Cleanup check_valid in commit-tree.
[git/git.git] / Documentation / user-manual.txt
1 Git User's Manual
2 _________________
3
4 This manual is designed to be readable by someone with basic unix
5 commandline skills, but no previous knowledge of git.
6
7 Chapter 1 gives a brief overview of git commands, without any
8 explanation; you may prefer to skip to chapter 2 on a first reading.
9
10 Chapters 2 and 3 explain how to fetch and study a project using
11 git--the tools you'd need to build and test a particular version of a
12 software project, to search for regressions, and so on.
13
14 Chapter 4 explains how to do development with git, and chapter 5 how
15 to share that development with others.
16
17 Further chapters cover more specialized topics.
18
19 Comprehensive reference documentation is available through the man
20 pages. For a command such as "git clone", just use
21
22 ------------------------------------------------
23 $ man git-clone
24 ------------------------------------------------
25
26 Git Quick Start
27 ===============
28
29 This is a quick summary of the major commands; the following chapters
30 will explain how these work in more detail.
31
32 Creating a new repository
33 -------------------------
34
35 From a tarball:
36
37 -----------------------------------------------
38 $ tar xzf project.tar.gz
39 $ cd project
40 $ git init
41 Initialized empty Git repository in .git/
42 $ git add .
43 $ git commit
44 -----------------------------------------------
45
46 From a remote repository:
47
48 -----------------------------------------------
49 $ git clone git://example.com/pub/project.git
50 $ cd project
51 -----------------------------------------------
52
53 Managing branches
54 -----------------
55
56 -----------------------------------------------
57 $ git branch # list all branches in this repo
58 $ git checkout test # switch working directory to branch "test"
59 $ git branch new # create branch "new" starting at current HEAD
60 $ git branch -d new # delete branch "new"
61 -----------------------------------------------
62
63 Instead of basing new branch on current HEAD (the default), use:
64
65 -----------------------------------------------
66 $ git branch new test # branch named "test"
67 $ git branch new v2.6.15 # tag named v2.6.15
68 $ git branch new HEAD^ # commit before the most recent
69 $ git branch new HEAD^^ # commit before that
70 $ git branch new test~10 # ten commits before tip of branch "test"
71 -----------------------------------------------
72
73 Create and switch to a new branch at the same time:
74
75 -----------------------------------------------
76 $ git checkout -b new v2.6.15
77 -----------------------------------------------
78
79 Update and examine branches from the repository you cloned from:
80
81 -----------------------------------------------
82 $ git fetch # update
83 $ git branch -r # list
84 origin/master
85 origin/next
86 ...
87 $ git branch checkout -b masterwork origin/master
88 -----------------------------------------------
89
90 Fetch a branch from a different repository, and give it a new
91 name in your repository:
92
93 -----------------------------------------------
94 $ git fetch git://example.com/project.git theirbranch:mybranch
95 $ git fetch git://example.com/project.git v2.6.15:mybranch
96 -----------------------------------------------
97
98 Keep a list of repositories you work with regularly:
99
100 -----------------------------------------------
101 $ git remote add example git://example.com/project.git
102 $ git remote # list remote repositories
103 example
104 origin
105 $ git remote show example # get details
106 * remote example
107 URL: git://example.com/project.git
108 Tracked remote branches
109 master next ...
110 $ git fetch example # update branches from example
111 $ git branch -r # list all remote branches
112 -----------------------------------------------
113
114
115 Exploring history
116 -----------------
117
118 -----------------------------------------------
119 $ gitk # visualize and browse history
120 $ git log # list all commits
121 $ git log src/ # ...modifying src/
122 $ git log v2.6.15..v2.6.16 # ...in v2.6.16, not in v2.6.15
123 $ git log master..test # ...in branch test, not in branch master
124 $ git log test..master # ...in branch master, but not in test
125 $ git log test...master # ...in one branch, not in both
126 $ git log -S'foo()' # ...where difference contain "foo()"
127 $ git log --since="2 weeks ago"
128 $ git log -p # show patches as well
129 $ git show # most recent commit
130 $ git diff v2.6.15..v2.6.16 # diff between two tagged versions
131 $ git diff v2.6.15..HEAD # diff with current head
132 $ git grep "foo()" # search working directory for "foo()"
133 $ git grep v2.6.15 "foo()" # search old tree for "foo()"
134 $ git show v2.6.15:a.txt # look at old version of a.txt
135 -----------------------------------------------
136
137 Search for regressions:
138
139 -----------------------------------------------
140 $ git bisect start
141 $ git bisect bad # current version is bad
142 $ git bisect good v2.6.13-rc2 # last known good revision
143 Bisecting: 675 revisions left to test after this
144 # test here, then:
145 $ git bisect good # if this revision is good, or
146 $ git bisect bad # if this revision is bad.
147 # repeat until done.
148 -----------------------------------------------
149
150 Making changes
151 --------------
152
153 Make sure git knows who to blame:
154
155 ------------------------------------------------
156 $ cat >~/.gitconfig <<\EOF
157 [user]
158 name = Your Name Comes Here
159 email = you@yourdomain.example.com
160 EOF
161 ------------------------------------------------
162
163 Select file contents to include in the next commit, then make the
164 commit:
165
166 -----------------------------------------------
167 $ git add a.txt # updated file
168 $ git add b.txt # new file
169 $ git rm c.txt # old file
170 $ git commit
171 -----------------------------------------------
172
173 Or, prepare and create the commit in one step:
174
175 -----------------------------------------------
176 $ git commit d.txt # use latest content only of d.txt
177 $ git commit -a # use latest content of all tracked files
178 -----------------------------------------------
179
180 Merging
181 -------
182
183 -----------------------------------------------
184 $ git merge test # merge branch "test" into the current branch
185 $ git pull git://example.com/project.git master
186 # fetch and merge in remote branch
187 $ git pull . test # equivalent to git merge test
188 -----------------------------------------------
189
190 Sharing your changes
191 --------------------
192
193 Importing or exporting patches:
194
195 -----------------------------------------------
196 $ git format-patch origin..HEAD # format a patch for each commit
197 # in HEAD but not in origin
198 $ git-am mbox # import patches from the mailbox "mbox"
199 -----------------------------------------------
200
201 Fetch a branch in a different git repository, then merge into the
202 current branch:
203
204 -----------------------------------------------
205 $ git pull git://example.com/project.git theirbranch
206 -----------------------------------------------
207
208 Store the fetched branch into a local branch before merging into the
209 current branch:
210
211 -----------------------------------------------
212 $ git pull git://example.com/project.git theirbranch:mybranch
213 -----------------------------------------------
214
215 After creating commits on a local branch, update the remote
216 branch with your commits:
217
218 -----------------------------------------------
219 $ git push ssh://example.com/project.git mybranch:theirbranch
220 -----------------------------------------------
221
222 When remote and local branch are both named "test":
223
224 -----------------------------------------------
225 $ git push ssh://example.com/project.git test
226 -----------------------------------------------
227
228 Shortcut version for a frequently used remote repository:
229
230 -----------------------------------------------
231 $ git remote add example ssh://example.com/project.git
232 $ git push example test
233 -----------------------------------------------
234
235 Repository maintenance
236 ----------------------
237
238 Check for corruption:
239
240 -----------------------------------------------
241 $ git fsck
242 -----------------------------------------------
243
244 Recompress, remove unused cruft:
245
246 -----------------------------------------------
247 $ git gc
248 -----------------------------------------------
249
250 Repositories and Branches
251 =========================
252
253 How to get a git repository
254 ---------------------------
255
256 It will be useful to have a git repository to experiment with as you
257 read this manual.
258
259 The best way to get one is by using the gitlink:git-clone[1] command
260 to download a copy of an existing repository for a project that you
261 are interested in. If you don't already have a project in mind, here
262 are some interesting examples:
263
264 ------------------------------------------------
265 # git itself (approx. 10MB download):
266 $ git clone git://git.kernel.org/pub/scm/git/git.git
267 # the linux kernel (approx. 150MB download):
268 $ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git
269 ------------------------------------------------
270
271 The initial clone may be time-consuming for a large project, but you
272 will only need to clone once.
273
274 The clone command creates a new directory named after the project
275 ("git" or "linux-2.6" in the examples above). After you cd into this
276 directory, you will see that it contains a copy of the project files,
277 together with a special top-level directory named ".git", which
278 contains all the information about the history of the project.
279
280 In most of the following, examples will be taken from one of the two
281 repositories above.
282
283 How to check out a different version of a project
284 -------------------------------------------------
285
286 Git is best thought of as a tool for storing the history of a
287 collection of files. It stores the history as a compressed
288 collection of interrelated snapshots (versions) of the project's
289 contents.
290
291 A single git repository may contain multiple branches. Each branch
292 is a bookmark referencing a particular point in the project history.
293 The gitlink:git-branch[1] command shows you the list of branches:
294
295 ------------------------------------------------
296 $ git branch
297 * master
298 ------------------------------------------------
299
300 A freshly cloned repository contains a single branch, named "master",
301 and the working directory contains the version of the project
302 referred to by the master branch.
303
304 Most projects also use tags. Tags, like branches, are references
305 into the project's history, and can be listed using the
306 gitlink:git-tag[1] command:
307
308 ------------------------------------------------
309 $ git tag -l
310 v2.6.11
311 v2.6.11-tree
312 v2.6.12
313 v2.6.12-rc2
314 v2.6.12-rc3
315 v2.6.12-rc4
316 v2.6.12-rc5
317 v2.6.12-rc6
318 v2.6.13
319 ...
