAD^ path/to/file
1487 -------------------------------------------------
1488
1489 replaces path/to/file by the contents it had in the commit HEAD^, and
1490 also updates the index to match. It does not change branches.
1491
1492 If you just want to look at an old version of the file, without
1493 modifying the working directory, you can do that with
1494 linkgit:git-show[1]:
1495
1496 -------------------------------------------------
1497 $ git show HEAD^:path/to/file
1498 -------------------------------------------------
1499
1500 which will display the given version of the file.
1501
1502 [[interrupted-work]]
1503 ==== Temporarily setting aside work in progress
1504
1505 While you are in the middle of working on something complicated, you
1506 find an unrelated but obvious and trivial bug. You would like to fix it
1507 before continuing. You can use linkgit:git-stash[1] to save the current
1508 state of your work, and after fixing the bug (or, optionally after doing
1509 so on a different branch and then coming back), unstash the
1510 work-in-progress changes.
1511
1512 ------------------------------------------------
1513 $ git stash push -m "work in progress for foo feature"
1514 ------------------------------------------------
1515
1516 This command will save your changes away to the `stash`, and
1517 reset your working tree and the index to match the tip of your
1518 current branch. Then you can make your fix as usual.
1519
1520 ------------------------------------------------
1521 ... edit and test ...
1522 $ git commit -a -m "blorpl: typofix"
1523 ------------------------------------------------
1524
1525 After that, you can go back to what you were working on with
1526 `git stash pop`:
1527
1528 ------------------------------------------------
1529 $ git stash pop
1530 ------------------------------------------------
1531
1532
1533 [[ensuring-good-performance]]
1534 === Ensuring good performance
1535
1536 On large repositories, Git depends on compression to keep the history
1537 information from taking up too much space on disk or in memory. Some
1538 Git commands may automatically run linkgit:git-gc[1], so you don't
1539 have to worry about running it manually. However, compressing a large
1540 repository may take a while, so you may want to call `gc` explicitly
1541 to avoid automatic compression kicking in when it is not convenient.
1542
1543
1544 [[ensuring-reliability]]
1545 === Ensuring reliability
1546
1547 [[checking-for-corruption]]
1548 ==== Checking the repository for corruption
1549
1550 The linkgit:git-fsck[1] command runs a number of self-consistency checks
1551 on the repository, and reports on any problems. This may take some
1552 time.
1553
1554 -------------------------------------------------
1555 $ git fsck
1556 dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
1557 dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
1558 dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
1559 dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb
1560 dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f
1561 dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e
1562 dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085
1563 dangling tree b24c2473f1fd3d91352a624795be026d64c8841f
1564 ...
1565 -------------------------------------------------
1566
1567 You will see informational messages on dangling objects. They are objects
1568 that still exist in the repository but are no longer referenced by any of
1569 your branches, and can (and will) be removed after a while with `gc`.
1570 You can run `git fsck --no-dangling` to suppress these messages, and still
1571 view real errors.
1572
1573 [[recovering-lost-changes]]
1574 ==== Recovering lost changes
1575
1576 [[reflogs]]
1577 ===== Reflogs
1578
1579 Say you modify a branch with <<fixing-mistakes,`git reset --hard`>>,
1580 and then realize that the branch was the only reference you had to
1581 that point in history.
1582
1583 Fortunately, Git also keeps a log, called a "reflog", of all the
1584 previous values of each branch. So in this case you can still find the
1585 old history using, for example,
1586
1587 -------------------------------------------------
1588 $ git log master@{1}
1589 -------------------------------------------------
1590
1591 This lists the commits reachable from the previous version of the
1592 `master` branch head. This syntax can be used with any Git command
1593 that accepts a commit, not just with `git log`. Some other examples:
1594
1595 -------------------------------------------------
1596 $ git show master@{2} # See where the branch pointed 2,
1597 $ git show master@{3} # 3, ... changes ago.
1598 $ gitk master@{yesterday} # See where it pointed yesterday,
1599 $ gitk master@{"1 week ago"} # ... or last week
1600 $ git log --walk-reflogs master # show reflog entries for master
1601 -------------------------------------------------
1602
1603 A separate reflog is kept for the HEAD, so
1604
1605 -------------------------------------------------
1606 $ git show HEAD@{"1 week ago"}
1607 -------------------------------------------------
1608
1609 will show what HEAD pointed to one week ago, not what the current branch
1610 pointed to one week ago. This allows you to see the history of what
1611 you've checked out.
1612
1613 The reflogs are kept by default for 30 days, after which they may be
1614 pruned. See linkgit:git-reflog[1] and linkgit:git-gc[1] to learn
1615 how to control this pruning, and see the "SPECIFYING REVISIONS"
1616 section of linkgit:gitrevisions[7] for details.
1617
1618 Note that the reflog history is very different from normal Git history.
1619 While normal history is shared by every repository that works on the
1620 same project, the reflog history is not shared: it tells you only about
1621 how the branches in your local repository have changed over time.
1622
1623 [[dangling-object-recovery]]
1624 ===== Examining dangling objects
1625
1626 In some situations the reflog may not be able to save you. For example,
1627 suppose you delete a branch, then realize you need the history it
1628 contained. The reflog is also deleted; however, if you have not yet
1629 pruned the repository, then you may still be able to find the lost
1630 commits in the dangling objects that `git fsck` reports. See
1631 <<dangling-objects>> for the details.
1632
1633 -------------------------------------------------
1634 $ git fsck
1635 dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
1636 dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
1637 dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
1638 ...
1639 -------------------------------------------------
1640
1641 You can examine
1642 one of those dangling commits with, for example,
1643
1644 ------------------------------------------------
1645 $ gitk 7281251ddd --not --all
1646 ------------------------------------------------
1647
1648 which does what it sounds like: it says that you want to see the commit
1649 history that is described by the dangling commit(s), but not the
1650 history that is described by all your existing branches and tags. Thus
1651 you get exactly the history reachable from that commit that is lost.
1652 (And notice that it might not be just one commit: we only report the
1653 "tip of the line" as being dangling, but there might be a whole deep
1654 and complex commit history that was dropped.)
1655
1656 If you decide you want the history back, you can always create a new
1657 reference pointing to it, for example, a new branch:
1658
1659 ------------------------------------------------
1660 $ git branch recovered-branch 7281251ddd
1661 ------------------------------------------------
1662
1663 Other types of dangling objects (blobs and trees) are also possible, and
1664 dangling objects can arise in other situations.
1665
1666
1667 [[sharing-development]]
1668 == Sharing development with others
1669
1670 [[getting-updates-With-git-pull]]
1671 === Getting updates with git pull
1672
1673 After you clone a repository and commit a few changes of your own, you
1674 may wish to check the original repository for updates and merge them
1675 into your own work.
1676
1677 We have already seen <<Updating-a-repository-With-git-fetch,how to
1678 keep remote-tracking branches up to date>> with linkgit:git-fetch[1],
1679 and how to merge two branches. So you can merge in changes from the
1680 original repository's master branch with:
1681
1682 -------------------------------------------------
1683 $ git fetch
1684 $ git merge origin/master
1685 -------------------------------------------------
1686
1687 However, the linkgit:git-pull[1] command provides a way to do this in
1688 one step:
1689
1690 -------------------------------------------------
1691 $ git pull origin master
1692 -------------------------------------------------
1693
1694 In fact, if you have `master` checked out, then this branch has been
1695 configured by `git clone` to get changes from the HEAD branch of the
1696 origin repository. So often you can
1697 accomplish the above with just a simple
1698
1699 -------------------------------------------------
1700 $ git pull
1701 -------------------------------------------------
1702
1703 This command will fetch changes from the remote branches to your
1704 remote-tracking branches `origin/*`, and merge the default branch into
1705 the current branch.
1706
1707 More generally, a branch that is created from a remote-tracking branch
1708 will pull
1709 by default from that branch. See the descriptions of the
1710 `branch.<name>.remote` and `branch.<name>.merge` options in
1711 linkgit:git-config[1], and the discussion of the `--track` option in
1712 linkgit:git-checkout[1], to learn how to control these defaults.
1713
1714 In addition to saving you keystrokes, `git pull` also helps you by
1715 producing a default commit message documenting the branch and
1716 repository that you pulled from.
1717
1718 (But note that no such commit will be created in the case of a
1719 <<fast-forwards,fast-forward>>; instead, your branch will just be
1720 updated to point to the latest commit from the upstream branch.)
1721
1722 The `git pull` command can also be given `.` as the "remote" repository,
1723 in which case it just merges in a branch from the current repository; so
1724 the commands
1725
1726 -------------------------------------------------
1727 $ git pull . branch
1728 $ git merge branch
1729 -------------------------------------------------
1730
1731 are roughly equivalent.
1732
1733 [[submitting-patches]]
1734 === Submitting patches to a project
1735
1736 If you just have a few changes, the simplest way to submit them may
1737 just be to send them as patches in email:
1738
1739 First, use linkgit:git-format-patch[1]; for example:
1740
1741 -------------------------------------------------
1742 $ git format-patch origin
1743 -------------------------------------------------
1744
1745 will produce a numbered series of files in the current directory, one
1746 for each patch in the current branch but not in `origin/HEAD`.
1747
1748 `git format-patch` can include an initial "cover letter". You can insert
1749 commentary on individual patches after the three dash line which
1750 `format-patch` places after the commit message but before the patch
1751 itself. If you use `git notes` to track your cover letter material,
1752 `git format-patch --notes` will include the commit's notes in a similar
1753 manner.
1754
1755 You can then import these into your mail client and send them by
1756 hand. However, if you have a lot to send at once, you may prefer to
1757 use the linkgit:git-send-email[1] script to automate the process.
1758 Consult the mailing list for your project first to determine
1759 their requirements for submitting patches.
1760
1761 [[importing-patches]]
1762 === Importing patches to a project
1763
1764 Git also provides a tool called linkgit:git-am[1] (am stands for
1765 "apply mailbox"), for importing such an emailed series of patches.
1766 Just save all of the patch-containing messages, in order, into a
1767 single mailbox file, say `patches.mbox`, then run
1768
1769 -------------------------------------------------
1770 $ git am -3 patches.mbox
1771 -------------------------------------------------
1772
1773 Git will apply each patch in order; if any conflicts are found, it
1774 will stop, and you can fix the conflicts as described in
1775 "<<resolving-a-merge,Resolving a merge>>". (The `-3` option tells
1776 Git to perform a merge; if you would prefer it just to abort and
1777 leave your tree and index untouched, you may omit that option.)
1778
1779 Once the index is updated with the results of the conflict
1780 resolution, instead of creating a new commit, just run
1781
1782 -------------------------------------------------
1783 $ git am --continue
1784 -------------------------------------------------
1785
1786 and Git will create the commit for you and continue applying the
1787 remaining patches from the mailbox.
1788
1789 The final result will be a series of commits, one for each patch in
1790 the original mailbox, with authorship and commit log message each
1791 taken from the message containing each patch.
1792
1793 [[public-repositories]]
1794 === Public Git repositories
1795
1796 Another way to submit changes to a project is to tell the maintainer
1797 of that project to pull the changes from your repository using
1798 linkgit:git-pull[1]. In the section "<<getting-updates-With-git-pull,
1799 Getting updates with `git pull`>>" we described this as a way to get
1800 updates from the "main" repository, but it works just as well in the
1801 other direction.
1802
1803 If you and the maintainer both have accounts on the same machine, then
1804 you can just pull changes from each other's repositories directly;
1805 commands that accept repository URLs as arguments will also accept a
1806 local directory name:
1807
1808 -------------------------------------------------
1809 $ git clone /path/to/repository
1810 $ git pull /path/to/other/repository
1811 -------------------------------------------------
1812
1813 or an ssh URL:
1814
1815 -------------------------------------------------
1816 $ git clone ssh://yourhost/~you/repository
1817 -------------------------------------------------
1818
1819 For projects with few developers, or for synchronizing a few private
1820 repositories, this may be all you need.
1821
1822 However, the more common way to do this is to maintain a separate public
1823 repository (usually on a different host) for others to pull changes
1824 from. This is usually more convenient, and allows you to cleanly
1825 separate private work in progress from publicly visible work.
1826
1827 You will continue to do your day-to-day work in your personal
1828 repository, but periodically "push" changes from your personal
1829 repository into your public repository, allowing other developers to
1830 pull from that repository. So the flow of changes, in a situation
1831 where there is one other developer with a public repository, looks
1832 like this:
1833
1834 you push
1835 your personal repo ------------------> your public repo
1836 ^ |
1837 | |
1838 | you pull | they pull
1839 | |
1840 | |
1841 | they push V
1842 their public repo <------------------- their repo
1843
1844 We explain how to do this in the following sections.
1845
1846 [[setting-up-a-public-repository]]
1847 ==== Setting up a public repository
1848
1849 Assume your personal repository is in the directory `~/proj`. We
1850 first create a new clone of the repository and tell `git daemon` that it
1851 is meant to be public:
1852
1853 -------------------------------------------------
1854 $ git clone --bare ~/proj proj.git
1855 $ touch proj.git/git-daemon-export-ok
1856 -------------------------------------------------
1857
1858 The resulting directory proj.git contains a "bare" git repository--it is
1859 just the contents of the `.git` directory, without any files checked out
1860 around it.
1861
1862 Next, copy `proj.git` to the server where you plan to host the
1863 public repository. You can use scp, rsync, or whatever is most
1864 convenient.
1865
1866 [[exporting-via-git]]
1867 ==== Exporting a Git repository via the Git protocol
1868
1869 This is the preferred method.
1870
1871 If someone else administers the server, they should tell you what
1872 directory to put the repository in, and what `git://` URL it will
1873 appear at. You can then skip to the section
1874 "<<pushing-changes-to-a-public-repository,Pushing changes to a public
1875 repository>>", below.
1876
1877 Otherwise, all you need to do is start linkgit:git-daemon[1]; it will
1878 listen on port 9418. By default, it will allow access to any directory
1879 that looks like a Git directory and contains the magic file
1880 git-daemon-export-ok. Passing some directory paths as `git daemon`
1881 arguments will further restrict the exports to those paths.
1882
1883 You can also run `git daemon` as an inetd service; see the
1884 linkgit:git-daemon[1] man page for details. (See especially the
1885 examples section.)
1886
1887 [[exporting-via-http]]
1888 ==== Exporting a git repository via HTTP
1889
1890 The Git protocol gives better performance and reliability, but on a
1891 host with a web server set up, HTTP exports may be simpler to set up.
1892
1893 All you need to do is place the newly created bare Git repository in
1894 a directory that is exported by the web server, and make some
1895 adjustments to give web clients some extra information they need:
1896
1897 -------------------------------------------------
1898 $ mv proj.git /home/you/public_html/proj.git
1899 $ cd proj.git
1900 $ git --bare update-server-info
1901 $ mv hooks/post-update.sample hooks/post-update
1902 -------------------------------------------------
1903
1904 (For an explanation of the last two lines, see
1905 linkgit:git-update-server-info[1] and linkgit:githooks[5].)
1906
1907 Advertise the URL of `proj.git`. Anybody else should then be able to
1908 clone or pull from that URL, for example with a command line like:
1909
1910 -------------------------------------------------
1911 $ git clone http://yourserver.com/~you/proj.git
1912 -------------------------------------------------
1913
1914 (See also
1915 link:howto/setup-git-server-over-http.html[setup-git-server-over-http]
1916 for a slightly more sophisticated setup using WebDAV which also
1917 allows pushing over HTTP.)
1918
1919 [[pushing-changes-to-a-public-repository]]
1920 ==== Pushing changes to a public repository
1921
1922 Note that the two techniques outlined above (exporting via
1923 <<exporting-via-http,http>> or <<exporting-via-git,git>>) allow other
1924 maintainers to fetch your latest changes, but they do not allow write
1925 access, which you will need to update the public repository with the
1926 latest changes created in your private repository.
1927
1928 The simplest way to do this is using linkgit:git-push[1] and ssh; to
1929 update the remote branch named `master` with the latest state of your
1930 branch named `master`, run
1931
1932 -------------------------------------------------
1933 $ git push ssh://yourserver.com/~you/proj.git master:master
1934 -------------------------------------------------
1935
1936 or just
1937
1938 -------------------------------------------------
1939 $ git push ssh://yourserver.com/~you/proj.git master
1940 -------------------------------------------------
1941
1942 As with `git fetch`, `git push` will complain if this does not result in a
1943 <<fast-forwards,fast-forward>>; see the following section for details on
1944 handling this case.
1945
1946 Note that the target of a `push` is normally a
1947 <<def_bare_repository,bare>> repository. You can also push to a
1948 repository that has a checked-out working tree, but a push to update the
1949 currently checked-out branch is denied by default to prevent confusion.
1950 See the description of the receive.denyCurrentBranch option
1951 in linkgit:git-config[1] for details.
1952
1953 As with `git fetch`, you may also set up configuration options to
1954 save typing; so, for example:
1955
1956 -------------------------------------------------
1957 $ git remote add public-repo ssh://yourserver.com/~you/proj.git
1958 -------------------------------------------------
1959
1960 adds the following to `.git/config`:
1961
1962 -------------------------------------------------
1963 [remote "public-repo"]
1964 url = yourserver.com:proj.git
1965 fetch = +refs/heads/*:refs/remotes/example/*
1966 -------------------------------------------------
1967
1968 which lets you do the same push with just
1969
1970 -------------------------------------------------
1971 $ git push public-repo master
1972 -------------------------------------------------
1973
1974 See the explanations of the `remote.<name>.url`,
1975 `branch.<name>.remote`, and `remote.<name>.push` options in
1976 linkgit:git-config[1] for details.
1977
1978 [[forcing-push]]
1979 ==== What to do when a push fails
1980
1981 If a push would not result in a <<fast-forwards,fast-forward>> of the
1982 remote branch, then it will fail with an error like:
1983
1984 -------------------------------------------------
1985 ! [rejected] master -> master (non-fast-forward)
1986 error: failed to push some refs to '...'
1987 hint: Updates were rejected because the tip of your current branch is behind
1988 hint: its remote counterpart. Integrate the remote changes (e.g.
1989 hint: 'git pull ...') before pushing again.
1990 hint: See the 'Note about fast-forwards' in 'git push --help' for details.
1991 -------------------------------------------------
1992
1993 This can happen, for example, if you:
1994
1995 - use `git reset --hard` to remove already-published commits, or
1996 - use `git commit --amend` to replace already-published commits
1997 (as in <<fixing-a-mistake-by-rewriting-history>>), or
1998 - use `git rebase` to rebase any already-published commits (as
1999 in <<using-git-rebase>>).
2000
2001 You may force `git push` to perform the update anyway by preceding the
2002 branch name with a plus sign:
2003
2004 -------------------------------------------------
2005 $ git push ssh://yourserver.com/~you/proj.git +master
2006 -------------------------------------------------
2007
2008 Note the addition of the `+` sign. Alternatively, you can use the
2009 `-f` flag to force the remote update, as in:
2010
2011 -------------------------------------------------
2012 $ git push -f ssh://yourserver.com/~you/proj.git master
2013 -------------------------------------------------
2014
2015 Normally whenever a branch head in a public repository is modified, it
2016 is modified to point to a descendant of the commit that it pointed to
2017 before. By forcing a push in this situation, you break that convention.
2018 (See <<problems-With-rewriting-history>>.)
2019
2020 Nevertheless, this is a common practice for people that need a simple
2021 way to publish a work-in-progress patch series, and it is an acceptable
2022 compromise as long as you warn other developers that this is how you
2023 intend to manage the branch.
2024
2025 It's also possible for a push to fail in this way when other people have
2026 the right to push to the same repository. In that case, the correct
2027 solution is to retry the push after first updating your work: either by a
2028 pull, or by a fetch followed by a rebase; see the
2029 <<setting-up-a-shared-repository,next section>> and
2030 linkgit:gitcvs-migration[7] for more.
2031
2032 [[setting-up-a-shared-repository]]
2033 ==== Setting up a shared repository
2034
2035 Another way to collaborate is by using a model similar to that
2036 commonly used in CVS, where several developers with special rights
2037 all push to and pull from a single shared repository. See
2038 linkgit:gitcvs-migration[7] for instructions on how to
2039 set this up.
2040
2041 However, while there is nothing wrong with Git's support for shared
2042 repositories, this mode of operation is not generally recommended,
2043 simply because the mode of collaboration that Git supports--by
2044 exchanging patches and pulling from public repositories--has so many
2045 advantages over the central shared repository:
2046
2047 - Git's ability to quickly import and merge patches allows a
2048 single maintainer to process incoming changes even at very
2049 high rates. And when that becomes too much, `git pull` provides
2050 an easy way for that maintainer to delegate this job to other
2051 maintainers while still allowing optional review of incoming
2052 changes.
2053 - Since every developer's repository has the same complete copy
2054 of the project history, no repository is special, and it is
2055 trivial for another developer to take over maintenance of a
2056 project, either by mutual agreement, or because a maintainer
2057 becomes unresponsive or difficult to work with.
2058 - The lack of a central group of "committers" means there is
2059 less need for formal decisions about who is "in" and who is
2060 "out".
2061
2062 [[setting-up-gitweb]]
2063 ==== Allowing web browsing of a repository
2064
2065 The gitweb cgi script provides users an easy way to browse your
2066 project's revisions, file contents and logs without having to install
2067 Git. Features like RSS/Atom feeds and blame/annotation details may
2068 optionally be enabled.
2069
2070 The linkgit:git-instaweb[1] command provides a simple way to start
2071 browsing the repository using gitweb. The default server when using
2072 instaweb is lighttpd.
2073
2074 See the file gitweb/INSTALL in the Git source tree and
2075 linkgit:gitweb[1] for instructions on details setting up a permanent
2076 installation with a CGI or Perl capable server.
2077
2078 [[how-to-get-a-git-repository-with-minimal-history]]
2079 === How to get a Git repository with minimal history
2080
2081 A <<def_shallow_clone,shallow clone>>, with its truncated
2082 history, is useful when one is interested only in recent history
2083 of a project and getting full history from the upstream is
2084 expensive.
2085
2086 A <<def_shallow_clone,shallow clone>> is created by specifying
2087 the linkgit:git-clone[1] `--depth` switch. The depth can later be
2088 changed with the linkgit:git-fetch[1] `--depth` switch, or full
2089 history restored with `--unshallow`.
2090
2091 Merging inside a <<def_shallow_clone,shallow clone>> will work as long
2092 as a merge base is in the recent history.
2093 Otherwise, it will be like merging unrelated histories and may
2094 have to result in huge conflicts. This limitation may make such
2095 a repository unsuitable to be used in merge based workflows.
2096
2097 [[sharing-development-examples]]
2098 === Examples
2099
2100 [[maintaining-topic-branches]]
2101 ==== Maintaining topic branches for a Linux subsystem maintainer
2102
2103 This describes how Tony Luck uses Git in his role as maintainer of the
2104 IA64 architecture for the Linux kernel.
2105
2106 He uses two public branches:
2107
2108 - A "test" tree into which patches are initially placed so that they
2109 can get some exposure when integrated with other ongoing development.
2110 This tree is available to Andrew for pulling into -mm whenever he
2111 wants.
2112
2113 - A "release" tree into which tested patches are moved for final sanity
2114 checking, and as a vehicle to send them upstream to Linus (by sending
2115 him a "please pull" request.)
2116
2117 He also uses a set of temporary branches ("topic branches"), each
2118 containing a logical grouping of patches.
2119
2120 To set this up, first create your work tree by cloning Linus's public
2121 tree:
2122
2123 -------------------------------------------------
2124 $ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git work
2125 $ cd work
2126 -------------------------------------------------
2127
2128 Linus's tree will be stored in the remote-tracking branch named origin/master,
2129 and can be updated using linkgit:git-fetch[1]; you can track other
2130 public trees using linkgit:git-remote[1] to set up a "remote" and
2131 linkgit:git-fetch[1] to keep them up to date; see
2132 <<repositories-and-branches>>.
2133
2134 Now create the branches in which you are going to work; these start out
2135 at the current tip of origin/master branch, and should be set up (using
2136 the `--track` option to linkgit:git-branch[1]) to merge changes in from
2137 Linus by default.
2138
2139 -------------------------------------------------
2140 $ git branch --track test origin/master
2141 $ git branch --track release origin/master
2142 -------------------------------------------------
2143
2144 These can be easily kept up to date using linkgit:git-pull[1].
2145
2146 -------------------------------------------------
2147 $ git switch test && git pull
2148 $ git switch release && git pull
2149 -------------------------------------------------
2150
2151 Important note! If you have any local changes in these branches, then
2152 this merge will create a commit object in the history (with no local
2153 changes Git will simply do a "fast-forward" merge). Many people dislike
2154 the "noise" that this creates in the Linux history, so you should avoid
2155 doing this capriciously in the `release` branch, as these noisy commits
2156 will become part of the permanent history when you ask Linus to pull
2157 from the release branch.
2158
2159 A few configuration variables (see linkgit:git-config[1]) can
2160 make it easy to push both branches to your public tree. (See
2161 <<setting-up-a-public-repository>>.)
2162
2163 -------------------------------------------------
2164 $ cat >> .git/config <<EOF
2165 [remote "mytree"]
2166 url = master.kernel.org:/pub/scm/linux/kernel/git/aegl/linux.git
2167 push = release
2168 push = test
2169 EOF
2170 -------------------------------------------------
2171
2172 Then you can push both the test and release trees using
2173 linkgit:git-push[1]:
2174
2175 -------------------------------------------------
2176 $ git push mytree
2177 -------------------------------------------------
2178
2179 or push just one of the test and release branches using:
2180
2181 -------------------------------------------------
2182 $ git push mytree test
2183 -------------------------------------------------
2184
2185 or
2186
2187 -------------------------------------------------
2188 $ git push mytree release
2189 -------------------------------------------------
2190
2191 Now to apply some patches from the community. Think of a short
2192 snappy name for a branch to hold this patch (or related group of
2193 patches), and create a new branch from a recent stable tag of
2194 Linus's branch. Picking a stable base for your branch will:
2195 1) help you: by avoiding inclusion of unrelated and perhaps lightly
2196 tested changes
2197 2) help future bug hunters that use `git bisect` to find problems
2198
2199 -------------------------------------------------
2200 $ git switch -c speed-up-spinlocks v2.6.35
2201 -------------------------------------------------
2202
2203 Now you apply the patch(es), run some tests, and commit the change(s). If
2204 the patch is a multi-part series, then you should apply each as a separate
2205 commit to this branch.
2206
2207 -------------------------------------------------
2208 $ ... patch ... test ... commit [ ... patch ... test ... commit ]*
2209 -------------------------------------------------
2210
2211 When you are happy with the state of this change, you can merge it into the
2212 "test" branch in preparation to make it public:
2213
2214 -------------------------------------------------
2215 $ git switch test && git merge speed-up-spinlocks
2216 -------------------------------------------------
2217
2218 It is unlikely that you would have any conflicts here ... but you might if you
2219 spent a while on this step and had also pulled new versions from upstream.
2220
2221 Sometime later when enough time has passed and testing done, you can pull the
2222 same branch into the `release` tree ready to go upstream. This is where you
2223 see the value of keeping each patch (or patch series) in its own branch. It
2224 means that the patches can be moved into the `release` tree in any order.
2225
2226 -------------------------------------------------
2227 $ git switch release && git merge speed-up-spinlocks
2228 -------------------------------------------------
2229
2230 After a while, you will have a number of branches, and despite the
2231 well chosen names you picked for each of them, you may forget what
2232 they are for, or what status they are in. To get a reminder of what
2233 changes are in a specific branch, use:
2234
2235 -------------------------------------------------
2236 $ git log linux..branchname | git shortlog
2237 -------------------------------------------------
2238
2239 To see whether it has already been merged into the test or release branches,
2240 use:
2241
2242 -------------------------------------------------
2243 $ git log test..branchname
2244 -------------------------------------------------
2245
2246 or
2247
2248 -------------------------------------------------
2249 $ git log release..branchname
2250 -------------------------------------------------
2251
2252 (If this branch has not yet been merged, you will see some log entries.
2253 If it has been merged, then there will be no output.)
2254
2255 Once a patch completes the great cycle (moving from test to release,
2256 then pulled by Linus, and finally coming back into your local
2257 `origin/master` branch), the branch for this change is no longer needed.