320 ------------------------------------------------
321
322 Tags are expected to always point at the same version of a project,
323 while branches are expected to advance as development progresses.
324
325 Create a new branch pointing to one of these versions and check it
326 out using gitlink:git-checkout[1]:
327
328 ------------------------------------------------
329 $ git checkout -b new v2.6.13
330 ------------------------------------------------
331
332 The working directory then reflects the contents that the project had
333 when it was tagged v2.6.13, and gitlink:git-branch[1] shows two
334 branches, with an asterisk marking the currently checked-out branch:
335
336 ------------------------------------------------
337 $ git branch
338 master
339 * new
340 ------------------------------------------------
341
342 If you decide that you'd rather see version 2.6.17, you can modify
343 the current branch to point at v2.6.17 instead, with
344
345 ------------------------------------------------
346 $ git reset --hard v2.6.17
347 ------------------------------------------------
348
349 Note that if the current branch was your only reference to a
350 particular point in history, then resetting that branch may leave you
351 with no way to find the history it used to point to; so use this
352 command carefully.
353
354 Understanding History: Commits
355 ------------------------------
356
357 Every change in the history of a project is represented by a commit.
358 The gitlink:git-show[1] command shows the most recent commit on the
359 current branch:
360
361 ------------------------------------------------
362 $ git show
363 commit 2b5f6dcce5bf94b9b119e9ed8d537098ec61c3d2
364 Author: Jamal Hadi Salim <hadi@cyberus.ca>
365 Date: Sat Dec 2 22:22:25 2006 -0800
366
367 [XFRM]: Fix aevent structuring to be more complete.
368
369 aevents can not uniquely identify an SA. We break the ABI with this
370 patch, but consensus is that since it is not yet utilized by any
371 (known) application then it is fine (better do it now than later).
372
373 Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
374 Signed-off-by: David S. Miller <davem@davemloft.net>
375
376 diff --git a/Documentation/networking/xfrm_sync.txt b/Documentation/networking/xfrm_sync.txt
377 index 8be626f..d7aac9d 100644
378 --- a/Documentation/networking/xfrm_sync.txt
379 +++ b/Documentation/networking/xfrm_sync.txt
380 @@ -47,10 +47,13 @@ aevent_id structure looks like:
381
382 struct xfrm_aevent_id {
383 struct xfrm_usersa_id sa_id;
384 + xfrm_address_t saddr;
385 __u32 flags;
386 + __u32 reqid;
387 };
388 ...
389 ------------------------------------------------
390
391 As you can see, a commit shows who made the latest change, what they
392 did, and why.
393
394 Every commit has a 40-hexdigit id, sometimes called the "object name"
395 or the "SHA1 id", shown on the first line of the "git show" output.
396 You can usually refer to a commit by a shorter name, such as a tag or a
397 branch name, but this longer name can also be useful. Most
398 importantly, it is a globally unique name for this commit: so if you
399 tell somebody else the object name (for example in email), then you are
400 guaranteed that name will refer to the same commit in their repository
401 that it does in yours (assuming their repository has that commit at
402 all).
403
404 Understanding history: commits, parents, and reachability
405 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
406
407 Every commit (except the very first commit in a project) also has a
408 parent commit which shows what happened before this commit.
409 Following the chain of parents will eventually take you back to the
410 beginning of the project.
411
412 However, the commits do not form a simple list; git allows lines of
413 development to diverge and then reconverge, and the point where two
414 lines of development reconverge is called a "merge". The commit
415 representing a merge can therefore have more than one parent, with
416 each parent representing the most recent commit on one of the lines
417 of development leading to that point.
418
419 The best way to see how this works is using the gitlink:gitk[1]
420 command; running gitk now on a git repository and looking for merge
421 commits will help understand how the git organizes history.
422
423 In the following, we say that commit X is "reachable" from commit Y
424 if commit X is an ancestor of commit Y. Equivalently, you could say
425 that Y is a descendent of X, or that there is a chain of parents
426 leading from commit Y to commit X.
427
428 Understanding history: History diagrams
429 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
430
431 We will sometimes represent git history using diagrams like the one
432 below. Commits are shown as "o", and the links between them with
433 lines drawn with - / and \. Time goes left to right:
434
435 o--o--o <-- Branch A
436 /
437 o--o--o <-- master
438 \
439 o--o--o <-- Branch B
440
441 If we need to talk about a particular commit, the character "o" may
442 be replaced with another letter or number.
443
444 Understanding history: What is a branch?
445 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
446
447 Though we've been using the word "branch" to mean a kind of reference
448 to a particular commit, the word branch is also commonly used to
449 refer to the line of commits leading up to that point. In the
450 example above, git may think of the branch named "A" as just a
451 pointer to one particular commit, but we may refer informally to the
452 line of three commits leading up to that point as all being part of
453 "branch A".
454
455 If we need to make it clear that we're just talking about the most
456 recent commit on the branch, we may refer to that commit as the
457 "head" of the branch.
458
459 Manipulating branches
460 ---------------------
461
462 Creating, deleting, and modifying branches is quick and easy; here's
463 a summary of the commands:
464
465 git branch::
466 list all branches
467 git branch <branch>::
468 create a new branch named <branch>, referencing the same
469 point in history as the current branch
470 git branch <branch> <start-point>::
471 create a new branch named <branch>, referencing
472 <start-point>, which may be specified any way you like,
473 including using a branch name or a tag name
474 git branch -d <branch>::
475 delete the branch <branch>; if the branch you are deleting
476 points to a commit which is not reachable from this branch,
477 this command will fail with a warning.
478 git branch -D <branch>::
479 even if the branch points to a commit not reachable
480 from the current branch, you may know that that commit
481 is still reachable from some other branch or tag. In that
482 case it is safe to use this command to force git to delete
483 the branch.
484 git checkout <branch>::
485 make the current branch <branch>, updating the working
486 directory to reflect the version referenced by <branch>
487 git checkout -b <new> <start-point>::
488 create a new branch <new> referencing <start-point>, and
489 check it out.
490
491 It is also useful to know that the special symbol "HEAD" can always
492 be used to refer to the current branch.
493
494 Examining branches from a remote repository
495 -------------------------------------------
496
497 The "master" branch that was created at the time you cloned is a copy
498 of the HEAD in the repository that you cloned from. That repository
499 may also have had other branches, though, and your local repository
500 keeps branches which track each of those remote branches, which you
501 can view using the "-r" option to gitlink:git-branch[1]:
502
503 ------------------------------------------------
504 $ git branch -r
505 origin/HEAD
506 origin/html
507 origin/maint
508 origin/man
509 origin/master
510 origin/next
511 origin/pu
512 origin/todo
513 ------------------------------------------------
514
515 You cannot check out these remote-tracking branches, but you can
516 examine them on a branch of your own, just as you would a tag:
517
518 ------------------------------------------------
519 $ git checkout -b my-todo-copy origin/todo
520 ------------------------------------------------
521
522 Note that the name "origin" is just the name that git uses by default
523 to refer to the repository that you cloned from.
524
525 [[how-git-stores-references]]
526 Naming branches, tags, and other references
527 -------------------------------------------
528
529 Branches, remote-tracking branches, and tags are all references to
530 commits. All references are named with a slash-separated path name
531 starting with "refs"; the names we've been using so far are actually
532 shorthand:
533
534 - The branch "test" is short for "refs/heads/test".
535 - The tag "v2.6.18" is short for "refs/tags/v2.6.18".
536 - "origin/master" is short for "refs/remotes/origin/master".
537
538 The full name is occasionally useful if, for example, there ever
539 exists a tag and a branch with the same name.
540
541 As another useful shortcut, if the repository "origin" posesses only
542 a single branch, you can refer to that branch as just "origin".
543
544 More generally, if you have defined a remote repository named
545 "example", you can refer to the branch in that repository as
546 "example". And for a repository with multiple branches, this will
547 refer to the branch designated as the "HEAD" branch.
548
549 For the complete list of paths which git checks for references, and
550 the order it uses to decide which to choose when there are multiple
551 references with the same shorthand name, see the "SPECIFYING
552 REVISIONS" section of gitlink:git-rev-parse[1].
553
554 [[Updating-a-repository-with-git-fetch]]
555 Updating a repository with git fetch
556 ------------------------------------
557
558 Eventually the developer cloned from will do additional work in her
559 repository, creating new commits and advancing the branches to point
560 at the new commits.
561
562 The command "git fetch", with no arguments, will update all of the
563 remote-tracking branches to the latest version found in her
564 repository. It will not touch any of your own branches--not even the
565 "master" branch that was created for you on clone.
566
567 Fetching branches from other repositories
568 -----------------------------------------
569
570 You can also track branches from repositories other than the one you
571 cloned from, using gitlink:git-remote[1]:
572
573 -------------------------------------------------
574 $ git remote add linux-nfs git://linux-nfs.org/pub/nfs-2.6.git
575 $ git fetch
576 * refs/remotes/linux-nfs/master: storing branch 'master' ...