2258 You detect this when the output from:
2259
2260 -------------------------------------------------
2261 $ git log origin..branchname
2262 -------------------------------------------------
2263
2264 is empty. At this point the branch can be deleted:
2265
2266 -------------------------------------------------
2267 $ git branch -d branchname
2268 -------------------------------------------------
2269
2270 Some changes are so trivial that it is not necessary to create a separate
2271 branch and then merge into each of the test and release branches. For
2272 these changes, just apply directly to the `release` branch, and then
2273 merge that into the `test` branch.
2274
2275 After pushing your work to `mytree`, you can use
2276 linkgit:git-request-pull[1] to prepare a "please pull" request message
2277 to send to Linus:
2278
2279 -------------------------------------------------
2280 $ git push mytree
2281 $ git request-pull origin mytree release
2282 -------------------------------------------------
2283
2284 Here are some of the scripts that simplify all this even further.
2285
2286 -------------------------------------------------
2287 ==== update script ====
2288 # Update a branch in my Git tree. If the branch to be updated
2289 # is origin, then pull from kernel.org. Otherwise merge
2290 # origin/master branch into test|release branch
2291
2292 case "$1" in
2293 test|release)
2294 git checkout $1 && git pull . origin
2295 ;;
2296 origin)
2297 before=$(git rev-parse refs/remotes/origin/master)
2298 git fetch origin
2299 after=$(git rev-parse refs/remotes/origin/master)
2300 if [ $before != $after ]
2301 then
2302 git log $before..$after | git shortlog
2303 fi
2304 ;;
2305 *)
2306 echo "usage: $0 origin|test|release" 1>&2
2307 exit 1
2308 ;;
2309 esac
2310 -------------------------------------------------
2311
2312 -------------------------------------------------
2313 ==== merge script ====
2314 # Merge a branch into either the test or release branch
2315
2316 pname=$0
2317
2318 usage()
2319 {
2320 echo "usage: $pname branch test|release" 1>&2
2321 exit 1
2322 }
2323
2324 git show-ref -q --verify -- refs/heads/"$1" || {
2325 echo "Can't see branch <$1>" 1>&2
2326 usage
2327 }
2328
2329 case "$2" in
2330 test|release)
2331 if [ $(git log $2..$1 | wc -c) -eq 0 ]
2332 then
2333 echo $1 already merged into $2 1>&2
2334 exit 1
2335 fi
2336 git checkout $2 && git pull . $1
2337 ;;
2338 *)
2339 usage
2340 ;;
2341 esac
2342 -------------------------------------------------
2343
2344 -------------------------------------------------
2345 ==== status script ====
2346 # report on status of my ia64 Git tree
2347
2348 gb=$(tput setab 2)
2349 rb=$(tput setab 1)
2350 restore=$(tput setab 9)
2351
2352 if [ `git rev-list test..release | wc -c` -gt 0 ]
2353 then
2354 echo $rb Warning: commits in release that are not in test $restore
2355 git log test..release
2356 fi
2357
2358 for branch in `git show-ref --heads | sed 's|^.*/||'`
2359 do
2360 if [ $branch = test -o $branch = release ]
2361 then
2362 continue
2363 fi
2364
2365 echo -n $gb ======= $branch ====== $restore " "
2366 status=
2367 for ref in test release origin/master
2368 do
2369 if [ `git rev-list $ref..$branch | wc -c` -gt 0 ]
2370 then
2371 status=$status${ref:0:1}
2372 fi
2373 done
2374 case $status in
2375 trl)
2376 echo $rb Need to pull into test $restore
2377 ;;
2378 rl)
2379 echo "In test"
2380 ;;
2381 l)
2382 echo "Waiting for linus"
2383 ;;
2384 "")
2385 echo $rb All done $restore
2386 ;;
2387 *)
2388 echo $rb "<$status>" $restore
2389 ;;
2390 esac
2391 git log origin/master..$branch | git shortlog
2392 done
2393 -------------------------------------------------
2394
2395
2396 [[cleaning-up-history]]
2397 == Rewriting history and maintaining patch series
2398
2399 Normally commits are only added to a project, never taken away or
2400 replaced. Git is designed with this assumption, and violating it will
2401 cause Git's merge machinery (for example) to do the wrong thing.
2402
2403 However, there is a situation in which it can be useful to violate this
2404 assumption.
2405
2406 [[patch-series]]
2407 === Creating the perfect patch series
2408
2409 Suppose you are a contributor to a large project, and you want to add a
2410 complicated feature, and to present it to the other developers in a way
2411 that makes it easy for them to read your changes, verify that they are
2412 correct, and understand why you made each change.
2413
2414 If you present all of your changes as a single patch (or commit), they
2415 may find that it is too much to digest all at once.
2416
2417 If you present them with the entire history of your work, complete with
2418 mistakes, corrections, and dead ends, they may be overwhelmed.
2419
2420 So the ideal is usually to produce a series of patches such that:
2421
2422 1. Each patch can be applied in order.
2423
2424 2. Each patch includes a single logical change, together with a
2425 message explaining the change.
2426
2427 3. No patch introduces a regression: after applying any initial
2428 part of the series, the resulting project still compiles and
2429 works, and has no bugs that it didn't have before.
2430
2431 4. The complete series produces the same end result as your own
2432 (probably much messier!) development process did.
2433
2434 We will introduce some tools that can help you do this, explain how to
2435 use them, and then explain some of the problems that can arise because
2436 you are rewriting history.
2437
2438 [[using-git-rebase]]
2439 === Keeping a patch series up to date using git rebase
2440
2441 Suppose that you create a branch `mywork` on a remote-tracking branch
2442 `origin`, and create some commits on top of it:
2443
2444 -------------------------------------------------
2445 $ git switch -c mywork origin
2446 $ vi file.txt
2447 $ git commit
2448 $ vi otherfile.txt
2449 $ git commit
2450 ...
2451 -------------------------------------------------
2452
2453 You have performed no merges into mywork, so it is just a simple linear
2454 sequence of patches on top of `origin`:
2455
2456 ................................................
2457 o--o--O <-- origin
2458 \
2459 a--b--c <-- mywork
2460 ................................................
2461
2462 Some more interesting work has been done in the upstream project, and
2463 `origin` has advanced:
2464
2465 ................................................
2466 o--o--O--o--o--o <-- origin
2467 \
2468 a--b--c <-- mywork
2469 ................................................
2470
2471 At this point, you could use `pull` to merge your changes back in;
2472 the result would create a new merge commit, like this:
2473
2474 ................................................
2475 o--o--O--o--o--o <-- origin
2476 \ \
2477 a--b--c--m <-- mywork
2478 ................................................
2479
2480 However, if you prefer to keep the history in mywork a simple series of
2481 commits without any merges, you may instead choose to use
2482 linkgit:git-rebase[1]:
2483
2484 -------------------------------------------------
2485 $ git switch mywork
2486 $ git rebase origin
2487 -------------------------------------------------
2488
2489 This will remove each of your commits from mywork, temporarily saving
2490 them as patches (in a directory named `.git/rebase-apply`), update mywork to
2491 point at the latest version of origin, then apply each of the saved
2492 patches to the new mywork. The result will look like:
2493
2494
2495 ................................................
2496 o--o--O--o--o--o <-- origin
2497 \
2498 a'--b'--c' <-- mywork
2499 ................................................
2500
2501 In the process, it may discover conflicts. In that case it will stop
2502 and allow you to fix the conflicts; after fixing conflicts, use `git add`
2503 to update the index with those contents, and then, instead of
2504 running `git commit`, just run
2505
2506 -------------------------------------------------
2507 $ git rebase --continue
2508 -------------------------------------------------
2509
2510 and Git will continue applying the rest of the patches.
2511
2512 At any point you may use the `--abort` option to abort this process and
2513 return mywork to the state it had before you started the rebase:
2514
2515 -------------------------------------------------
2516 $ git rebase --abort
2517 -------------------------------------------------
2518
2519 If you need to reorder or edit a number of commits in a branch, it may
2520 be easier to use `git rebase -i`, which allows you to reorder and
2521 squash commits, as well as marking them for individual editing during
2522 the rebase. See <<interactive-rebase>> for details, and
2523 <<reordering-patch-series>> for alternatives.
2524
2525 [[rewriting-one-commit]]
2526 === Rewriting a single commit
2527
2528 We saw in <<fixing-a-mistake-by-rewriting-history>> that you can replace the
2529 most recent commit using
2530
2531 -------------------------------------------------
2532 $ git commit --amend
2533 -------------------------------------------------
2534
2535 which will replace the old commit by a new commit incorporating your
2536 changes, giving you a chance to edit the old commit message first.
2537 This is useful for fixing typos in your last commit, or for adjusting
2538 the patch contents of a poorly staged commit.
2539
2540 If you need to amend commits from deeper in your history, you can
2541 use <<interactive-rebase,interactive rebase's `edit` instruction>>.
2542
2543 [[reordering-patch-series]]
2544 === Reordering or selecting from a patch series
2545
2546 Sometimes you want to edit a commit deeper in your history. One
2547 approach is to use `git format-patch` to create a series of patches
2548 and then reset the state to before the patches:
2549
2550 -------------------------------------------------
2551 $ git format-patch origin
2552 $ git reset --hard origin
2553 -------------------------------------------------
2554
2555 Then modify, reorder, or eliminate patches as needed before applying
2556 them again with linkgit:git-am[1]:
2557
2558 -------------------------------------------------
2559 $ git am *.patch
2560 -------------------------------------------------
2561
2562 [[interactive-rebase]]
2563 === Using interactive rebases
2564
2565 You can also edit a patch series with an interactive rebase. This is
2566 the same as <<reordering-patch-series,reordering a patch series using
2567 `format-patch`>>, so use whichever interface you like best.
2568
2569 Rebase your current HEAD on the last commit you want to retain as-is.
2570 For example, if you want to reorder the last 5 commits, use:
2571
2572 -------------------------------------------------
2573 $ git rebase -i HEAD~5
2574 -------------------------------------------------
2575
2576 This will open your editor with a list of steps to be taken to perform
2577 your rebase.
2578
2579 -------------------------------------------------
2580 pick deadbee The oneline of this commit
2581 pick fa1afe1 The oneline of the next commit
2582 ...
2583
2584 # Rebase c0ffeee..deadbee onto c0ffeee
2585 #
2586 # Commands:
2587 # p, pick = use commit
2588 # r, reword = use commit, but edit the commit message
2589 # e, edit = use commit, but stop for amending
2590 # s, squash = use commit, but meld into previous commit
2591 # f, fixup = like "squash", but discard this commit's log message
2592 # x, exec = run command (the rest of the line) using shell
2593 #
2594 # These lines can be re-ordered; they are executed from top to bottom.
2595 #
2596 # If you remove a line here THAT COMMIT WILL BE LOST.
2597 #
2598 # However, if you remove everything, the rebase will be aborted.
2599 #
2600 # Note that empty commits are commented out
2601 -------------------------------------------------
2602
2603 As explained in the comments, you can reorder commits, squash them
2604 together, edit commit messages, etc. by editing the list. Once you
2605 are satisfied, save the list and close your editor, and the rebase
2606 will begin.
2607
2608 The rebase will stop where `pick` has been replaced with `edit` or
2609 when a step in the list fails to mechanically resolve conflicts and
2610 needs your help. When you are done editing and/or resolving conflicts
2611 you can continue with `git rebase --continue`. If you decide that
2612 things are getting too hairy, you can always bail out with `git rebase
2613 --abort`. Even after the rebase is complete, you can still recover
2614 the original branch by using the <<reflogs,reflog>>.
2615
2616 For a more detailed discussion of the procedure and additional tips,
2617 see the "INTERACTIVE MODE" section of linkgit:git-rebase[1].
2618
2619 [[patch-series-tools]]
2620 === Other tools
2621
2622 There are numerous other tools, such as StGit, which exist for the
2623 purpose of maintaining a patch series. These are outside of the scope of
2624 this manual.
2625
2626 [[problems-With-rewriting-history]]
2627 === Problems with rewriting history
2628
2629 The primary problem with rewriting the history of a branch has to do
2630 with merging. Suppose somebody fetches your branch and merges it into
2631 their branch, with a result something like this:
2632
2633 ................................................
2634 o--o--O--o--o--o <-- origin
2635 \ \
2636 t--t--t--m <-- their branch:
2637 ................................................
2638
2639 Then suppose you modify the last three commits:
2640
2641 ................................................
2642 o--o--o <-- new head of origin
2643 /
2644 o--o--O--o--o--o <-- old head of origin
2645 ................................................
2646
2647 If we examined all this history together in one repository, it will
2648 look like:
2649
2650 ................................................
2651 o--o--o <-- new head of origin
2652 /
2653 o--o--O--o--o--o <-- old head of origin
2654 \ \
2655 t--t--t--m <-- their branch:
2656 ................................................
2657
2658 Git has no way of knowing that the new head is an updated version of
2659 the old head; it treats this situation exactly the same as it would if
2660 two developers had independently done the work on the old and new heads
2661 in parallel. At this point, if someone attempts to merge the new head
2662 in to their branch, Git will attempt to merge together the two (old and
2663 new) lines of development, instead of trying to replace the old by the
2664 new. The results are likely to be unexpected.
2665
2666 You may still choose to publish branches whose history is rewritten,
2667 and it may be useful for others to be able to fetch those branches in
2668 order to examine or test them, but they should not attempt to pull such
2669 branches into their own work.
2670
2671 For true distributed development that supports proper merging,
2672 published branches should never be rewritten.
2673
2674 [[bisect-merges]]
2675 === Why bisecting merge commits can be harder than bisecting linear history
2676
2677 The linkgit:git-bisect[1] command correctly handles history that
2678 includes merge commits. However, when the commit that it finds is a
2679 merge commit, the user may need to work harder than usual to figure out
2680 why that commit introduced a problem.
2681
2682 Imagine this history:
2683
2684 ................................................
2685 ---Z---o---X---...---o---A---C---D
2686 \ /
2687 o---o---Y---...---o---B
2688 ................................................
2689
2690 Suppose that on the upper line of development, the meaning of one
2691 of the functions that exists at Z is changed at commit X. The
2692 commits from Z leading to A change both the function's
2693 implementation and all calling sites that exist at Z, as well
2694 as new calling sites they add, to be consistent. There is no
2695 bug at A.
2696
2697 Suppose that in the meantime on the lower line of development somebody
2698 adds a new calling site for that function at commit Y. The
2699 commits from Z leading to B all assume the old semantics of that
2700 function and the callers and the callee are consistent with each
2701 other. There is no bug at B, either.
2702
2703 Suppose further that the two development lines merge cleanly at C,
2704 so no conflict resolution is required.
2705
2706 Nevertheless, the code at C is broken, because the callers added
2707 on the lower line of development have not been converted to the new
2708 semantics introduced on the upper line of development. So if all
2709 you know is that D is bad, that Z is good, and that
2710 linkgit:git-bisect[1] identifies C as the culprit, how will you
2711 figure out that the problem is due to this change in semantics?
2712
2713 When the result of a `git bisect` is a non-merge commit, you should
2714 normally be able to discover the problem by examining just that commit.
2715 Developers can make this easy by breaking their changes into small
2716 self-contained commits. That won't help in the case above, however,
2717 because the problem isn't obvious from examination of any single
2718 commit; instead, a global view of the development is required. To
2719 make matters worse, the change in semantics in the problematic
2720 function may be just one small part of the changes in the upper
2721 line of development.
2722
2723 On the other hand, if instead of merging at C you had rebased the
2724 history between Z to B on top of A, you would have gotten this
2725 linear history:
2726
2727 ................................................................
2728 ---Z---o---X--...---o---A---o---o---Y*--...---o---B*--D*
2729 ................................................................
2730
2731 Bisecting between Z and D* would hit a single culprit commit Y*,
2732 and understanding why Y* was broken would probably be easier.
2733
2734 Partly for this reason, many experienced Git users, even when
2735 working on an otherwise merge-heavy project, keep the history
2736 linear by rebasing against the latest upstream version before
2737 publishing.
2738
2739 [[advanced-branch-management]]
2740 == Advanced branch management
2741
2742 [[fetching-individual-branches]]
2743 === Fetching individual branches
2744
2745 Instead of using linkgit:git-remote[1], you can also choose just
2746 to update one branch at a time, and to store it locally under an
2747 arbitrary name:
2748
2749 -------------------------------------------------
2750 $ git fetch origin todo:my-todo-work
2751 -------------------------------------------------
2752
2753 The first argument, `origin`, just tells Git to fetch from the
2754 repository you originally cloned from. The second argument tells Git
2755 to fetch the branch named `todo` from the remote repository, and to
2756 store it locally under the name `refs/heads/my-todo-work`.
2757
2758 You can also fetch branches from other repositories; so
2759
2760 -------------------------------------------------
2761 $ git fetch git://example.com/proj.git master:example-master
2762 -------------------------------------------------
2763
2764 will create a new branch named `example-master` and store in it the
2765 branch named `master` from the repository at the given URL. If you
2766 already have a branch named example-master, it will attempt to
2767 <<fast-forwards,fast-forward>> to the commit given by example.com's
2768 master branch. In more detail:
2769
2770 [[fetch-fast-forwards]]
2771 === git fetch and fast-forwards
2772
2773 In the previous example, when updating an existing branch, `git fetch`
2774 checks to make sure that the most recent commit on the remote
2775 branch is a descendant of the most recent commit on your copy of the
2776 branch before updating your copy of the branch to point at the new
2777 commit. Git calls this process a <<fast-forwards,fast-forward>>.
2778
2779 A fast-forward looks something like this:
2780
2781 ................................................
2782 o--o--o--o <-- old head of the branch
2783 \
2784 o--o--o <-- new head of the branch
2785 ................................................
2786
2787
2788 In some cases it is possible that the new head will *not* actually be
2789 a descendant of the old head. For example, the developer may have
2790 realized she made a serious mistake, and decided to backtrack,
2791 resulting in a situation like:
2792
2793 ................................................
2794 o--o--o--o--a--b <-- old head of the branch
2795 \
2796 o--o--o <-- new head of the branch
2797 ................................................
2798
2799 In this case, `git fetch` will fail, and print out a warning.
2800
2801 In that case, you can still force Git to update to the new head, as
2802 described in the following section. However, note that in the
2803 situation above this may mean losing the commits labeled `a` and `b`,
2804 unless you've already created a reference of your own pointing to
2805 them.
2806
2807 [[forcing-fetch]]
2808 === Forcing git fetch to do non-fast-forward updates
2809
2810 If git fetch fails because the new head of a branch is not a
2811 descendant of the old head, you may force the update with:
2812
2813 -------------------------------------------------
2814 $ git fetch git://example.com/proj.git +master:refs/remotes/example/master
2815 -------------------------------------------------
2816
2817 Note the addition of the `+` sign. Alternatively, you can use the `-f`
2818 flag to force updates of all the fetched branches, as in:
2819
2820 -------------------------------------------------
2821 $ git fetch -f origin
2822 -------------------------------------------------
2823
2824 Be aware that commits that the old version of example/master pointed at
2825 may be lost, as we saw in the previous section.
2826
2827 [[remote-branch-configuration]]
2828 === Configuring remote-tracking branches
2829
2830 We saw above that `origin` is just a shortcut to refer to the
2831 repository that you originally cloned from. This information is
2832 stored in Git configuration variables, which you can see using
2833 linkgit:git-config[1]:
2834
2835 -------------------------------------------------
2836 $ git config -l
2837 core.repositoryformatversion=0
2838 core.filemode=true
2839 core.logallrefupdates=true
2840 remote.origin.url=git://git.kernel.org/pub/scm/git/git.git
2841 remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
2842 branch.master.remote=origin
2843 branch.master.merge=refs/heads/master
2844 -------------------------------------------------
2845
2846 If there are other repositories that you also use frequently, you can
2847 create similar configuration options to save typing; for example,
2848
2849 -------------------------------------------------
2850 $ git remote add example git://example.com/proj.git
2851 -------------------------------------------------
2852
2853 adds the following to `.git/config`:
2854
2855 -------------------------------------------------
2856 [remote "example"]
2857 url = git://example.com/proj.git
2858 fetch = +refs/heads/*:refs/remotes/example/*
2859 -------------------------------------------------
2860
2861 Also note that the above configuration can be performed by directly
2862 editing the file `.git/config` instead of using linkgit:git-remote[1].
2863
2864 After configuring the remote, the following three commands will do the
2865 same thing:
2866
2867 -------------------------------------------------
2868 $ git fetch git://example.com/proj.git +refs/heads/*:refs/remotes/example/*
2869 $ git fetch example +refs/heads/*:refs/remotes/example/*
2870 $ git fetch example
2871 -------------------------------------------------
2872
2873 See linkgit:git-config[1] for more details on the configuration
2874 options mentioned above and linkgit:git-fetch[1] for more details on
2875 the refspec syntax.
2876
2877
2878 [[git-concepts]]
2879 == Git concepts
2880
2881 Git is built on a small number of simple but powerful ideas. While it
2882 is possible to get things done without understanding them, you will find
2883 Git much more intuitive if you do.
2884
2885 We start with the most important, the <<def_object_database,object
2886 database>> and the <<def_index,index>>.
2887
2888 [[the-object-database]]
2889 === The Object Database
2890
2891
2892 We already saw in <<understanding-commits>> that all commits are stored
2893 under a 40-digit "object name". In fact, all the information needed to
2894 represent the history of a project is stored in objects with such names.
2895 In each case the name is calculated by taking the SHA-1 hash of the
2896 contents of the object. The SHA-1 hash is a cryptographic hash function.
2897 What that means to us is that it is impossible to find two different
2898 objects with the same name. This has a number of advantages; among
2899 others:
2900
2901 - Git can quickly determine whether two objects are identical or not,
2902 just by comparing names.
2903 - Since object names are computed the same way in every repository, the
2904 same content stored in two repositories will always be stored under
2905 the same name.
2906 - Git can detect errors when it reads an object, by checking that the
2907 object's name is still the SHA-1 hash of its contents.
2908
2909 (See <<object-details>> for the details of the object formatting and
2910 SHA-1 calculation.)
2911
2912 There are four different types of objects: "blob", "tree", "commit", and
2913 "tag".
2914
2915 - A <<def_blob_object,"blob" object>> is used to store file data.
2916 - A <<def_tree_object,"tree" object>> ties one or more
2917 "blob" objects into a directory structure. In addition, a tree object
2918 can refer to other tree objects, thus creating a directory hierarchy.
2919 - A <<def_commit_object,"commit" object>> ties such directory hierarchies
2920 together into a <<def_DAG,directed acyclic graph>> of revisions--each
2921 commit contains the object name of exactly one tree designating the
2922 directory hierarchy at the time of the commit. In addition, a commit
2923 refers to "parent" commit objects that describe the history of how we
2924 arrived at that directory hierarchy.
2925 - A <<def_tag_object,"tag" object>> symbolically identifies and can be
2926 used to sign other objects. It contains the object name and type of
2927 another object, a symbolic name (of course!) and, optionally, a
2928 signature.
2929
2930 The object types in some more detail:
2931
2932 [[commit-object]]
2933 ==== Commit Object
2934
2935 The "commit" object links a physical state of a tree with a description
2936 of how we got there and why. Use the `--pretty=raw` option to
2937 linkgit:git-show[1] or linkgit:git-log[1] to examine your favorite
2938 commit:
2939
2940 ------------------------------------------------
2941 $ git show -s --pretty=raw 2be7fcb476
2942 commit 2be7fcb4764f2dbcee52635b91fedb1b3dcf7ab4
2943 tree fb3a8bdd0ceddd019615af4d57a53f43d8cee2bf
2944 parent 257a84d9d02e90447b149af58b271c19405edb6a
2945 author Dave Watson <dwatson@mimvista.com> 1187576872 -0400
2946 committer Junio C Hamano <gitster@pobox.com> 1187591163 -0700
2947
2948 Fix misspelling of 'suppress' in docs
2949
2950 Signed-off-by: Junio C Hamano <gitster@pobox.com>
2951 ------------------------------------------------
2952
2953 As you can see, a commit is defined by:
2954
2955 - a tree: The SHA-1 name of a tree object (as defined below), representing
2956 the contents of a directory at a certain point in time.
2957 - parent(s): The SHA-1 name(s) of some number of commits which represent the
2958 immediately previous step(s) in the history of the project. The
2959 example above has one parent; merge commits may have more than
2960 one. A commit with no parents is called a "root" commit, and
2961 represents the initial revision of a project. Each project must have
2962 at least one root. A project can also have multiple roots, though
2963 that isn't common (or necessarily a good idea).
2964 - an author: The name of the person responsible for this change, together
2965 with its date.
2966 - a committer: The name of the person who actually created the commit,
2967 with the date it was done. This may be different from the author, for
2968 example, if the author was someone who wrote a patch and emailed it
2969 to the person who used it to create the commit.
2970 - a comment describing this commit.
2971
2972 Note that a commit does not itself contain any information about what
2973 actually changed; all changes are calculated by comparing the contents
2974 of the tree referred to by this commit with the trees associated with
2975 its parents. In particular, Git does not attempt to record file renames
2976 explicitly, though it can identify cases where the existence of the same
2977 file data at changing paths suggests a rename. (See, for example, the
2978 `-M` option to linkgit:git-diff[1]).
2979
2980 A commit is usually created by linkgit:git-commit[1], which creates a
2981 commit whose parent is normally the current HEAD, and whose tree is
2982 taken from the content currently stored in the index.
2983
2984 [[tree-object]]
2985 ==== Tree Object
2986
2987 The ever-versatile linkgit:git-show[1] command can also be used to
2988 examine tree objects, but linkgit:git-ls-tree[1] will give you more
2989 details:
2990
2991 ------------------------------------------------
2992 $ git ls-tree fb3a8bdd0ce
2993 100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c .gitignore
2994 100644 blob 5529b198e8d14decbe4ad99db3f7fb632de0439d .mailmap
2995 100644 blob 6ff87c4664981e4397625791c8ea3bbb5f2279a3 COPYING
2996 040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745 Documentation
2997 100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200 GIT-VERSION-GEN
2998 100644 blob 289b046a443c0647624607d471289b2c7dcd470b INSTALL
2999 100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1 Makefile
3000 100644 blob 548142c327a6790ff8821d67c2ee1eff7a656b52 README
3001 ...
3002 ------------------------------------------------
3003
3004 As you can see, a tree object contains a list of entries, each with a
3005 mode, object type, SHA-1 name, and name, sorted by name. It represents
3006 the contents of a single directory tree.
3007
3008 The object type may be a blob, representing the contents of a file, or
3009 another tree, representing the contents of a subdirectory. Since trees
3010 and blobs, like all other objects, are named by the SHA-1 hash of their
3011 contents, two trees have the same SHA-1 name if and only if their
3012 contents (including, recursively, the contents of all subdirectories)
3013 are identical. This allows Git to quickly determine the differences
3014 between two related tree objects, since it can ignore any entries with
3015 identical object names.
3016
3017 (Note: in the presence of submodules, trees may also have commits as
3018 entries. See <<submodules>> for documentation.)
3019
3020 Note that the files all have mode 644 or 755: Git actually only pays
3021 attention to the executable bit.
3022
3023 [[blob-object]]
3024 ==== Blob Object
3025
3026 You can use linkgit:git-show[1] to examine the contents of a blob; take,
3027 for example, the blob in the entry for `COPYING` from the tree above:
3028
3029 ------------------------------------------------
3030 $ git show 6ff87c4664
3031
3032 Note that the only valid version of the GPL as far as this project
3033 is concerned is _this_ particular version of the license (ie v2, not
3034 v2.2 or v3.x or whatever), unless explicitly otherwise stated.
3035 ...
3036 ------------------------------------------------
3037
3038 A "blob" object is nothing but a binary blob of data. It doesn't refer
3039 to anything else or have attributes of any kind.
3040
3041 Since the blob is entirely defined by its data, if two files in a
3042 directory tree (or in multiple different versions of the repository)
3043 have the same contents, they will share the same blob object. The object
3044 is totally independent of its location in the directory tree, and
3045 renaming a file does not change the object that file is associated with.
3046
3047 Note that any tree or blob object can be examined using
3048 linkgit:git-show[1] with the <revision>:<path> syntax. This can
3049 sometimes be useful for browsing the contents of a tree that is not
3050 currently checked out.