577 commit: bf81b46
578 -------------------------------------------------
579
580 New remote-tracking branches will be stored under the shorthand name
581 that you gave "git remote add", in this case linux-nfs:
582
583 -------------------------------------------------
584 $ git branch -r
585 linux-nfs/master
586 origin/master
587 -------------------------------------------------
588
589 If you run "git fetch <remote>" later, the tracking branches for the
590 named <remote> will be updated.
591
592 If you examine the file .git/config, you will see that git has added
593 a new stanza:
594
595 -------------------------------------------------
596 $ cat .git/config
597 ...
598 [remote "linux-nfs"]
599 url = git://linux-nfs.org/~bfields/git.git
600 fetch = +refs/heads/*:refs/remotes/linux-nfs-read/*
601 ...
602 -------------------------------------------------
603
604 This is what causes git to track the remote's branches; you may modify
605 or delete these configuration options by editing .git/config with a
606 text editor. (See the "CONFIGURATION FILE" section of
607 gitlink:git-config[1] for details.)
608
609 Exploring git history
610 =====================
611
612 Git is best thought of as a tool for storing the history of a
613 collection of files. It does this by storing compressed snapshots of
614 the contents of a file heirarchy, together with "commits" which show
615 the relationships between these snapshots.
616
617 Git provides extremely flexible and fast tools for exploring the
618 history of a project.
619
620 We start with one specialized tool that is useful for finding the
621 commit that introduced a bug into a project.
622
623 How to use bisect to find a regression
624 --------------------------------------
625
626 Suppose version 2.6.18 of your project worked, but the version at
627 "master" crashes. Sometimes the best way to find the cause of such a
628 regression is to perform a brute-force search through the project's
629 history to find the particular commit that caused the problem. The
630 gitlink:git-bisect[1] command can help you do this:
631
632 -------------------------------------------------
633 $ git bisect start
634 $ git bisect good v2.6.18
635 $ git bisect bad master
636 Bisecting: 3537 revisions left to test after this
637 [65934a9a028b88e83e2b0f8b36618fe503349f8e] BLOCK: Make USB storage depend on SCSI rather than selecting it [try #6]
638 -------------------------------------------------
639
640 If you run "git branch" at this point, you'll see that git has
641 temporarily moved you to a new branch named "bisect". This branch
642 points to a commit (with commit id 65934...) that is reachable from
643 v2.6.19 but not from v2.6.18. Compile and test it, and see whether
644 it crashes. Assume it does crash. Then:
645
646 -------------------------------------------------
647 $ git bisect bad
648 Bisecting: 1769 revisions left to test after this
649 [7eff82c8b1511017ae605f0c99ac275a7e21b867] i2c-core: Drop useless bitmaskings
650 -------------------------------------------------
651
652 checks out an older version. Continue like this, telling git at each
653 stage whether the version it gives you is good or bad, and notice
654 that the number of revisions left to test is cut approximately in
655 half each time.
656
657 After about 13 tests (in this case), it will output the commit id of
658 the guilty commit. You can then examine the commit with
659 gitlink:git-show[1], find out who wrote it, and mail them your bug
660 report with the commit id. Finally, run
661
662 -------------------------------------------------
663 $ git bisect reset
664 -------------------------------------------------
665
666 to return you to the branch you were on before and delete the
667 temporary "bisect" branch.
668
669 Note that the version which git-bisect checks out for you at each
670 point is just a suggestion, and you're free to try a different
671 version if you think it would be a good idea. For example,
672 occasionally you may land on a commit that broke something unrelated;
673 run
674
675 -------------------------------------------------
676 $ git bisect-visualize
677 -------------------------------------------------
678
679 which will run gitk and label the commit it chose with a marker that
680 says "bisect". Chose a safe-looking commit nearby, note its commit
681 id, and check it out with:
682
683 -------------------------------------------------
684 $ git reset --hard fb47ddb2db...
685 -------------------------------------------------
686
687 then test, run "bisect good" or "bisect bad" as appropriate, and
688 continue.
689
690 Naming commits
691 --------------
692
693 We have seen several ways of naming commits already:
694
695 - 40-hexdigit object name
696 - branch name: refers to the commit at the head of the given
697 branch
698 - tag name: refers to the commit pointed to by the given tag
699 (we've seen branches and tags are special cases of
700 <<how-git-stores-references,references>>).
701 - HEAD: refers to the head of the current branch
702
703 There are many more; see the "SPECIFYING REVISIONS" section of the
704 gitlink:git-rev-parse[1] man page for the complete list of ways to
705 name revisions. Some examples:
706
707 -------------------------------------------------
708 $ git show fb47ddb2 # the first few characters of the object name
709 # are usually enough to specify it uniquely
710 $ git show HEAD^ # the parent of the HEAD commit
711 $ git show HEAD^^ # the grandparent
712 $ git show HEAD~4 # the great-great-grandparent
713 -------------------------------------------------
714
715 Recall that merge commits may have more than one parent; by default,
716 ^ and ~ follow the first parent listed in the commit, but you can
717 also choose:
718
719 -------------------------------------------------
720 $ git show HEAD^1 # show the first parent of HEAD
721 $ git show HEAD^2 # show the second parent of HEAD
722 -------------------------------------------------
723
724 In addition to HEAD, there are several other special names for
725 commits:
726
727 Merges (to be discussed later), as well as operations such as
728 git-reset, which change the currently checked-out commit, generally
729 set ORIG_HEAD to the value HEAD had before the current operation.
730
731 The git-fetch operation always stores the head of the last fetched
732 branch in FETCH_HEAD. For example, if you run git fetch without
733 specifying a local branch as the target of the operation
734
735 -------------------------------------------------
736 $ git fetch git://example.com/proj.git theirbranch
737 -------------------------------------------------
738
739 the fetched commits will still be available from FETCH_HEAD.
740
741 When we discuss merges we'll also see the special name MERGE_HEAD,
742 which refers to the other branch that we're merging in to the current
743 branch.
744
745 The gitlink:git-rev-parse[1] command is a low-level command that is
746 occasionally useful for translating some name for a commit to the object
747 name for that commit:
748
749 -------------------------------------------------
750 $ git rev-parse origin
751 e05db0fd4f31dde7005f075a84f96b360d05984b
752 -------------------------------------------------
753
754 Creating tags
755 -------------
756
757 We can also create a tag to refer to a particular commit; after
758 running
759
760 -------------------------------------------------
761 $ git-tag stable-1 1b2e1d63ff
762 -------------------------------------------------
763
764 You can use stable-1 to refer to the commit 1b2e1d63ff.
765
766 This creates a "lightweight" tag. If the tag is a tag you wish to
767 share with others, and possibly sign cryptographically, then you
768 should create a tag object instead; see the gitlink:git-tag[1] man
769 page for details.
770
771 Browsing revisions
772 ------------------
773
774 The gitlink:git-log[1] command can show lists of commits. On its
775 own, it shows all commits reachable from the parent commit; but you
776 can also make more specific requests:
777
778 -------------------------------------------------
779 $ git log v2.5.. # commits since (not reachable from) v2.5
780 $ git log test..master # commits reachable from master but not test
781 $ git log master..test # ...reachable from test but not master
782 $ git log master...test # ...reachable from either test or master,
783 # but not both
784 $ git log --since="2 weeks ago" # commits from the last 2 weeks
785 $ git log Makefile # commits which modify Makefile
786 $ git log fs/ # ... which modify any file under fs/
787 $ git log -S'foo()' # commits which add or remove any file data
788 # matching the string 'foo()'
789 -------------------------------------------------
790
791 And of course you can combine all of these; the following finds
792 commits since v2.5 which touch the Makefile or any file under fs:
793
794 -------------------------------------------------
795 $ git log v2.5.. Makefile fs/
796 -------------------------------------------------
797
798 You can also ask git log to show patches:
799
800 -------------------------------------------------
801 $ git log -p
802 -------------------------------------------------
803
804 See the "--pretty" option in the gitlink:git-log[1] man page for more
805 display options.
806
807 Note that git log starts with the most recent commit and works
808 backwards through the parents; however, since git history can contain
809 multiple independent lines of development, the particular order that
810 commits are listed in may be somewhat arbitrary.
811
812 Generating diffs
813 ----------------
814
815 You can generate diffs between any two versions using
816 gitlink:git-diff[1]:
817
818 -------------------------------------------------
819 $ git diff master..test
820 -------------------------------------------------
821
822 Sometimes what you want instead is a set of patches:
823
824 -------------------------------------------------
825 $ git format-patch master..test
826 -------------------------------------------------
827
828 will generate a file with a patch for each commit reachable from test
829 but not from master. Note that if master also has commits which are
830 not reachable from test, then the combined result of these patches
831 will not be the same as the diff produced by the git-diff example.
832
833 Viewing old file versions
834 -------------------------
835
836 You can always view an old version of a file by just checking out the
837 correct revision first. But sometimes it is more convenient to be
838 able to view an old version of a single file without checking
839 anything out; this command does that:
840
841 -------------------------------------------------
842 $ git show v2.5:fs/locks.c
843 -------------------------------------------------
844
845 Before the colon may be anything that names a commit, and after it
846 may be any path to a file tracked by git.
847
848 Examples
849 --------
850
851 Check whether two branches point at the same history
852 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
853
854 Suppose you want to check whether two branches point at the same point
855 in history.