3051
3052 [[trust]]
3053 ==== Trust
3054
3055 If you receive the SHA-1 name of a blob from one source, and its contents
3056 from another (possibly untrusted) source, you can still trust that those
3057 contents are correct as long as the SHA-1 name agrees. This is because
3058 the SHA-1 is designed so that it is infeasible to find different contents
3059 that produce the same hash.
3060
3061 Similarly, you need only trust the SHA-1 name of a top-level tree object
3062 to trust the contents of the entire directory that it refers to, and if
3063 you receive the SHA-1 name of a commit from a trusted source, then you
3064 can easily verify the entire history of commits reachable through
3065 parents of that commit, and all of those contents of the trees referred
3066 to by those commits.
3067
3068 So to introduce some real trust in the system, the only thing you need
3069 to do is to digitally sign just 'one' special note, which includes the
3070 name of a top-level commit. Your digital signature shows others
3071 that you trust that commit, and the immutability of the history of
3072 commits tells others that they can trust the whole history.
3073
3074 In other words, you can easily validate a whole archive by just
3075 sending out a single email that tells the people the name (SHA-1 hash)
3076 of the top commit, and digitally sign that email using something
3077 like GPG/PGP.
3078
3079 To assist in this, Git also provides the tag object...
3080
3081 [[tag-object]]
3082 ==== Tag Object
3083
3084 A tag object contains an object, object type, tag name, the name of the
3085 person ("tagger") who created the tag, and a message, which may contain
3086 a signature, as can be seen using linkgit:git-cat-file[1]:
3087
3088 ------------------------------------------------
3089 $ git cat-file tag v1.5.0
3090 object 437b1b20df4b356c9342dac8d38849f24ef44f27
3091 type commit
3092 tag v1.5.0
3093 tagger Junio C Hamano <junkio@cox.net> 1171411200 +0000
3094
3095 GIT 1.5.0
3096 -----BEGIN PGP SIGNATURE-----
3097 Version: GnuPG v1.4.6 (GNU/Linux)
3098
3099 iD8DBQBF0lGqwMbZpPMRm5oRAuRiAJ9ohBLd7s2kqjkKlq1qqC57SbnmzQCdG4ui
3100 nLE/L9aUXdWeTFPron96DLA=
3101 =2E+0
3102 -----END PGP SIGNATURE-----
3103 ------------------------------------------------
3104
3105 See the linkgit:git-tag[1] command to learn how to create and verify tag
3106 objects. (Note that linkgit:git-tag[1] can also be used to create
3107 "lightweight tags", which are not tag objects at all, but just simple
3108 references whose names begin with `refs/tags/`).
3109
3110 [[pack-files]]
3111 ==== How Git stores objects efficiently: pack files
3112
3113 Newly created objects are initially created in a file named after the
3114 object's SHA-1 hash (stored in `.git/objects`).
3115
3116 Unfortunately this system becomes inefficient once a project has a
3117 lot of objects. Try this on an old project:
3118
3119 ------------------------------------------------
3120 $ git count-objects
3121 6930 objects, 47620 kilobytes
3122 ------------------------------------------------
3123
3124 The first number is the number of objects which are kept in
3125 individual files. The second is the amount of space taken up by
3126 those "loose" objects.
3127
3128 You can save space and make Git faster by moving these loose objects in
3129 to a "pack file", which stores a group of objects in an efficient
3130 compressed format; the details of how pack files are formatted can be
3131 found in link:technical/pack-format.html[pack format].
3132
3133 To put the loose objects into a pack, just run git repack:
3134
3135 ------------------------------------------------
3136 $ git repack
3137 Counting objects: 6020, done.
3138 Delta compression using up to 4 threads.
3139 Compressing objects: 100% (6020/6020), done.
3140 Writing objects: 100% (6020/6020), done.
3141 Total 6020 (delta 4070), reused 0 (delta 0)
3142 ------------------------------------------------
3143
3144 This creates a single "pack file" in .git/objects/pack/
3145 containing all currently unpacked objects. You can then run
3146
3147 ------------------------------------------------
3148 $ git prune
3149 ------------------------------------------------
3150
3151 to remove any of the "loose" objects that are now contained in the
3152 pack. This will also remove any unreferenced objects (which may be
3153 created when, for example, you use `git reset` to remove a commit).
3154 You can verify that the loose objects are gone by looking at the
3155 `.git/objects` directory or by running
3156
3157 ------------------------------------------------
3158 $ git count-objects
3159 0 objects, 0 kilobytes
3160 ------------------------------------------------
3161
3162 Although the object files are gone, any commands that refer to those
3163 objects will work exactly as they did before.
3164
3165 The linkgit:git-gc[1] command performs packing, pruning, and more for
3166 you, so is normally the only high-level command you need.
3167
3168 [[dangling-objects]]
3169 ==== Dangling objects
3170
3171 The linkgit:git-fsck[1] command will sometimes complain about dangling
3172 objects. They are not a problem.
3173
3174 The most common cause of dangling objects is that you've rebased a
3175 branch, or you have pulled from somebody else who rebased a branch--see
3176 <<cleaning-up-history>>. In that case, the old head of the original
3177 branch still exists, as does everything it pointed to. The branch
3178 pointer itself just doesn't, since you replaced it with another one.
3179
3180 There are also other situations that cause dangling objects. For
3181 example, a "dangling blob" may arise because you did a `git add` of a
3182 file, but then, before you actually committed it and made it part of the
3183 bigger picture, you changed something else in that file and committed
3184 that *updated* thing--the old state that you added originally ends up
3185 not being pointed to by any commit or tree, so it's now a dangling blob
3186 object.
3187
3188 Similarly, when the "recursive" merge strategy runs, and finds that
3189 there are criss-cross merges and thus more than one merge base (which is
3190 fairly unusual, but it does happen), it will generate one temporary
3191 midway tree (or possibly even more, if you had lots of criss-crossing
3192 merges and more than two merge bases) as a temporary internal merge
3193 base, and again, those are real objects, but the end result will not end
3194 up pointing to them, so they end up "dangling" in your repository.
3195
3196 Generally, dangling objects aren't anything to worry about. They can
3197 even be very useful: if you screw something up, the dangling objects can
3198 be how you recover your old tree (say, you did a rebase, and realized
3199 that you really didn't want to--you can look at what dangling objects
3200 you have, and decide to reset your head to some old dangling state).
3201
3202 For commits, you can just use:
3203
3204 ------------------------------------------------
3205 $ gitk <dangling-commit-sha-goes-here> --not --all
3206 ------------------------------------------------
3207
3208 This asks for all the history reachable from the given commit but not
3209 from any branch, tag, or other reference. If you decide it's something
3210 you want, you can always create a new reference to it, e.g.,
3211
3212 ------------------------------------------------
3213 $ git branch recovered-branch <dangling-commit-sha-goes-here>
3214 ------------------------------------------------
3215
3216 For blobs and trees, you can't do the same, but you can still examine
3217 them. You can just do
3218
3219 ------------------------------------------------
3220 $ git show <dangling-blob/tree-sha-goes-here>
3221 ------------------------------------------------
3222
3223 to show what the contents of the blob were (or, for a tree, basically
3224 what the `ls` for that directory was), and that may give you some idea
3225 of what the operation was that left that dangling object.
3226
3227 Usually, dangling blobs and trees aren't very interesting. They're
3228 almost always the result of either being a half-way mergebase (the blob
3229 will often even have the conflict markers from a merge in it, if you
3230 have had conflicting merges that you fixed up by hand), or simply
3231 because you interrupted a `git fetch` with ^C or something like that,
3232 leaving _some_ of the new objects in the object database, but just
3233 dangling and useless.
3234
3235 Anyway, once you are sure that you're not interested in any dangling
3236 state, you can just prune all unreachable objects:
3237
3238 ------------------------------------------------
3239 $ git prune
3240 ------------------------------------------------
3241
3242 and they'll be gone. (You should only run `git prune` on a quiescent
3243 repository--it's kind of like doing a filesystem fsck recovery: you
3244 don't want to do that while the filesystem is mounted.
3245 `git prune` is designed not to cause any harm in such cases of concurrent
3246 accesses to a repository but you might receive confusing or scary messages.)
3247
3248 [[recovering-from-repository-corruption]]
3249 ==== Recovering from repository corruption
3250
3251 By design, Git treats data trusted to it with caution. However, even in
3252 the absence of bugs in Git itself, it is still possible that hardware or
3253 operating system errors could corrupt data.
3254
3255 The first defense against such problems is backups. You can back up a
3256 Git directory using clone, or just using cp, tar, or any other backup
3257 mechanism.
3258
3259 As a last resort, you can search for the corrupted objects and attempt
3260 to replace them by hand. Back up your repository before attempting this
3261 in case you corrupt things even more in the process.
3262
3263 We'll assume that the problem is a single missing or corrupted blob,
3264 which is sometimes a solvable problem. (Recovering missing trees and
3265 especially commits is *much* harder).
3266
3267 Before starting, verify that there is corruption, and figure out where
3268 it is with linkgit:git-fsck[1]; this may be time-consuming.
3269
3270 Assume the output looks like this:
3271
3272 ------------------------------------------------
3273 $ git fsck --full --no-dangling
3274 broken link from tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
3275 to blob 4b9458b3786228369c63936db65827de3cc06200
3276 missing blob 4b9458b3786228369c63936db65827de3cc06200
3277 ------------------------------------------------
3278
3279 Now you know that blob 4b9458b3 is missing, and that the tree 2d9263c6
3280 points to it. If you could find just one copy of that missing blob
3281 object, possibly in some other repository, you could move it into
3282 `.git/objects/4b/9458b3...` and be done. Suppose you can't. You can
3283 still examine the tree that pointed to it with linkgit:git-ls-tree[1],
3284 which might output something like:
3285
3286 ------------------------------------------------
3287 $ git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
3288 100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8 .gitignore
3289 100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883 .mailmap
3290 100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c COPYING
3291 ...
3292 100644 blob 4b9458b3786228369c63936db65827de3cc06200 myfile
3293 ...
3294 ------------------------------------------------
3295
3296 So now you know that the missing blob was the data for a file named
3297 `myfile`. And chances are you can also identify the directory--let's
3298 say it's in `somedirectory`. If you're lucky the missing copy might be
3299 the same as the copy you have checked out in your working tree at
3300 `somedirectory/myfile`; you can test whether that's right with
3301 linkgit:git-hash-object[1]:
3302
3303 ------------------------------------------------
3304 $ git hash-object -w somedirectory/myfile
3305 ------------------------------------------------
3306
3307 which will create and store a blob object with the contents of
3308 somedirectory/myfile, and output the SHA-1 of that object. if you're
3309 extremely lucky it might be 4b9458b3786228369c63936db65827de3cc06200, in
3310 which case you've guessed right, and the corruption is fixed!
3311
3312 Otherwise, you need more information. How do you tell which version of
3313 the file has been lost?
3314
3315 The easiest way to do this is with:
3316
3317 ------------------------------------------------
3318 $ git log --raw --all --full-history -- somedirectory/myfile
3319 ------------------------------------------------
3320
3321 Because you're asking for raw output, you'll now get something like
3322
3323 ------------------------------------------------
3324 commit abc
3325 Author:
3326 Date:
3327 ...
3328 :100644 100644 4b9458b newsha M somedirectory/myfile
3329
3330
3331 commit xyz
3332 Author:
3333 Date:
3334
3335 ...
3336 :100644 100644 oldsha 4b9458b M somedirectory/myfile
3337 ------------------------------------------------
3338
3339 This tells you that the immediately following version of the file was
3340 "newsha", and that the immediately preceding version was "oldsha".
3341 You also know the commit messages that went with the change from oldsha
3342 to 4b9458b and with the change from 4b9458b to newsha.
3343
3344 If you've been committing small enough changes, you may now have a good
3345 shot at reconstructing the contents of the in-between state 4b9458b.
3346
3347 If you can do that, you can now recreate the missing object with
3348
3349 ------------------------------------------------
3350 $ git hash-object -w <recreated-file>
3351 ------------------------------------------------
3352
3353 and your repository is good again!
3354
3355 (Btw, you could have ignored the `fsck`, and started with doing a
3356
3357 ------------------------------------------------
3358 $ git log --raw --all
3359 ------------------------------------------------
3360
3361 and just looked for the sha of the missing object (4b9458b) in that
3362 whole thing. It's up to you--Git does *have* a lot of information, it is
3363 just missing one particular blob version.
3364
3365 [[the-index]]
3366 === The index
3367
3368 The index is a binary file (generally kept in `.git/index`) containing a
3369 sorted list of path names, each with permissions and the SHA-1 of a blob
3370 object; linkgit:git-ls-files[1] can show you the contents of the index:
3371
3372 -------------------------------------------------
3373 $ git ls-files --stage
3374 100644 63c918c667fa005ff12ad89437f2fdc80926e21c 0 .gitignore
3375 100644 5529b198e8d14decbe4ad99db3f7fb632de0439d 0 .mailmap
3376 100644 6ff87c4664981e4397625791c8ea3bbb5f2279a3 0 COPYING
3377 100644 a37b2152bd26be2c2289e1f57a292534a51a93c7 0 Documentation/.gitignore
3378 100644 fbefe9a45b00a54b58d94d06eca48b03d40a50e0 0 Documentation/Makefile
3379 ...
3380 100644 2511aef8d89ab52be5ec6a5e46236b4b6bcd07ea 0 xdiff/xtypes.h
3381 100644 2ade97b2574a9f77e7ae4002a4e07a6a38e46d07 0 xdiff/xutils.c
3382 100644 d5de8292e05e7c36c4b68857c1cf9855e3d2f70a 0 xdiff/xutils.h
3383 -------------------------------------------------
3384
3385 Note that in older documentation you may see the index called the
3386 "current directory cache" or just the "cache". It has three important
3387 properties:
3388
3389 1. The index contains all the information necessary to generate a single
3390 (uniquely determined) tree object.
3391 +
3392 For example, running linkgit:git-commit[1] generates this tree object
3393 from the index, stores it in the object database, and uses it as the
3394 tree object associated with the new commit.
3395
3396 2. The index enables fast comparisons between the tree object it defines
3397 and the working tree.