856
857 -------------------------------------------------
858 $ git diff origin..master
859 -------------------------------------------------
860
861 will tell you whether the contents of the project are the same at the
862 two branches; in theory, however, it's possible that the same project
863 contents could have been arrived at by two different historical
864 routes. You could compare the object names:
865
866 -------------------------------------------------
867 $ git rev-list origin
868 e05db0fd4f31dde7005f075a84f96b360d05984b
869 $ git rev-list master
870 e05db0fd4f31dde7005f075a84f96b360d05984b
871 -------------------------------------------------
872
873 Or you could recall that the ... operator selects all commits
874 contained reachable from either one reference or the other but not
875 both: so
876
877 -------------------------------------------------
878 $ git log origin...master
879 -------------------------------------------------
880
881 will return no commits when the two branches are equal.
882
883 Find first tagged version including a given fix
884 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
885
886 Suppose you know that the commit e05db0fd fixed a certain problem.
887 You'd like to find the earliest tagged release that contains that
888 fix.
889
890 Of course, there may be more than one answer--if the history branched
891 after commit e05db0fd, then there could be multiple "earliest" tagged
892 releases.
893
894 You could just visually inspect the commits since e05db0fd:
895
896 -------------------------------------------------
897 $ gitk e05db0fd..
898 -------------------------------------------------
899
900 Or you can use gitlink:git-name-rev[1], which will give the commit a
901 name based on any tag it finds pointing to one of the commit's
902 descendants:
903
904 -------------------------------------------------
905 $ git name-rev e05db0fd
906 e05db0fd tags/v1.5.0-rc1^0~23
907 -------------------------------------------------
908
909 The gitlink:git-describe[1] command does the opposite, naming the
910 revision using a tag on which the given commit is based:
911
912 -------------------------------------------------
913 $ git describe e05db0fd
914 v1.5.0-rc0-ge05db0f
915 -------------------------------------------------
916
917 but that may sometimes help you guess which tags might come after the
918 given commit.
919
920 If you just want to verify whether a given tagged version contains a
921 given commit, you could use gitlink:git-merge-base[1]:
922
923 -------------------------------------------------
924 $ git merge-base e05db0fd v1.5.0-rc1
925 e05db0fd4f31dde7005f075a84f96b360d05984b
926 -------------------------------------------------
927
928 The merge-base command finds a common ancestor of the given commits,
929 and always returns one or the other in the case where one is a
930 descendant of the other; so the above output shows that e05db0fd
931 actually is an ancestor of v1.5.0-rc1.
932
933 Alternatively, note that
934
935 -------------------------------------------------
936 $ git log v1.5.0-rc1..e05db0fd
937 -------------------------------------------------
938
939 will produce empty output if and only if v1.5.0-rc1 includes e05db0fd,
940 because it outputs only commits that are not reachable from v1.5.0-rc1.
941
942 As yet another alternative, the gitlink:git-show-branch[1] command lists
943 the commits reachable from its arguments with a display on the left-hand
944 side that indicates which arguments that commit is reachable from. So,
945 you can run something like
946
947 -------------------------------------------------
948 $ git show-branch e05db0fd v1.5.0-rc0 v1.5.0-rc1 v1.5.0-rc2
949 ! [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
950 available
951 ! [v1.5.0-rc0] GIT v1.5.0 preview
952 ! [v1.5.0-rc1] GIT v1.5.0-rc1
953 ! [v1.5.0-rc2] GIT v1.5.0-rc2
954 ...
955 -------------------------------------------------
956
957 then search for a line that looks like
958
959 -------------------------------------------------
960 + ++ [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
961 available
962 -------------------------------------------------
963
964 Which shows that e05db0fd is reachable from itself, from v1.5.0-rc1, and
965 from v1.5.0-rc2, but not from v1.5.0-rc0.
966
967
968 Developing with git
969 ===================
970
971 Telling git your name
972 ---------------------
973
974 Before creating any commits, you should introduce yourself to git. The
975 easiest way to do so is:
976
977 ------------------------------------------------
978 $ cat >~/.gitconfig <<\EOF
979 [user]
980 name = Your Name Comes Here
981 email = you@yourdomain.example.com
982 EOF
983 ------------------------------------------------
984
985 (See the "CONFIGURATION FILE" section of gitlink:git-config[1] for
986 details on the configuration file.)
987
988
989 Creating a new repository
990 -------------------------
991
992 Creating a new repository from scratch is very easy:
993
994 -------------------------------------------------
995 $ mkdir project
996 $ cd project
997 $ git init
998 -------------------------------------------------
999
1000 If you have some initial content (say, a tarball):
1001
1002 -------------------------------------------------
1003 $ tar -xzvf project.tar.gz
1004 $ cd project
1005 $ git init
1006 $ git add . # include everything below ./ in the first commit:
1007 $ git commit
1008 -------------------------------------------------
1009
1010 [[how-to-make-a-commit]]
1011 how to make a commit
1012 --------------------
1013
1014 Creating a new commit takes three steps:
1015
1016 1. Making some changes to the working directory using your
1017 favorite editor.
1018 2. Telling git about your changes.
1019 3. Creating the commit using the content you told git about
1020 in step 2.
1021
1022 In practice, you can interleave and repeat steps 1 and 2 as many
1023 times as you want: in order to keep track of what you want committed
1024 at step 3, git maintains a snapshot of the tree's contents in a
1025 special staging area called "the index."
1026
1027 At the beginning, the content of the index will be identical to
1028 that of the HEAD. The command "git diff --cached", which shows
1029 the difference between the HEAD and the index, should therefore
1030 produce no output at that point.
1031
1032 Modifying the index is easy:
1033
1034 To update the index with the new contents of a modified file, use
1035
1036 -------------------------------------------------
1037 $ git add path/to/file
1038 -------------------------------------------------
1039
1040 To add the contents of a new file to the index, use
1041
1042 -------------------------------------------------
1043 $ git add path/to/file
1044 -------------------------------------------------
1045
1046 To remove a file from the index and from the working tree,
1047
1048 -------------------------------------------------
1049 $ git rm path/to/file
1050 -------------------------------------------------
1051
1052 After each step you can verify that
1053
1054 -------------------------------------------------
1055 $ git diff --cached
1056 -------------------------------------------------
1057
1058 always shows the difference between the HEAD and the index file--this
1059 is what you'd commit if you created the commit now--and that
1060
1061 -------------------------------------------------
1062 $ git diff
1063 -------------------------------------------------
1064
1065 shows the difference between the working tree and the index file.
1066
1067 Note that "git add" always adds just the current contents of a file
1068 to the index; further changes to the same file will be ignored unless
1069 you run git-add on the file again.
1070
1071 When you're ready, just run
1072
1073 -------------------------------------------------
1074 $ git commit
1075 -------------------------------------------------
1076
1077 and git will prompt you for a commit message and then create the new
1078 commit. Check to make sure it looks like what you expected with
1079
1080 -------------------------------------------------
1081 $ git show
1082 -------------------------------------------------
1083
1084 As a special shortcut,
1085
1086 -------------------------------------------------
1087 $ git commit -a
1088 -------------------------------------------------
1089
1090 will update the index with any files that you've modified or removed
1091 and create a commit, all in one step.
1092
1093 A number of commands are useful for keeping track of what you're
1094 about to commit:
1095
1096 -------------------------------------------------
1097 $ git diff --cached # difference between HEAD and the index; what
1098 # would be commited if you ran "commit" now.
1099 $ git diff # difference between the index file and your
1100 # working directory; changes that would not
1101 # be included if you ran "commit" now.
1102 $ git status # a brief per-file summary of the above.
1103 -------------------------------------------------
1104
1105 creating good commit messages
1106 -----------------------------
1107
1108 Though not required, it's a good idea to begin the commit message
1109 with a single short (less than 50 character) line summarizing the
1110 change, followed by a blank line and then a more thorough
1111 description. Tools that turn commits into email, for example, use
1112 the first line on the Subject line and the rest of the commit in the
1113 body.
1114
1115 how to merge
1116 ------------
1117
1118 You can rejoin two diverging branches of development using
1119 gitlink:git-merge[1]:
1120
1121 -------------------------------------------------
1122 $ git merge branchname
1123 -------------------------------------------------
1124
1125 merges the development in the branch "branchname" into the current
1126 branch. If there are conflicts--for example, if the same file is
1127 modified in two different ways in the remote branch and the local
1128 branch--then you are warned; the output may look something like this:
1129
1130 -------------------------------------------------
1131 $ git pull . next
1132 Trying really trivial in-index merge...
1133 fatal: Merge requires file-level merging
1134 Nope.
1135 Merging HEAD with 77976da35a11db4580b80ae27e8d65caf5208086
1136 Merging:
1137 15e2162 world
1138 77976da goodbye
1139 found 1 common ancestor(s):
1140 d122ed4 initial
1141 Auto-merging file.txt
1142 CONFLICT (content): Merge conflict in file.txt
1143 Automatic merge failed; fix conflicts and then commit the result.
1144 -------------------------------------------------
1145
1146 Conflict markers are left in the problematic files, and after
1147 you resolve the conflicts manually, you can update the index
1148 with the contents and run git commit, as you normally would when
1149 creating a new file.
1150
1151 If you examine the resulting commit using gitk, you will see that it
1152 has two parents, one pointing to the top of the current branch, and
1153 one to the top of the other branch.
1154
1155 In more detail:
1156
1157 [[resolving-a-merge]]
1158 Resolving a merge
1159 -----------------
1160
1161 When a merge isn't resolved automatically, git leaves the index and
1162 the working tree in a special state that gives you all the
1163 information you need to help resolve the merge.
1164
1165 Files with conflicts are marked specially in the index, so until you
1166 resolve the problem and update the index, git commit will fail:
1167
1168 -------------------------------------------------
1169 $ git commit
1170 file.txt: needs merge
1171 -------------------------------------------------
1172
1173 Also, git status will list those files as "unmerged".
1174
1175 All of the changes that git was able to merge automatically are
1176 already added to the index file, so gitlink:git-diff[1] shows only
1177 the conflicts. Also, it uses a somewhat unusual syntax:
1178
1179 -------------------------------------------------
1180 $ git diff
1181 diff --cc file.txt
1182 index 802992c,2b60207..0000000
1183 --- a/file.txt
1184 +++ b/file.txt
1185 @@@ -1,1 -1,1 +1,5 @@@
1186 ++<<<<<<< HEAD:file.txt
1187 +Hello world
1188 ++=======
1189 + Goodbye
1190 ++>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
1191 -------------------------------------------------
1192
1193 Recall that the commit which will be commited after we resolve this
1194 conflict will have two parents instead of the usual one: one parent
1195 will be HEAD, the tip of the current branch; the other will be the
1196 tip of the other branch, which is stored temporarily in MERGE_HEAD.
1197
1198 The diff above shows the differences between the working-tree version
1199 of file.txt and two previous version: one version from HEAD, and one
1200 from MERGE_HEAD. So instead of preceding each line by a single "+"
1201 or "-", it now uses two columns: the first column is used for
1202 differences between the first parent and the working directory copy,
1203 and the second for differences between the second parent and the
1204 working directory copy. Thus after resolving the conflict in the
1205 obvious way, the diff will look like:
1206
1207 -------------------------------------------------
1208 $ git diff
1209 diff --cc file.txt
1210 index 802992c,2b60207..0000000
1211 --- a/file.txt
1212 +++ b/file.txt
1213 @@@ -1,1 -1,1 +1,1 @@@
1214 - Hello world
1215 -Goodbye
1216 ++Goodbye world
1217 -------------------------------------------------
1218
1219 This shows that our resolved version deleted "Hello world" from the
1220 first parent, deleted "Goodbye" from the second parent, and added
1221 "Goodbye world", which was previously absent from both.
1222
1223 The gitlink:git-log[1] command also provides special help for merges:
1224
1225 -------------------------------------------------
1226 $ git log --merge
1227 -------------------------------------------------
1228
1229 This will list all commits which exist only on HEAD or on MERGE_HEAD,
1230 and which touch an unmerged file.
1231
1232 We can now add the resolved version to the index and commit:
1233
1234 -------------------------------------------------
1235 $ git add file.txt
1236 $ git commit
1237 -------------------------------------------------
1238
1239 Note that the commit message will already be filled in for you with
1240 some information about the merge. Normally you can just use this
1241 default message unchanged, but you may add additional commentary of
1242 your own if desired.
1243
1244 [[undoing-a-merge]]
1245 undoing a merge
1246 ---------------
1247
1248 If you get stuck and decide to just give up and throw the whole mess
1249 away, you can always return to the pre-merge state with
1250
1251 -------------------------------------------------
1252 $ git reset --hard HEAD
1253 -------------------------------------------------
1254
1255 Or, if you've already commited the merge that you want to throw away,
1256
1257 -------------------------------------------------
1258 $ git reset --hard HEAD^
1259 -------------------------------------------------
1260
1261 However, this last command can be dangerous in some cases--never
1262 throw away a commit you have already committed if that commit may
1263 itself have been merged into another branch, as doing so may confuse
1264 further merges.
1265
1266 Fast-forward merges
1267 -------------------
1268
1269 There is one special case not mentioned above, which is treated
1270 differently. Normally, a merge results in a merge commit, with two
1271 parents, one pointing at each of the two lines of development that
1272 were merged.
1273
1274 However, if one of the two lines of development is completely
1275 contained within the other--so every commit present in the one is
1276 already contained in the other--then git just performs a
1277 <<fast-forwards,fast forward>>; the head of the current branch is
1278 moved forward to point at the head of the merged-in branch, without
1279 any new commits being created.
1280
1281 Fixing mistakes
1282 ---------------
1283
1284 If you've messed up the working tree, but haven't yet committed your
1285 mistake, you can return the entire working tree to the last committed
1286 state with
1287
1288 -------------------------------------------------
1289 $ git reset --hard HEAD
1290 -------------------------------------------------
1291
1292 If you make a commit that you later wish you hadn't, there are two
1293 fundamentally different ways to fix the problem:
1294
1295 1. You can create a new commit that undoes whatever was done
1296 by the previous commit. This is the correct thing if your
1297 mistake has already been made public.
1298
1299 2. You can go back and modify the old commit. You should
1300 never do this if you have already made the history public;
1301 git does not normally expect the "history" of a project to
1302 change, and cannot correctly perform repeated merges from
1303 a branch that has had its history changed.
1304
1305 Fixing a mistake with a new commit
1306 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1307
1308 Creating a new commit that reverts an earlier change is very easy;
1309 just pass the gitlink:git-revert[1] command a reference to the bad
1310 commit; for example, to revert the most recent commit:
1311
1312 -------------------------------------------------
1313 $ git revert HEAD
1314 -------------------------------------------------
1315
1316 This will create a new commit which undoes the change in HEAD. You
1317 will be given a chance to edit the commit message for the new commit.
1318
1319 You can also revert an earlier change, for example, the next-to-last:
1320
1321 -------------------------------------------------
1322 $ git revert HEAD^
1323 -------------------------------------------------
1324
1325 In this case git will attempt to undo the old change while leaving
1326 intact any changes made since then. If more recent changes overlap
1327 with the changes to be reverted, then you will be asked to fix
1328 conflicts manually, just as in the case of <<resolving-a-merge,
1329 resolving a merge>>.
1330
1331 Fixing a mistake by editing history
1332 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1333
1334 If the problematic commit is the most recent commit, and you have not
1335 yet made that commit public, then you may just
1336 <<undoing-a-merge,destroy it using git-reset>>.
1337
1338 Alternatively, you
1339 can edit the working directory and update the index to fix your
1340 mistake, just as if you were going to <<how-to-make-a-commit,create a
1341 new commit>>, then run
1342
1343 -------------------------------------------------
1344 $ git commit --amend
1345 -------------------------------------------------
1346
1347 which will replace the old commit by a new commit incorporating your
1348 changes, giving you a chance to edit the old commit message first.
1349
1350 Again, you should never do this to a commit that may already have
1351 been merged into another branch; use gitlink:git-revert[1] instead in
1352 that case.
1353
1354 It is also possible to edit commits further back in the history, but
1355 this is an advanced topic to be left for
1356 <<cleaning-up-history,another chapter>>.
1357
1358 Checking out an old version of a file
1359 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1360
1361 In the process of undoing a previous bad change, you may find it
1362 useful to check out an older version of a particular file using
1363 gitlink:git-checkout[1]. We've used git checkout before to switch
1364 branches, but it has quite different behavior if it is given a path
1365 name: the command
1366
1367 -------------------------------------------------
1368 $ git checkout HEAD^ path/to/file
1369 -------------------------------------------------
1370
1371 replaces path/to/file by the contents it had in the commit HEAD^, and
1372 also updates the index to match. It does not change branches.
1373
1374 If you just want to look at an old version of the file, without
1375 modifying the working directory, you can do that with
1376 gitlink:git-show[1]:
1377
1378 -------------------------------------------------
1379 $ git show HEAD^ path/to/file
1380 -------------------------------------------------
1381
1382 which will display the given version of the file.
1383
1384 Ensuring good performance
1385 -------------------------
1386
1387 On large repositories, git depends on compression to keep the history
1388 information from taking up to much space on disk or in memory.
1389
1390 This compression is not performed automatically. Therefore you
1391 should occasionally run gitlink:git-gc[1]:
1392
1393 -------------------------------------------------
1394 $ git gc
1395 -------------------------------------------------
1396
1397 to recompress the archive. This can be very time-consuming, so
1398 you may prefer to run git-gc when you are not doing other work.
1399
1400 Ensuring reliability
1401 --------------------
1402
1403 Checking the repository for corruption
1404 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1405
1406 The gitlink:git-fsck[1] command runs a number of self-consistency checks
1407 on the repository, and reports on any problems. This may take some
1408 time. The most common warning by far is about "dangling" objects:
1409
1410 -------------------------------------------------
1411 $ git fsck
1412 dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
1413 dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
1414 dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
1415 dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb
1416 dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f
1417 dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e
1418 dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085
1419 dangling tree b24c2473f1fd3d91352a624795be026d64c8841f
1420 ...
1421 -------------------------------------------------
1422
1423 Dangling objects are objects that are harmless, but also unnecessary;
1424 you can remove them at any time with gitlink:git-prune[1] or the --prune
1425 option to gitlink:git-gc[1]:
1426
1427 -------------------------------------------------
1428 $ git gc --prune
1429 -------------------------------------------------
1430
1431 This may be time-consuming. Unlike most other git operations (including
1432 git-gc when run without any options), it is not safe to prune while
1433 other git operations are in progress in the same repository.