3398 +
3399 It does this by storing some additional data for each entry (such as
3400 the last modified time). This data is not displayed above, and is not
3401 stored in the created tree object, but it can be used to determine
3402 quickly which files in the working directory differ from what was
3403 stored in the index, and thus save Git from having to read all of the
3404 data from such files to look for changes.
3405
3406 3. It can efficiently represent information about merge conflicts
3407 between different tree objects, allowing each pathname to be
3408 associated with sufficient information about the trees involved that
3409 you can create a three-way merge between them.
3410 +
3411 We saw in <<conflict-resolution>> that during a merge the index can
3412 store multiple versions of a single file (called "stages"). The third
3413 column in the linkgit:git-ls-files[1] output above is the stage
3414 number, and will take on values other than 0 for files with merge
3415 conflicts.
3416
3417 The index is thus a sort of temporary staging area, which is filled with
3418 a tree which you are in the process of working on.
3419
3420 If you blow the index away entirely, you generally haven't lost any
3421 information as long as you have the name of the tree that it described.
3422
3423 [[submodules]]
3424 == Submodules
3425
3426 Large projects are often composed of smaller, self-contained modules. For
3427 example, an embedded Linux distribution's source tree would include every
3428 piece of software in the distribution with some local modifications; a movie
3429 player might need to build against a specific, known-working version of a
3430 decompression library; several independent programs might all share the same
3431 build scripts.
3432
3433 With centralized revision control systems this is often accomplished by
3434 including every module in one single repository. Developers can check out
3435 all modules or only the modules they need to work with. They can even modify
3436 files across several modules in a single commit while moving things around
3437 or updating APIs and translations.
3438
3439 Git does not allow partial checkouts, so duplicating this approach in Git
3440 would force developers to keep a local copy of modules they are not
3441 interested in touching. Commits in an enormous checkout would be slower
3442 than you'd expect as Git would have to scan every directory for changes.
3443 If modules have a lot of local history, clones would take forever.
3444
3445 On the plus side, distributed revision control systems can much better
3446 integrate with external sources. In a centralized model, a single arbitrary
3447 snapshot of the external project is exported from its own revision control
3448 and then imported into the local revision control on a vendor branch. All
3449 the history is hidden. With distributed revision control you can clone the
3450 entire external history and much more easily follow development and re-merge
3451 local changes.
3452
3453 Git's submodule support allows a repository to contain, as a subdirectory, a
3454 checkout of an external project. Submodules maintain their own identity;
3455 the submodule support just stores the submodule repository location and
3456 commit ID, so other developers who clone the containing project
3457 ("superproject") can easily clone all the submodules at the same revision.
3458 Partial checkouts of the superproject are possible: you can tell Git to
3459 clone none, some or all of the submodules.
3460
3461 The linkgit:git-submodule[1] command is available since Git 1.5.3. Users
3462 with Git 1.5.2 can look up the submodule commits in the repository and
3463 manually check them out; earlier versions won't recognize the submodules at
3464 all.
3465
3466 To see how submodule support works, create four example
3467 repositories that can be used later as a submodule:
3468
3469 -------------------------------------------------
3470 $ mkdir ~/git
3471 $ cd ~/git
3472 $ for i in a b c d
3473 do
3474 mkdir $i
3475 cd $i
3476 git init
3477 echo "module $i" > $i.txt
3478 git add $i.txt
3479 git commit -m "Initial commit, submodule $i"
3480 cd ..
3481 done
3482 -------------------------------------------------
3483
3484 Now create the superproject and add all the submodules:
3485
3486 -------------------------------------------------
3487 $ mkdir super
3488 $ cd super
3489 $ git init
3490 $ for i in a b c d
3491 do
3492 git submodule add ~/git/$i $i
3493 done
3494 -------------------------------------------------
3495
3496 NOTE: Do not use local URLs here if you plan to publish your superproject!
3497
3498 See what files `git submodule` created:
3499
3500 -------------------------------------------------
3501 $ ls -a
3502 . .. .git .gitmodules a b c d
3503 -------------------------------------------------
3504
3505 The `git submodule add <repo> <path>` command does a couple of things:
3506
3507 - It clones the submodule from `<repo>` to the given `<path>` under the
3508 current directory and by default checks out the master branch.
3509 - It adds the submodule's clone path to the linkgit:gitmodules[5] file and
3510 adds this file to the index, ready to be committed.
3511 - It adds the submodule's current commit ID to the index, ready to be
3512 committed.
3513
3514 Commit the superproject:
3515
3516 -------------------------------------------------
3517 $ git commit -m "Add submodules a, b, c and d."
3518 -------------------------------------------------
3519
3520 Now clone the superproject:
3521
3522 -------------------------------------------------
3523 $ cd ..
3524 $ git clone super cloned
3525 $ cd cloned
3526 -------------------------------------------------
3527
3528 The submodule directories are there, but they're empty:
3529
3530 -------------------------------------------------
3531 $ ls -a a
3532 . ..
3533 $ git submodule status
3534 -d266b9873ad50488163457f025db7cdd9683d88b a
3535 -e81d457da15309b4fef4249aba9b50187999670d b
3536 -c1536a972b9affea0f16e0680ba87332dc059146 c
3537 -d96249ff5d57de5de093e6baff9e0aafa5276a74 d
3538 -------------------------------------------------
3539
3540 NOTE: The commit object names shown above would be different for you, but they
3541 should match the HEAD commit object names of your repositories. You can check
3542 it by running `git ls-remote ../a`.
3543
3544 Pulling down the submodules is a two-step process. First run `git submodule
3545 init` to add the submodule repository URLs to `.git/config`:
3546
3547 -------------------------------------------------
3548 $ git submodule init
3549 -------------------------------------------------
3550
3551 Now use `git submodule update` to clone the repositories and check out the
3552 commits specified in the superproject:
3553
3554 -------------------------------------------------
3555 $ git submodule update
3556 $ cd a
3557 $ ls -a
3558 . .. .git a.txt
3559 -------------------------------------------------
3560
3561 One major difference between `git submodule update` and `git submodule add` is
3562 that `git submodule update` checks out a specific commit, rather than the tip
3563 of a branch. It's like checking out a tag: the head is detached, so you're not
3564 working on a branch.
3565
3566 -------------------------------------------------
3567 $ git branch
3568 * (detached from d266b98)
3569 master
3570 -------------------------------------------------
3571
3572 If you want to make a change within a submodule and you have a detached head,
3573 then you should create or checkout a branch, make your changes, publish the
3574 change within the submodule, and then update the superproject to reference the
3575 new commit:
3576
3577 -------------------------------------------------
3578 $ git switch master
3579 -------------------------------------------------
3580
3581 or
3582
3583 -------------------------------------------------
3584 $ git switch -c fix-up
3585 -------------------------------------------------
3586
3587 then
3588
3589 -------------------------------------------------
3590 $ echo "adding a line again" >> a.txt
3591 $ git commit -a -m "Updated the submodule from within the superproject."
3592 $ git push
3593 $ cd ..
3594 $ git diff
3595 diff --git a/a b/a
3596 index d266b98..261dfac 160000
3597 --- a/a
3598 +++ b/a
3599 @@ -1 +1 @@
3600 -Subproject commit d266b9873ad50488163457f025db7cdd9683d88b
3601 +Subproject commit 261dfac35cb99d380eb966e102c1197139f7fa24
3602 $ git add a
3603 $ git commit -m "Updated submodule a."
3604 $ git push
3605 -------------------------------------------------
3606
3607 You have to run `git submodule update` after `git pull` if you want to update
3608 submodules, too.
3609
3610 [[pitfalls-with-submodules]]
3611 === Pitfalls with submodules
3612
3613 Always publish the submodule change before publishing the change to the
3614 superproject that references it. If you forget to publish the submodule change,
3615 others won't be able to clone the repository:
3616
3617 -------------------------------------------------
3618 $ cd ~/git/super/a
3619 $ echo i added another line to this file >> a.txt
3620 $ git commit -a -m "doing it wrong this time"
3621 $ cd ..
3622 $ git add a
3623 $ git commit -m "Updated submodule a again."
3624 $ git push
3625 $ cd ~/git/cloned
3626 $ git pull
3627 $ git submodule update
3628 error: pathspec '261dfac35cb99d380eb966e102c1197139f7fa24' did not match any file(s) known to git.
3629 Did you forget to 'git add'?
3630 Unable to checkout '261dfac35cb99d380eb966e102c1197139f7fa24' in submodule path 'a'
3631 -------------------------------------------------
3632
3633 In older Git versions it could be easily forgotten to commit new or modified
3634 files in a submodule, which silently leads to similar problems as not pushing
3635 the submodule changes. Starting with Git 1.7.0 both `git status` and `git diff`
3636 in the superproject show submodules as modified when they contain new or
3637 modified files to protect against accidentally committing such a state. `git
3638 diff` will also add a `-dirty` to the work tree side when generating patch
3639 output or used with the `--submodule` option:
3640
3641 -------------------------------------------------
3642 $ git diff
3643 diff --git a/sub b/sub
3644 --- a/sub
3645 +++ b/sub
3646 @@ -1 +1 @@
3647 -Subproject commit 3f356705649b5d566d97ff843cf193359229a453
3648 +Subproject commit 3f356705649b5d566d97ff843cf193359229a453-dirty
3649 $ git diff --submodule
3650 Submodule sub 3f35670..3f35670-dirty:
3651 -------------------------------------------------
3652
3653 You also should not rewind branches in a submodule beyond commits that were
3654 ever recorded in any superproject.
3655
3656 It's not safe to run `git submodule update` if you've made and committed
3657 changes within a submodule without checking out a branch first. They will be
3658 silently overwritten:
3659
3660 -------------------------------------------------
3661 $ cat a.txt
3662 module a
3663 $ echo line added from private2 >> a.txt
3664 $ git commit -a -m "line added inside private2"
3665 $ cd ..
3666 $ git submodule update
3667 Submodule path 'a': checked out 'd266b9873ad50488163457f025db7cdd9683d88b'
3668 $ cd a
3669 $ cat a.txt
3670 module a
3671 -------------------------------------------------
3672
3673 NOTE: The changes are still visible in the submodule's reflog.
3674
3675 If you have uncommitted changes in your submodule working tree, `git
3676 submodule update` will not overwrite them. Instead, you get the usual
3677 warning about not being able switch from a dirty branch.
3678
3679 [[low-level-operations]]
3680 == Low-level Git operations
3681
3682 Many of the higher-level commands were originally implemented as shell
3683 scripts using a smaller core of low-level Git commands. These can still
3684 be useful when doing unusual things with Git, or just as a way to
3685 understand its inner workings.
3686
3687 [[object-manipulation]]
3688 === Object access and manipulation
3689
3690 The linkgit:git-cat-file[1] command can show the contents of any object,
3691 though the higher-level linkgit:git-show[1] is usually more useful.
3692
3693 The linkgit:git-commit-tree[1] command allows constructing commits with
3694 arbitrary parents and trees.
3695
3696 A tree can be created with linkgit:git-write-tree[1] and its data can be
3697 accessed by linkgit:git-ls-tree[1]. Two trees can be compared with
3698 linkgit:git-diff-tree[1].
3699
3700 A tag is created with linkgit:git-mktag[1], and the signature can be
3701 verified by linkgit:git-verify-tag[1], though it is normally simpler to
3702 use linkgit:git-tag[1] for both.
3703
3704 [[the-workflow]]
3705 === The Workflow
3706
3707 High-level operations such as linkgit:git-commit[1] and
3708 linkgit:git-restore[1] work by moving data
3709 between the working tree, the index, and the object database. Git
3710 provides low-level operations which perform each of these steps
3711 individually.
3712
3713 Generally, all Git operations work on the index file. Some operations
3714 work *purely* on the index file (showing the current state of the
3715 index), but most operations move data between the index file and either
3716 the database or the working directory. Thus there are four main
3717 combinations:
3718
3719 [[working-directory-to-index]]
3720 ==== working directory -> index
3721
3722 The linkgit:git-update-index[1] command updates the index with
3723 information from the working directory. You generally update the
3724 index information by just specifying the filename you want to update,
3725 like so:
3726
3727 -------------------------------------------------
3728 $ git update-index filename
3729 -------------------------------------------------
3730
3731 but to avoid common mistakes with filename globbing etc., the command
3732 will not normally add totally new entries or remove old entries,
3733 i.e. it will normally just update existing cache entries.
3734
3735 To tell Git that yes, you really do realize that certain files no
3736 longer exist, or that new files should be added, you
3737 should use the `--remove` and `--add` flags respectively.
3738
3739 NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
3740 necessarily be removed: if the files still exist in your directory
3741 structure, the index will be updated with their new status, not
3742 removed. The only thing `--remove` means is that update-index will be
3743 considering a removed file to be a valid thing, and if the file really
3744 does not exist any more, it will update the index accordingly.
3745
3746 As a special case, you can also do `git update-index --refresh`, which
3747 will refresh the "stat" information of each index to match the current
3748 stat information. It will 'not' update the object status itself, and
3749 it will only update the fields that are used to quickly test whether
3750 an object still matches its old backing store object.
3751
3752 The previously introduced linkgit:git-add[1] is just a wrapper for
3753 linkgit:git-update-index[1].
3754
3755 [[index-to-object-database]]
3756 ==== index -> object database
3757
3758 You write your current index file to a "tree" object with the program
3759
3760 -------------------------------------------------
3761 $ git write-tree
3762 -------------------------------------------------
3763
3764 that doesn't come with any options--it will just write out the
3765 current index into the set of tree objects that describe that state,
3766 and it will return the name of the resulting top-level tree. You can
3767 use that tree to re-generate the index at any time by going in the
3768 other direction:
3769
3770 [[object-database-to-index]]
3771 ==== object database -> index
3772
3773 You read a "tree" file from the object database, and use that to
3774 populate (and overwrite--don't do this if your index contains any
3775 unsaved state that you might want to restore later!) your current
3776 index. Normal operation is just
3777
3778 -------------------------------------------------
3779 $ git read-tree <SHA-1 of tree>
3780 -------------------------------------------------
3781
3782 and your index file will now be equivalent to the tree that you saved
3783 earlier. However, that is only your 'index' file: your working
3784 directory contents have not been modified.