1434
1435 For more about dangling objects, see <<dangling-objects>>.
1436
1437
1438 Recovering lost changes
1439 ~~~~~~~~~~~~~~~~~~~~~~~
1440
1441 Reflogs
1442 ^^^^^^^
1443
1444 Say you modify a branch with gitlink:git-reset[1] --hard, and then
1445 realize that the branch was the only reference you had to that point in
1446 history.
1447
1448 Fortunately, git also keeps a log, called a "reflog", of all the
1449 previous values of each branch. So in this case you can still find the
1450 old history using, for example,
1451
1452 -------------------------------------------------
1453 $ git log master@{1}
1454 -------------------------------------------------
1455
1456 This lists the commits reachable from the previous version of the head.
1457 This syntax can be used to with any git command that accepts a commit,
1458 not just with git log. Some other examples:
1459
1460 -------------------------------------------------
1461 $ git show master@{2} # See where the branch pointed 2,
1462 $ git show master@{3} # 3, ... changes ago.
1463 $ gitk master@{yesterday} # See where it pointed yesterday,
1464 $ gitk master@{"1 week ago"} # ... or last week
1465 -------------------------------------------------
1466
1467 The reflogs are kept by default for 30 days, after which they may be
1468 pruned. See gitlink:git-reflog[1] and gitlink:git-gc[1] to learn
1469 how to control this pruning, and see the "SPECIFYING REVISIONS"
1470 section of gitlink:git-rev-parse[1] for details.
1471
1472 Note that the reflog history is very different from normal git history.
1473 While normal history is shared by every repository that works on the
1474 same project, the reflog history is not shared: it tells you only about
1475 how the branches in your local repository have changed over time.
1476
1477 Examining dangling objects
1478 ^^^^^^^^^^^^^^^^^^^^^^^^^^
1479
1480 In some situations the reflog may not be able to save you. For
1481 example, suppose you delete a branch, then realize you need the history
1482 it pointed you. The reflog is also deleted; however, if you have not
1483 yet pruned the repository, then you may still be able to find
1484 the lost commits; run git-fsck and watch for output that mentions
1485 "dangling commits":
1486
1487 -------------------------------------------------
1488 $ git fsck
1489 dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
1490 dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
1491 dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
1492 ...
1493 -------------------------------------------------
1494
1495 You can examine
1496 one of those dangling commits with, for example,
1497
1498 ------------------------------------------------
1499 $ gitk 7281251ddd --not --all
1500 ------------------------------------------------
1501
1502 which does what it sounds like: it says that you want to see the commit
1503 history that is described by the dangling commit(s), but not the
1504 history that is described by all your existing branches and tags. Thus
1505 you get exactly the history reachable from that commit that is lost.
1506 (And notice that it might not be just one commit: we only report the
1507 "tip of the line" as being dangling, but there might be a whole deep
1508 and complex commit history that was gotten dropped.)
1509
1510 If you decide you want the history back, you can always create a new
1511 reference pointing to it, for example, a new branch:
1512
1513 ------------------------------------------------
1514 $ git branch recovered-branch 7281251ddd
1515 ------------------------------------------------
1516
1517
1518 Sharing development with others
1519 ===============================
1520
1521 [[getting-updates-with-git-pull]]
1522 Getting updates with git pull
1523 -----------------------------
1524
1525 After you clone a repository and make a few changes of your own, you
1526 may wish to check the original repository for updates and merge them
1527 into your own work.
1528
1529 We have already seen <<Updating-a-repository-with-git-fetch,how to
1530 keep remote tracking branches up to date>> with gitlink:git-fetch[1],
1531 and how to merge two branches. So you can merge in changes from the
1532 original repository's master branch with:
1533
1534 -------------------------------------------------
1535 $ git fetch
1536 $ git merge origin/master
1537 -------------------------------------------------
1538
1539 However, the gitlink:git-pull[1] command provides a way to do this in
1540 one step:
1541
1542 -------------------------------------------------
1543 $ git pull origin master
1544 -------------------------------------------------
1545
1546 In fact, "origin" is normally the default repository to pull from,
1547 and the default branch is normally the HEAD of the remote repository,
1548 so often you can accomplish the above with just
1549
1550 -------------------------------------------------
1551 $ git pull
1552 -------------------------------------------------
1553
1554 See the descriptions of the branch.<name>.remote and
1555 branch.<name>.merge options in gitlink:git-config[1] to learn
1556 how to control these defaults depending on the current branch.
1557
1558 In addition to saving you keystrokes, "git pull" also helps you by
1559 producing a default commit message documenting the branch and
1560 repository that you pulled from.
1561
1562 (But note that no such commit will be created in the case of a
1563 <<fast-forwards,fast forward>>; instead, your branch will just be
1564 updated to point to the latest commit from the upstream branch).
1565
1566 The git-pull command can also be given "." as the "remote" repository,
1567 in which case it just merges in a branch from the current repository; so
1568 the commands
1569
1570 -------------------------------------------------
1571 $ git pull . branch
1572 $ git merge branch
1573 -------------------------------------------------
1574
1575 are roughly equivalent. The former is actually very commonly used.
1576
1577 Submitting patches to a project
1578 -------------------------------
1579
1580 If you just have a few changes, the simplest way to submit them may
1581 just be to send them as patches in email:
1582
1583 First, use gitlink:git-format-patch[1]; for example:
1584
1585 -------------------------------------------------
1586 $ git format-patch origin
1587 -------------------------------------------------
1588
1589 will produce a numbered series of files in the current directory, one
1590 for each patch in the current branch but not in origin/HEAD.
1591
1592 You can then import these into your mail client and send them by
1593 hand. However, if you have a lot to send at once, you may prefer to
1594 use the gitlink:git-send-email[1] script to automate the process.
1595 Consult the mailing list for your project first to determine how they
1596 prefer such patches be handled.
1597
1598 Importing patches to a project
1599 ------------------------------
1600
1601 Git also provides a tool called gitlink:git-am[1] (am stands for
1602 "apply mailbox"), for importing such an emailed series of patches.
1603 Just save all of the patch-containing messages, in order, into a
1604 single mailbox file, say "patches.mbox", then run
1605
1606 -------------------------------------------------
1607 $ git am -3 patches.mbox
1608 -------------------------------------------------
1609
1610 Git will apply each patch in order; if any conflicts are found, it
1611 will stop, and you can fix the conflicts as described in
1612 "<<resolving-a-merge,Resolving a merge>>". (The "-3" option tells
1613 git to perform a merge; if you would prefer it just to abort and
1614 leave your tree and index untouched, you may omit that option.)
1615
1616 Once the index is updated with the results of the conflict
1617 resolution, instead of creating a new commit, just run
1618
1619 -------------------------------------------------
1620 $ git am --resolved
1621 -------------------------------------------------
1622
1623 and git will create the commit for you and continue applying the
1624 remaining patches from the mailbox.
1625
1626 The final result will be a series of commits, one for each patch in
1627 the original mailbox, with authorship and commit log message each
1628 taken from the message containing each patch.
1629
1630 [[setting-up-a-public-repository]]
1631 Setting up a public repository
1632 ------------------------------
1633
1634 Another way to submit changes to a project is to simply tell the
1635 maintainer of that project to pull from your repository, exactly as
1636 you did in the section "<<getting-updates-with-git-pull, Getting
1637 updates with git pull>>".
1638
1639 If you and maintainer both have accounts on the same machine, then
1640 then you can just pull changes from each other's repositories
1641 directly; note that all of the command (gitlink:git-clone[1],
1642 git-fetch[1], git-pull[1], etc.) which accept a URL as an argument
1643 will also accept a local file patch; so, for example, you can
1644 use
1645
1646 -------------------------------------------------
1647 $ git clone /path/to/repository
1648 $ git pull /path/to/other/repository
1649 -------------------------------------------------
1650
1651 If this sort of setup is inconvenient or impossible, another (more
1652 common) option is to set up a public repository on a public server.
1653 This also allows you to cleanly separate private work in progress
1654 from publicly visible work.
1655
1656 You will continue to do your day-to-day work in your personal
1657 repository, but periodically "push" changes from your personal
1658 repository into your public repository, allowing other developers to
1659 pull from that repository. So the flow of changes, in a situation
1660 where there is one other developer with a public repository, looks
1661 like this:
1662
1663 you push
1664 your personal repo ------------------> your public repo
1665 ^ |
1666 | |
1667 | you pull | they pull
1668 | |
1669 | |
1670 | they push V
1671 their public repo <------------------- their repo
1672
1673 Now, assume your personal repository is in the directory ~/proj. We
1674 first create a new clone of the repository:
1675
1676 -------------------------------------------------
1677 $ git clone --bare proj-clone.git
1678 -------------------------------------------------
1679
1680 The resulting directory proj-clone.git will contains a "bare" git
1681 repository--it is just the contents of the ".git" directory, without
1682 a checked-out copy of a working directory.
1683
1684 Next, copy proj-clone.git to the server where you plan to host the
1685 public repository. You can use scp, rsync, or whatever is most
1686 convenient.
1687
1688 If somebody else maintains the public server, they may already have
1689 set up a git service for you, and you may skip to the section
1690 "<<pushing-changes-to-a-public-repository,Pushing changes to a public
1691 repository>>", below.
1692
1693 Otherwise, the following sections explain how to export your newly
1694 created public repository:
1695
1696 [[exporting-via-http]]
1697 Exporting a git repository via http
1698 -----------------------------------
1699
1700 The git protocol gives better performance and reliability, but on a
1701 host with a web server set up, http exports may be simpler to set up.
1702
1703 All you need to do is place the newly created bare git repository in
1704 a directory that is exported by the web server, and make some
1705 adjustments to give web clients some extra information they need:
1706
1707 -------------------------------------------------
1708 $ mv proj.git /home/you/public_html/proj.git
1709 $ cd proj.git
1710 $ git update-server-info
1711 $ chmod a+x hooks/post-update
1712 -------------------------------------------------
1713
1714 (For an explanation of the last two lines, see
1715 gitlink:git-update-server-info[1], and the documentation
1716 link:hooks.txt[Hooks used by git].)
1717
1718 Advertise the url of proj.git. Anybody else should then be able to
1719 clone or pull from that url, for example with a commandline like:
1720
1721 -------------------------------------------------
1722 $ git clone http://yourserver.com/~you/proj.git
1723 -------------------------------------------------
1724
1725 (See also
1726 link:howto/setup-git-server-over-http.txt[setup-git-server-over-http]
1727 for a slightly more sophisticated setup using WebDAV which also
1728 allows pushing over http.)
1729
1730 [[exporting-via-git]]
1731 Exporting a git repository via the git protocol
1732 -----------------------------------------------
1733
1734 This is the preferred method.
1735
1736 For now, we refer you to the gitlink:git-daemon[1] man page for
1737 instructions. (See especially the examples section.)
1738
1739 [[pushing-changes-to-a-public-repository]]
1740 Pushing changes to a public repository
1741 --------------------------------------
1742
1743 Note that the two techniques outline above (exporting via
1744 <<exporting-via-http,http>> or <<exporting-via-git,git>>) allow other
1745 maintainers to fetch your latest changes, but they do not allow write
1746 access, which you will need to update the public repository with the
1747 latest changes created in your private repository.
1748
1749 The simplest way to do this is using gitlink:git-push[1] and ssh; to
1750 update the remote branch named "master" with the latest state of your
1751 branch named "master", run
1752
1753 -------------------------------------------------
1754 $ git push ssh://yourserver.com/~you/proj.git master:master
1755 -------------------------------------------------
1756
1757 or just
1758
1759 -------------------------------------------------
1760 $ git push ssh://yourserver.com/~you/proj.git master
1761 -------------------------------------------------
1762
1763 As with git-fetch, git-push will complain if this does not result in
1764 a <<fast-forwards,fast forward>>. Normally this is a sign of
1765 something wrong. However, if you are sure you know what you're
1766 doing, you may force git-push to perform the update anyway by
1767 proceeding the branch name by a plus sign:
1768
1769 -------------------------------------------------
1770 $ git push ssh://yourserver.com/~you/proj.git +master
1771 -------------------------------------------------
1772
1773 As with git-fetch, you may also set up configuration options to
1774 save typing; so, for example, after
1775
1776 -------------------------------------------------
1777 $ cat >.git/config <<EOF
1778 [remote "public-repo"]
1779 url = ssh://yourserver.com/~you/proj.git
1780 EOF
1781 -------------------------------------------------
1782
1783 you should be able to perform the above push with just
1784
1785 -------------------------------------------------
1786 $ git push public-repo master
1787 -------------------------------------------------
1788
1789 See the explanations of the remote.<name>.url, branch.<name>.remote,
1790 and remote.<name>.push options in gitlink:git-config[1] for
1791 details.
1792
1793 Setting up a shared repository
1794 ------------------------------
1795
1796 Another way to collaborate is by using a model similar to that
1797 commonly used in CVS, where several developers with special rights
1798 all push to and pull from a single shared repository. See
1799 link:cvs-migration.txt[git for CVS users] for instructions on how to
1800 set this up.
1801
1802 Allow web browsing of a repository
1803 ----------------------------------
1804
1805 The gitweb cgi script provides users an easy way to browse your
1806 project's files and history without having to install git; see the file
1807 gitweb/README in the git source tree for instructions on setting it up.
1808
1809 Examples
1810 --------
1811
1812 TODO: topic branches, typical roles as in everyday.txt, ?
1813
1814
1815 [[cleaning-up-history]]
1816 Rewriting history and maintaining patch series
1817 ==============================================
1818
1819 Normally commits are only added to a project, never taken away or
1820 replaced. Git is designed with this assumption, and violating it will
1821 cause git's merge machinery (for example) to do the wrong thing.
1822
1823 However, there is a situation in which it can be useful to violate this
1824 assumption.
1825
1826 Creating the perfect patch series
1827 ---------------------------------
1828
1829 Suppose you are a contributor to a large project, and you want to add a
1830 complicated feature, and to present it to the other developers in a way
1831 that makes it easy for them to read your changes, verify that they are
1832 correct, and understand why you made each change.
1833
1834 If you present all of your changes as a single patch (or commit), they
1835 may find it is too much to digest all at once.
1836
1837 If you present them with the entire history of your work, complete with
1838 mistakes, corrections, and dead ends, they may be overwhelmed.
1839
1840 So the ideal is usually to produce a series of patches such that:
1841
1842 1. Each patch can be applied in order.
1843
1844 2. Each patch includes a single logical change, together with a
1845 message explaining the change.
1846
1847 3. No patch introduces a regression: after applying any initial
1848 part of the series, the resulting project still compiles and
1849 works, and has no bugs that it didn't have before.
1850
1851 4. The complete series produces the same end result as your own
1852 (probably much messier!) development process did.
1853
1854 We will introduce some tools that can help you do this, explain how to
1855 use them, and then explain some of the problems that can arise because
1856 you are rewriting history.
1857
1858 Keeping a patch series up to date using git-rebase
1859 --------------------------------------------------
1860
1861 Suppose you have a series of commits in a branch "mywork", which
1862 originally branched off from "origin".
1863
1864 Suppose you create a branch "mywork" on a remote-tracking branch
1865 "origin", and created some commits on top of it:
1866
1867 -------------------------------------------------
1868 $ git checkout -b mywork origin
1869 $ vi file.txt
1870 $ git commit
1871 $ vi otherfile.txt
1872 $ git commit
1873 ...
1874 -------------------------------------------------
1875
1876 You have performed no merges into mywork, so it is just a simple linear
1877 sequence of patches on top of "origin":
1878
1879
1880 o--o--o <-- origin
1881 \
1882 o--o--o <-- mywork
1883
1884 Some more interesting work has been done in the upstream project, and
1885 "origin" has advanced:
1886
1887 o--o--O--o--o--o <-- origin
1888 \
1889 a--b--c <-- mywork
1890
1891 At this point, you could use "pull" to merge your changes back in;
1892 the result would create a new merge commit, like this:
1893
1894
1895 o--o--O--o--o--o <-- origin
1896 \ \
1897 a--b--c--m <-- mywork
1898
1899 However, if you prefer to keep the history in mywork a simple series of
1900 commits without any merges, you may instead choose to use
1901 gitlink:git-rebase[1]:
1902
1903 -------------------------------------------------
1904 $ git checkout mywork
1905 $ git rebase origin
1906 -------------------------------------------------
1907
1908 This will remove each of your commits from mywork, temporarily saving
1909 them as patches (in a directory named ".dotest"), update mywork to
1910 point at the latest version of origin, then apply each of the saved
1911 patches to the new mywork. The result will look like:
1912
1913
1914 o--o--O--o--o--o <-- origin
1915 \
1916 a'--b'--c' <-- mywork
1917
1918 In the process, it may discover conflicts. In that case it will stop
1919 and allow you to fix the conflicts; after fixing conflicts, use "git
1920 add" to update the index with those contents, and then, instead of
1921 running git-commit, just run
1922
1923 -------------------------------------------------
1924 $ git rebase --continue
1925 -------------------------------------------------
1926
1927 and git will continue applying the rest of the patches.
1928
1929 At any point you may use the --abort option to abort this process and
1930 return mywork to the state it had before you started the rebase:
1931
1932 -------------------------------------------------
1933 $ git rebase --abort
1934 -------------------------------------------------
1935
1936 Reordering or selecting from a patch series
1937 -------------------------------------------
1938
1939 Given one existing commit, the gitlink:git-cherry-pick[1] command
1940 allows you to apply the change introduced by that commit and create a
1941 new commit that records it. So, for example, if "mywork" points to a
1942 series of patches on top of "origin", you might do something like:
1943
1944 -------------------------------------------------
1945 $ git checkout -b mywork-new origin
1946 $ gitk origin..mywork &
1947 -------------------------------------------------
1948
1949 And browse through the list of patches in the mywork branch using gitk,
1950 applying them (possibly in a different order) to mywork-new using
1951 cherry-pick, and possibly modifying them as you go using commit
1952 --amend.
1953
1954 Another technique is to use git-format-patch to create a series of
1955 patches, then reset the state to before the patches:
1956
1957 -------------------------------------------------
1958 $ git format-patch origin
1959 $ git reset --hard origin
1960 -------------------------------------------------
1961
1962 Then modify, reorder, or eliminate patches as preferred before applying
1963 them again with gitlink:git-am[1].
1964
1965 Other tools
1966 -----------
1967
1968 There are numerous other tools, such as stgit, which exist for the
1969 purpose of maintaining a patch series. These are out of the scope of
1970 this manual.