3785
3786 [[index-to-working-directory]]
3787 ==== index -> working directory
3788
3789 You update your working directory from the index by "checking out"
3790 files. This is not a very common operation, since normally you'd just
3791 keep your files updated, and rather than write to your working
3792 directory, you'd tell the index files about the changes in your
3793 working directory (i.e. `git update-index`).
3794
3795 However, if you decide to jump to a new version, or check out somebody
3796 else's version, or just restore a previous tree, you'd populate your
3797 index file with read-tree, and then you need to check out the result
3798 with
3799
3800 -------------------------------------------------
3801 $ git checkout-index filename
3802 -------------------------------------------------
3803
3804 or, if you want to check out all of the index, use `-a`.
3805
3806 NOTE! `git checkout-index` normally refuses to overwrite old files, so
3807 if you have an old version of the tree already checked out, you will
3808 need to use the `-f` flag ('before' the `-a` flag or the filename) to
3809 'force' the checkout.
3810
3811
3812 Finally, there are a few odds and ends which are not purely moving
3813 from one representation to the other:
3814
3815 [[tying-it-all-together]]
3816 ==== Tying it all together
3817
3818 To commit a tree you have instantiated with `git write-tree`, you'd
3819 create a "commit" object that refers to that tree and the history
3820 behind it--most notably the "parent" commits that preceded it in
3821 history.
3822
3823 Normally a "commit" has one parent: the previous state of the tree
3824 before a certain change was made. However, sometimes it can have two
3825 or more parent commits, in which case we call it a "merge", due to the
3826 fact that such a commit brings together ("merges") two or more
3827 previous states represented by other commits.
3828
3829 In other words, while a "tree" represents a particular directory state
3830 of a working directory, a "commit" represents that state in time,
3831 and explains how we got there.
3832
3833 You create a commit object by giving it the tree that describes the
3834 state at the time of the commit, and a list of parents:
3835
3836 -------------------------------------------------
3837 $ git commit-tree <tree> -p <parent> [(-p <parent2>)...]
3838 -------------------------------------------------
3839
3840 and then giving the reason for the commit on stdin (either through
3841 redirection from a pipe or file, or by just typing it at the tty).
3842
3843 `git commit-tree` will return the name of the object that represents
3844 that commit, and you should save it away for later use. Normally,
3845 you'd commit a new `HEAD` state, and while Git doesn't care where you
3846 save the note about that state, in practice we tend to just write the
3847 result to the file pointed at by `.git/HEAD`, so that we can always see
3848 what the last committed state was.
3849
3850 Here is a picture that illustrates how various pieces fit together:
3851
3852 ------------
3853
3854 commit-tree
3855 commit obj
3856 +----+
3857 | |
3858 | |
3859 V V
3860 +-----------+
3861 | Object DB |
3862 | Backing |
3863 | Store |
3864 +-----------+
3865 ^
3866 write-tree | |
3867 tree obj | |
3868 | | read-tree
3869 | | tree obj
3870 V
3871 +-----------+
3872 | Index |
3873 | "cache" |
3874 +-----------+
3875 update-index ^
3876 blob obj | |
3877 | |
3878 checkout-index -u | | checkout-index
3879 stat | | blob obj
3880 V
3881 +-----------+
3882 | Working |
3883 | Directory |
3884 +-----------+
3885
3886 ------------
3887
3888
3889 [[examining-the-data]]
3890 === Examining the data
3891
3892 You can examine the data represented in the object database and the
3893 index with various helper tools. For every object, you can use
3894 linkgit:git-cat-file[1] to examine details about the
3895 object:
3896
3897 -------------------------------------------------
3898 $ git cat-file -t <objectname>
3899 -------------------------------------------------
3900
3901 shows the type of the object, and once you have the type (which is
3902 usually implicit in where you find the object), you can use
3903
3904 -------------------------------------------------
3905 $ git cat-file blob|tree|commit|tag <objectname>
3906 -------------------------------------------------
3907
3908 to show its contents. NOTE! Trees have binary content, and as a result
3909 there is a special helper for showing that content, called
3910 `git ls-tree`, which turns the binary content into a more easily
3911 readable form.
3912
3913 It's especially instructive to look at "commit" objects, since those
3914 tend to be small and fairly self-explanatory. In particular, if you
3915 follow the convention of having the top commit name in `.git/HEAD`,
3916 you can do
3917
3918 -------------------------------------------------
3919 $ git cat-file commit HEAD
3920 -------------------------------------------------
3921
3922 to see what the top commit was.
3923
3924 [[merging-multiple-trees]]
3925 === Merging multiple trees
3926
3927 Git can help you perform a three-way merge, which can in turn be
3928 used for a many-way merge by repeating the merge procedure several
3929 times. The usual situation is that you only do one three-way merge
3930 (reconciling two lines of history) and commit the result, but if
3931 you like to, you can merge several branches in one go.
3932
3933 To perform a three-way merge, you start with the two commits you
3934 want to merge, find their closest common parent (a third commit),
3935 and compare the trees corresponding to these three commits.
3936
3937 To get the "base" for the merge, look up the common parent of two
3938 commits:
3939
3940 -------------------------------------------------
3941 $ git merge-base <commit1> <commit2>
3942 -------------------------------------------------
3943
3944 This prints the name of a commit they are both based on. You should
3945 now look up the tree objects of those commits, which you can easily
3946 do with
3947
3948 -------------------------------------------------
3949 $ git cat-file commit <commitname> | head -1
3950 -------------------------------------------------
3951
3952 since the tree object information is always the first line in a commit
3953 object.
3954
3955 Once you know the three trees you are going to merge (the one "original"
3956 tree, aka the common tree, and the two "result" trees, aka the branches
3957 you want to merge), you do a "merge" read into the index. This will
3958 complain if it has to throw away your old index contents, so you should
3959 make sure that you've committed those--in fact you would normally
3960 always do a merge against your last commit (which should thus match what
3961 you have in your current index anyway).
3962
3963 To do the merge, do
3964
3965 -------------------------------------------------
3966 $ git read-tree -m -u <origtree> <yourtree> <targettree>
3967 -------------------------------------------------
3968
3969 which will do all trivial merge operations for you directly in the
3970 index file, and you can just write the result out with
3971 `git write-tree`.
3972
3973
3974 [[merging-multiple-trees-2]]
3975 === Merging multiple trees, continued
3976
3977 Sadly, many merges aren't trivial. If there are files that have
3978 been added, moved or removed, or if both branches have modified the
3979 same file, you will be left with an index tree that contains "merge
3980 entries" in it. Such an index tree can 'NOT' be written out to a tree
3981 object, and you will have to resolve any such merge clashes using
3982 other tools before you can write out the result.
3983
3984 You can examine such index state with `git ls-files --unmerged`
3985 command. An example:
3986
3987 ------------------------------------------------
3988 $ git read-tree -m $orig HEAD $target
3989 $ git ls-files --unmerged
3990 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello.c
3991 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello.c
3992 100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello.c
3993 ------------------------------------------------
3994
3995 Each line of the `git ls-files --unmerged` output begins with
3996 the blob mode bits, blob SHA-1, 'stage number', and the
3997 filename. The 'stage number' is Git's way to say which tree it
3998 came from: stage 1 corresponds to the `$orig` tree, stage 2 to
3999 the `HEAD` tree, and stage 3 to the `$target` tree.
4000
4001 Earlier we said that trivial merges are done inside
4002 `git read-tree -m`. For example, if the file did not change
4003 from `$orig` to `HEAD` or `$target`, or if the file changed
4004 from `$orig` to `HEAD` and `$orig` to `$target` the same way,
4005 obviously the final outcome is what is in `HEAD`. What the
4006 above example shows is that file `hello.c` was changed from
4007 `$orig` to `HEAD` and `$orig` to `$target` in a different way.
4008 You could resolve this by running your favorite 3-way merge
4009 program, e.g. `diff3`, `merge`, or Git's own merge-file, on
4010 the blob objects from these three stages yourself, like this:
4011
4012 ------------------------------------------------
4013 $ git cat-file blob 263414f >hello.c~1
4014 $ git cat-file blob 06fa6a2 >hello.c~2
4015 $ git cat-file blob cc44c73 >hello.c~3
4016 $ git merge-file hello.c~2 hello.c~1 hello.c~3
4017 ------------------------------------------------
4018
4019 This would leave the merge result in `hello.c~2` file, along
4020 with conflict markers if there are conflicts. After verifying
4021 the merge result makes sense, you can tell Git what the final
4022 merge result for this file is by:
4023
4024 -------------------------------------------------
4025 $ mv -f hello.c~2 hello.c
4026 $ git update-index hello.c
4027 -------------------------------------------------
4028
4029 When a path is in the "unmerged" state, running `git update-index` for
4030 that path tells Git to mark the path resolved.
4031
4032 The above is the description of a Git merge at the lowest level,
4033 to help you understand what conceptually happens under the hood.
4034 In practice, nobody, not even Git itself, runs `git cat-file` three times
4035 for this. There is a `git merge-index` program that extracts the
4036 stages to temporary files and calls a "merge" script on it:
4037
4038 -------------------------------------------------
4039 $ git merge-index git-merge-one-file hello.c
4040 -------------------------------------------------
4041
4042 and that is what higher level `git merge -s resolve` is implemented with.
4043
4044 [[hacking-git]]
4045 == Hacking Git
4046
4047 This chapter covers internal details of the Git implementation which
4048 probably only Git developers need to understand.
4049
4050 [[object-details]]
4051 === Object storage format
4052
4053 All objects have a statically determined "type" which identifies the
4054 format of the object (i.e. how it is used, and how it can refer to other
4055 objects). There are currently four different object types: "blob",
4056 "tree", "commit", and "tag".
4057
4058 Regardless of object type, all objects share the following
4059 characteristics: they are all deflated with zlib, and have a header
4060 that not only specifies their type, but also provides size information
4061 about the data in the object. It's worth noting that the SHA-1 hash
4062 that is used to name the object is the hash of the original data
4063 plus this header, so `sha1sum` 'file' does not match the object name
4064 for 'file'.
4065
4066 As a result, the general consistency of an object can always be tested
4067 independently of the contents or the type of the object: all objects can
4068 be validated by verifying that (a) their hashes match the content of the
4069 file and (b) the object successfully inflates to a stream of bytes that
4070 forms a sequence of
4071 `<ascii type without space> + <space> + <ascii decimal size> +
4072 <byte\0> + <binary object data>`.
4073
4074 The structured objects can further have their structure and
4075 connectivity to other objects verified. This is generally done with
4076 the `git fsck` program, which generates a full dependency graph
4077 of all objects, and verifies their internal consistency (in addition
4078 to just verifying their superficial consistency through the hash).
4079
4080 [[birdview-on-the-source-code]]
4081 === A birds-eye view of Git's source code
4082
4083 It is not always easy for new developers to find their way through Git's
4084 source code. This section gives you a little guidance to show where to
4085 start.
4086
4087 A good place to start is with the contents of the initial commit, with:
4088
4089 ----------------------------------------------------
4090 $ git switch --detach e83c5163
4091 ----------------------------------------------------
4092
4093 The initial revision lays the foundation for almost everything Git has
4094 today, but is small enough to read in one sitting.
4095
4096 Note that terminology has changed since that revision. For example, the
4097 README in that revision uses the word "changeset" to describe what we
4098 now call a <<def_commit_object,commit>>.
4099
4100 Also, we do not call it "cache" any more, but rather "index"; however, the
4101 file is still called `cache.h`. Remark: Not much reason to change it now,
4102 especially since there is no good single name for it anyway, because it is
4103 basically _the_ header file which is included by _all_ of Git's C sources.
4104
4105 If you grasp the ideas in that initial commit, you should check out a
4106 more recent version and skim `cache.h`, `object.h` and `commit.h`.
4107
4108 In the early days, Git (in the tradition of UNIX) was a bunch of programs
4109 which were extremely simple, and which you used in scripts, piping the
4110 output of one into another. This turned out to be good for initial
4111 development, since it was easier to test new things. However, recently
4112 many of these parts have become builtins, and some of the core has been
4113 "libified", i.e. put into libgit.a for performance, portability reasons,
4114 and to avoid code duplication.
4115
4116 By now, you know what the index is (and find the corresponding data
4117 structures in `cache.h`), and that there are just a couple of object types
4118 (blobs, trees, commits and tags) which inherit their common structure from
4119 `struct object`, which is their first member (and thus, you can cast e.g.
4120 `(struct object *)commit` to achieve the _same_ as `&commit->object`, i.e.
4121 get at the object name and flags).
4122
4123 Now is a good point to take a break to let this information sink in.
4124
4125 Next step: get familiar with the object naming. Read <<naming-commits>>.
4126 There are quite a few ways to name an object (and not only revisions!).
4127 All of these are handled in `sha1_name.c`. Just have a quick look at
4128 the function `get_sha1()`. A lot of the special handling is done by
4129 functions like `get_sha1_basic()` or the likes.
4130
4131 This is just to get you into the groove for the most libified part of Git:
4132 the revision walker.
4133
4134 Basically, the initial version of `git log` was a shell script:
4135
4136 ----------------------------------------------------------------
4137 $ git-rev-list --pretty $(git-rev-parse --default HEAD "$@") | \
4138 LESS=-S ${PAGER:-less}
4139 ----------------------------------------------------------------
4140
4141 What does this mean?
4142
4143 `git rev-list` is the original version of the revision walker, which
4144 _always_ printed a list of revisions to stdout. It is still functional,
4145 and needs to, since most new Git commands start out as scripts using
4146 `git rev-list`.
4147
4148 `git rev-parse` is not as important any more; it was only used to filter out
4149 options that were relevant for the different plumbing commands that were
4150 called by the script.
4151
4152 Most of what `git rev-list` did is contained in `revision.c` and
4153 `revision.h`. It wraps the options in a struct named `rev_info`, which
4154 controls how and what revisions are walked, and more.
4155
4156 The original job of `git rev-parse` is now taken by the function
4157 `setup_revisions()`, which parses the revisions and the common command-line
4158 options for the revision walker. This information is stored in the struct
4159 `rev_info` for later consumption. You can do your own command-line option
4160 parsing after calling `setup_revisions()`. After that, you have to call
4161 `prepare_revision_walk()` for initialization, and then you can get the
4162 commits one by one with the function `get_revision()`.
4163
4164 If you are interested in more details of the revision walking process,
4165 just have a look at the first implementation of `cmd_log()`; call
4166 `git show v1.3.0~155^2~4` and scroll down to that function (note that you
4167 no longer need to call `setup_pager()` directly).
4168
4169 Nowadays, `git log` is a builtin, which means that it is _contained_ in the
4170 command `git`. The source side of a builtin is
4171
4172 - a function called `cmd_<bla>`, typically defined in `builtin/<bla.c>`
4173 (note that older versions of Git used to have it in `builtin-<bla>.c`
4174 instead), and declared in `builtin.h`.
4175
4176 - an entry in the `commands[]` array in `git.c`, and
4177
4178 - an entry in `BUILTIN_OBJECTS` in the `Makefile`.
4179
4180 Sometimes, more than one builtin is contained in one source file. For
4181 example, `cmd_whatchanged()` and `cmd_log()` both reside in `builtin/log.c`,
4182 since they share quite a bit of code. In that case, the commands which are
4183 _not_ named like the `.c` file in which they live have to be listed in
4184 `BUILT_INS` in the `Makefile`.
4185
4186 `git log` looks more complicated in C than it does in the original script,
4187 but that allows for a much greater flexibility and performance.
4188
4189 Here again it is a good point to take a pause.
4190
4191 Lesson three is: study the code. Really, it is the best way to learn about
4192 the organization of Git (after you know the basic concepts).
4193
4194 So, think about something which you are interested in, say, "how can I
4195 access a blob just knowing the object name of it?". The first step is to
4196 find a Git command with which you can do it. In this example, it is either
4197 `git show` or `git cat-file`.
4198
4199 For the sake of clarity, let's stay with `git cat-file`, because it
4200
4201 - is plumbing, and
4202
4203 - was around even in the initial commit (it literally went only through
4204 some 20 revisions as `cat-file.c`, was renamed to `builtin/cat-file.c`
4205 when made a builtin, and then saw less than 10 versions).
4206
4207 So, look into `builtin/cat-file.c`, search for `cmd_cat_file()` and look what
4208 it does.
4209
4210 ------------------------------------------------------------------
4211 git_config(git_default_config);
4212 if (argc != 3)
4213 usage("git cat-file [-t|-s|-e|-p|<type>] <sha1>");
4214 if (get_sha1(argv[2], sha1))
4215 die("Not a valid object name %s", argv[2]);
4216 ------------------------------------------------------------------
4217
4218 Let's skip over the obvious details; the only really interesting part
4219 here is the call to `get_sha1()`. It tries to interpret `argv[2]` as an
4220 object name, and if it refers to an object which is present in the current
4221 repository, it writes the resulting SHA-1 into the variable `sha1`.
4222
4223 Two things are interesting here:
4224
4225 - `get_sha1()` returns 0 on _success_. This might surprise some new
4226 Git hackers, but there is a long tradition in UNIX to return different
4227 negative numbers in case of different errors--and 0 on success.
4228
4229 - the variable `sha1` in the function signature of `get_sha1()` is `unsigned
4230 char *`, but is actually expected to be a pointer to `unsigned
4231 char[20]`. This variable will contain the 160-bit SHA-1 of the given
4232 commit. Note that whenever a SHA-1 is passed as `unsigned char *`, it
4233 is the binary representation, as opposed to the ASCII representation in
4234 hex characters, which is passed as `char *`.
4235
4236 You will see both of these things throughout the code.
4237
4238 Now, for the meat:
4239
4240 -----------------------------------------------------------------------------
4241 case 0:
4242 buf = read_object_with_reference(sha1, argv[1], &size, NULL);
4243 -----------------------------------------------------------------------------
4244
4245 This is how you read a blob (actually, not only a blob, but any type of
4246 object). To know how the function `read_object_with_reference()` actually
4247 works, find the source code for it (something like `git grep
4248 read_object_with | grep ":[a-z]"` in the Git repository), and read
4249 the source.
4250
4251 To find out how the result can be used, just read on in `cmd_cat_file()`:
4252
4253 -----------------------------------
4254 write_or_die(1, buf, size);
4255 -----------------------------------
4256
4257 Sometimes, you do not know where to look for a feature. In many such cases,
4258 it helps to search through the output of `git log`, and then `git show` the
4259 corresponding commit.
4260
4261 Example: If you know that there was some test case for `git bundle`, but
4262 do not remember where it was (yes, you _could_ `git grep bundle t/`, but that
4263 does not illustrate the point!):
4264
4265 ------------------------
4266 $ git log --no-merges t/
4267 ------------------------
4268
4269 In the pager (`less`), just search for "bundle", go a few lines back,
4270 and see that it is in commit 18449ab0. Now just copy this object name,
4271 and paste it into the command line
4272
4273 -------------------
4274 $ git show 18449ab0
4275 -------------------
4276
4277 Voila.
4278
4279 Another example: Find out what to do in order to make some script a
4280 builtin:
4281
4282 -------------------------------------------------
4283 $ git log --no-merges --diff-filter=A builtin/*.c
4284 -------------------------------------------------
4285
4286 You see, Git is actually the best tool to find out about the source of Git
4287 itself!
4288
4289 [[glossary]]
4290 == Git Glossary
4291
4292 [[git-explained]]
4293 === Git explained
4294
4295 include::glossary-content.txt[]
4296
4297 [[git-quick-start]]
4298 [appendix]
4299 == Git Quick Reference
4300
4301 This is a quick summary of the major commands; the previous chapters
4302 explain how these work in more detail.
4303
4304 [[quick-creating-a-new-repository]]
4305 === Creating a new repository
4306
4307 From a tarball:
4308
4309 -----------------------------------------------
4310 $ tar xzf project.tar.gz
4311 $ cd project
4312 $ git init
4313 Initialized empty Git repository in .git/
4314 $ git add .
4315 $ git commit
4316 -----------------------------------------------
4317
4318 From a remote repository:
4319
4320 -----------------------------------------------
4321 $ git clone git://example.com/pub/project.git
4322 $ cd project
4323 -----------------------------------------------
4324
4325 [[managing-branches]]
4326 === Managing branches
4327
4328 -----------------------------------------------
4329 $ git branch # list all local branches in this repo
4330 $ git switch test # switch working directory to branch "test"
4331 $ git branch new # create branch "new" starting at current HEAD
4332 $ git branch -d new # delete branch "new"
4333 -----------------------------------------------
4334
4335 Instead of basing a new branch on current HEAD (the default), use:
4336
4337 -----------------------------------------------
4338 $ git branch new test # branch named "test"
4339 $ git branch new v2.6.15 # tag named v2.6.15
4340 $ git branch new HEAD^ # commit before the most recent
4341 $ git branch new HEAD^^ # commit before that
4342 $ git branch new test~10 # ten commits before tip of branch "test"
4343 -----------------------------------------------
4344
4345 Create and switch to a new branch at the same time:
4346
4347 -----------------------------------------------
4348 $ git switch -c new v2.6.15
4349 -----------------------------------------------
4350
4351 Update and examine branches from the repository you cloned from:
4352
4353 -----------------------------------------------
4354 $ git fetch # update
4355 $ git branch -r # list
4356 origin/master
4357 origin/next
4358 ...
4359 $ git switch -c masterwork origin/master
4360 -----------------------------------------------
4361
4362 Fetch a branch from a different repository, and give it a new
4363 name in your repository:
4364
4365 -----------------------------------------------
4366 $ git fetch git://example.com/project.git theirbranch:mybranch
4367 $ git fetch git://example.com/project.git v2.6.15:mybranch
4368 -----------------------------------------------
4369
4370 Keep a list of repositories you work with regularly:
4371
4372 -----------------------------------------------
4373 $ git remote add example git://example.com/project.git
4374 $ git remote # list remote repositories
4375 example
4376 origin
4377 $ git remote show example # get details
4378 * remote example
4379 URL: git://example.com/project.git
4380 Tracked remote branches
4381 master
4382 next
4383 ...
4384 $ git fetch example # update branches from example
4385 $ git branch -r # list all remote branches
4386 -----------------------------------------------
4387
4388
4389 [[exploring-history]]
4390 === Exploring history
4391
4392 -----------------------------------------------
4393 $ gitk # visualize and browse history
4394 $ git log # list all commits
4395 $ git log src/ # ...modifying src/
4396 $ git log v2.6.15..v2.6.16 # ...in v2.6.16, not in v2.6.15
4397 $ git log master..test # ...in branch test, not in branch master
4398 $ git log test..master # ...in branch master, but not in test
4399 $ git log test...master # ...in one branch, not in both
4400 $ git log -S'foo()' # ...where difference contain "foo()"
4401 $ git log --since="2 weeks ago"
4402 $ git log -p # show patches as well
4403 $ git show # most recent commit
4404 $ git diff v2.6.15..v2.6.16 # diff between two tagged versions
4405 $ git diff v2.6.15..HEAD # diff with current head
4406 $ git grep "foo()" # search working directory for "foo()"
4407 $ git grep v2.6.15 "foo()" # search old tree for "foo()"
4408 $ git show v2.6.15:a.txt # look at old version of a.txt
4409 -----------------------------------------------
4410
4411 Search for regressions:
4412
4413 -----------------------------------------------
4414 $ git bisect start
4415 $ git bisect bad # current version is bad
4416 $ git bisect good v2.6.13-rc2 # last known good revision
4417 Bisecting: 675 revisions left to test after this
4418 # test here, then:
4419 $ git bisect good # if this revision is good, or
4420 $ git bisect bad # if this revision is bad.
4421 # repeat until done.
4422 -----------------------------------------------
4423
4424 [[making-changes]]
4425 === Making changes
4426
4427 Make sure Git knows who to blame:
4428
4429 ------------------------------------------------
4430 $ cat >>~/.gitconfig <<\EOF
4431 [user]
4432 name = Your Name Comes Here
4433 email = you@yourdomain.example.com
4434 EOF
4435 ------------------------------------------------
4436
4437 Select file contents to include in the next commit, then make the
4438 commit:
4439
4440 -----------------------------------------------
4441 $ git add a.txt # updated file
4442 $ git add b.txt # new file
4443 $ git rm c.txt # old file
4444 $ git commit
4445 -----------------------------------------------
4446
4447 Or, prepare and create the commit in one step:
4448
4449 -----------------------------------------------
4450 $ git commit d.txt # use latest content only of d.txt
4451 $ git commit -a # use latest content of all tracked files
4452 -----------------------------------------------
4453
4454 [[merging]]
4455 === Merging
4456
4457 -----------------------------------------------
4458 $ git merge test # merge branch "test" into the current branch
4459 $ git pull git://example.com/project.git master
4460 # fetch and merge in remote branch
4461 $ git pull . test # equivalent to git merge test
4462 -----------------------------------------------
4463
4464 [[sharing-your-changes]]
4465 === Sharing your changes
4466
4467 Importing or exporting patches:
4468
4469 -----------------------------------------------
4470 $ git format-patch origin..HEAD # format a patch for each commit
4471 # in HEAD but not in origin
4472 $ git am mbox # import patches from the mailbox "mbox"
4473 -----------------------------------------------
4474
4475 Fetch a branch in a different Git repository, then merge into the
4476 current branch:
4477
4478 -----------------------------------------------
4479 $ git pull git://example.com/project.git theirbranch
4480 -----------------------------------------------
4481
4482 Store the fetched branch into a local branch before merging into the
4483 current branch:
4484
4485 -----------------------------------------------
4486 $ git pull git://example.com/project.git theirbranch:mybranch
4487 -----------------------------------------------
4488
4489 After creating commits on a local branch, update the remote
4490 branch with your commits:
4491
4492 -----------------------------------------------
4493 $ git push ssh://example.com/project.git mybranch:theirbranch
4494 -----------------------------------------------
4495
4496 When remote and local branch are both named "test":
4497
4498 -----------------------------------------------
4499 $ git push ssh://example.com/project.git test
4500 -----------------------------------------------
4501
4502 Shortcut version for a frequently used remote repository:
4503
4504 -----------------------------------------------
4505 $ git remote add example ssh://example.com/project.git
4506 $ git push example test
4507 -----------------------------------------------
4508
4509 [[repository-maintenance]]
4510 === Repository maintenance
4511
4512 Check for corruption:
4513
4514 -----------------------------------------------
4515 $ git fsck
4516 -----------------------------------------------
4517
4518 Recompress, remove unused cruft:
4519
4520 -----------------------------------------------
4521 $ git gc
4522 -----------------------------------------------
4523
4524
4525 [[todo]]
4526 [appendix]
4527 == Notes and todo list for this manual
4528
4529 [[todo-list]]
4530 === Todo list
4531
4532 This is a work in progress.
4533
4534 The basic requirements:
4535
4536 - It must be readable in order, from beginning to end, by someone
4537 intelligent with a basic grasp of the UNIX command line, but without
4538 any special knowledge of Git. If necessary, any other prerequisites
4539 should be specifically mentioned as they arise.
4540 - Whenever possible, section headings should clearly describe the task
4541 they explain how to do, in language that requires no more knowledge
4542 than necessary: for example, "importing patches into a project" rather
4543 than "the `git am` command"
4544
4545 Think about how to create a clear chapter dependency graph that will
4546 allow people to get to important topics without necessarily reading
4547 everything in between.
4548
4549 Scan `Documentation/` for other stuff left out; in particular:
4550
4551 - howto's
4552 - some of `technical/`?
4553 - hooks
4554 - list of commands in linkgit:git[1]
4555
4556 Scan email archives for other stuff left out
4557
4558 Scan man pages to see if any assume more background than this manual
4559 provides.
4560
4561 Add more good examples. Entire sections of just cookbook examples
4562 might be a good idea; maybe make an "advanced examples" section a
4563 standard end-of-chapter section?
4564
4565 Include cross-references to the glossary, where appropriate.
4566
4567 Add a section on working with other version control systems, including
4568 CVS, Subversion, and just imports of series of release tarballs.
4569
4570 Write a chapter on using plumbing and writing scripts.
4571
4572 Alternates, clone -reference, etc.
4573
4574 More on recovery from repository corruption. See:
4575 http://marc.info/?l=git&m=117263864820799&w=2
4576 http://marc.info/?l=git&m=117147855503798&w=2