1971
1972 Problems with rewriting history
1973 -------------------------------
1974
1975 The primary problem with rewriting the history of a branch has to do
1976 with merging. Suppose somebody fetches your branch and merges it into
1977 their branch, with a result something like this:
1978
1979 o--o--O--o--o--o <-- origin
1980 \ \
1981 t--t--t--m <-- their branch:
1982
1983 Then suppose you modify the last three commits:
1984
1985 o--o--o <-- new head of origin
1986 /
1987 o--o--O--o--o--o <-- old head of origin
1988
1989 If we examined all this history together in one repository, it will
1990 look like:
1991
1992 o--o--o <-- new head of origin
1993 /
1994 o--o--O--o--o--o <-- old head of origin
1995 \ \
1996 t--t--t--m <-- their branch:
1997
1998 Git has no way of knowing that the new head is an updated version of
1999 the old head; it treats this situation exactly the same as it would if
2000 two developers had independently done the work on the old and new heads
2001 in parallel. At this point, if someone attempts to merge the new head
2002 in to their branch, git will attempt to merge together the two (old and
2003 new) lines of development, instead of trying to replace the old by the
2004 new. The results are likely to be unexpected.
2005
2006 You may still choose to publish branches whose history is rewritten,
2007 and it may be useful for others to be able to fetch those branches in
2008 order to examine or test them, but they should not attempt to pull such
2009 branches into their own work.
2010
2011 For true distributed development that supports proper merging,
2012 published branches should never be rewritten.
2013
2014 Advanced branch management
2015 ==========================
2016
2017 Fetching individual branches
2018 ----------------------------
2019
2020 Instead of using gitlink:git-remote[1], you can also choose just
2021 to update one branch at a time, and to store it locally under an
2022 arbitrary name:
2023
2024 -------------------------------------------------
2025 $ git fetch origin todo:my-todo-work
2026 -------------------------------------------------
2027
2028 The first argument, "origin", just tells git to fetch from the
2029 repository you originally cloned from. The second argument tells git
2030 to fetch the branch named "todo" from the remote repository, and to
2031 store it locally under the name refs/heads/my-todo-work.
2032
2033 You can also fetch branches from other repositories; so
2034
2035 -------------------------------------------------
2036 $ git fetch git://example.com/proj.git master:example-master
2037 -------------------------------------------------
2038
2039 will create a new branch named "example-master" and store in it the
2040 branch named "master" from the repository at the given URL. If you
2041 already have a branch named example-master, it will attempt to
2042 "fast-forward" to the commit given by example.com's master branch. So
2043 next we explain what a fast-forward is:
2044
2045 [[fast-forwards]]
2046 Understanding git history: fast-forwards
2047 ----------------------------------------
2048
2049 In the previous example, when updating an existing branch, "git
2050 fetch" checks to make sure that the most recent commit on the remote
2051 branch is a descendant of the most recent commit on your copy of the
2052 branch before updating your copy of the branch to point at the new
2053 commit. Git calls this process a "fast forward".
2054
2055 A fast forward looks something like this:
2056
2057 o--o--o--o <-- old head of the branch
2058 \
2059 o--o--o <-- new head of the branch
2060
2061
2062 In some cases it is possible that the new head will *not* actually be
2063 a descendant of the old head. For example, the developer may have
2064 realized she made a serious mistake, and decided to backtrack,
2065 resulting in a situation like:
2066
2067 o--o--o--o--a--b <-- old head of the branch
2068 \
2069 o--o--o <-- new head of the branch
2070
2071
2072
2073 In this case, "git fetch" will fail, and print out a warning.
2074
2075 In that case, you can still force git to update to the new head, as
2076 described in the following section. However, note that in the
2077 situation above this may mean losing the commits labeled "a" and "b",
2078 unless you've already created a reference of your own pointing to
2079 them.
2080
2081 Forcing git fetch to do non-fast-forward updates
2082 ------------------------------------------------
2083
2084 If git fetch fails because the new head of a branch is not a
2085 descendant of the old head, you may force the update with:
2086
2087 -------------------------------------------------
2088 $ git fetch git://example.com/proj.git +master:refs/remotes/example/master
2089 -------------------------------------------------
2090
2091 Note the addition of the "+" sign. Be aware that commits which the
2092 old version of example/master pointed at may be lost, as we saw in
2093 the previous section.
2094
2095 Configuring remote branches
2096 ---------------------------
2097
2098 We saw above that "origin" is just a shortcut to refer to the
2099 repository which you originally cloned from. This information is
2100 stored in git configuration variables, which you can see using
2101 gitlink:git-config[1]:
2102
2103 -------------------------------------------------
2104 $ git config -l
2105 core.repositoryformatversion=0
2106 core.filemode=true
2107 core.logallrefupdates=true
2108 remote.origin.url=git://git.kernel.org/pub/scm/git/git.git
2109 remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
2110 branch.master.remote=origin
2111 branch.master.merge=refs/heads/master
2112 -------------------------------------------------
2113
2114 If there are other repositories that you also use frequently, you can
2115 create similar configuration options to save typing; for example,
2116 after
2117
2118 -------------------------------------------------
2119 $ git config remote.example.url git://example.com/proj.git
2120 -------------------------------------------------
2121
2122 then the following two commands will do the same thing:
2123
2124 -------------------------------------------------
2125 $ git fetch git://example.com/proj.git master:refs/remotes/example/master
2126 $ git fetch example master:refs/remotes/example/master
2127 -------------------------------------------------
2128
2129 Even better, if you add one more option:
2130
2131 -------------------------------------------------
2132 $ git config remote.example.fetch master:refs/remotes/example/master
2133 -------------------------------------------------
2134
2135 then the following commands will all do the same thing:
2136
2137 -------------------------------------------------
2138 $ git fetch git://example.com/proj.git master:ref/remotes/example/master
2139 $ git fetch example master:ref/remotes/example/master
2140 $ git fetch example example/master
2141 $ git fetch example
2142 -------------------------------------------------
2143
2144 You can also add a "+" to force the update each time:
2145
2146 -------------------------------------------------
2147 $ git config remote.example.fetch +master:ref/remotes/example/master
2148 -------------------------------------------------
2149
2150 Don't do this unless you're sure you won't mind "git fetch" possibly
2151 throwing away commits on mybranch.
2152
2153 Also note that all of the above configuration can be performed by
2154 directly editing the file .git/config instead of using
2155 gitlink:git-config[1].
2156
2157 See gitlink:git-config[1] for more details on the configuration
2158 options mentioned above.
2159
2160
2161 Git internals
2162 =============
2163
2164 There are two object abstractions: the "object database", and the
2165 "current directory cache" aka "index".
2166
2167 The Object Database
2168 -------------------
2169
2170 The object database is literally just a content-addressable collection
2171 of objects. All objects are named by their content, which is
2172 approximated by the SHA1 hash of the object itself. Objects may refer
2173 to other objects (by referencing their SHA1 hash), and so you can
2174 build up a hierarchy of objects.
2175
2176 All objects have a statically determined "type" aka "tag", which is
2177 determined at object creation time, and which identifies the format of
2178 the object (i.e. how it is used, and how it can refer to other
2179 objects). There are currently four different object types: "blob",
2180 "tree", "commit" and "tag".
2181
2182 A "blob" object cannot refer to any other object, and is, like the type
2183 implies, a pure storage object containing some user data. It is used to
2184 actually store the file data, i.e. a blob object is associated with some
2185 particular version of some file.
2186
2187 A "tree" object is an object that ties one or more "blob" objects into a
2188 directory structure. In addition, a tree object can refer to other tree
2189 objects, thus creating a directory hierarchy.
2190
2191 A "commit" object ties such directory hierarchies together into
2192 a DAG of revisions - each "commit" is associated with exactly one tree
2193 (the directory hierarchy at the time of the commit). In addition, a
2194 "commit" refers to one or more "parent" commit objects that describe the
2195 history of how we arrived at that directory hierarchy.
2196
2197 As a special case, a commit object with no parents is called the "root"
2198 object, and is the point of an initial project commit. Each project
2199 must have at least one root, and while you can tie several different
2200 root objects together into one project by creating a commit object which
2201 has two or more separate roots as its ultimate parents, that's probably
2202 just going to confuse people. So aim for the notion of "one root object
2203 per project", even if git itself does not enforce that.
2204
2205 A "tag" object symbolically identifies and can be used to sign other
2206 objects. It contains the identifier and type of another object, a
2207 symbolic name (of course!) and, optionally, a signature.
2208
2209 Regardless of object type, all objects share the following
2210 characteristics: they are all deflated with zlib, and have a header
2211 that not only specifies their type, but also provides size information
2212 about the data in the object. It's worth noting that the SHA1 hash
2213 that is used to name the object is the hash of the original data
2214 plus this header, so `sha1sum` 'file' does not match the object name
2215 for 'file'.
2216 (Historical note: in the dawn of the age of git the hash
2217 was the sha1 of the 'compressed' object.)
2218
2219 As a result, the general consistency of an object can always be tested
2220 independently of the contents or the type of the object: all objects can
2221 be validated by verifying that (a) their hashes match the content of the
2222 file and (b) the object successfully inflates to a stream of bytes that
2223 forms a sequence of <ascii type without space> + <space> + <ascii decimal
2224 size> + <byte\0> + <binary object data>.
2225
2226 The structured objects can further have their structure and
2227 connectivity to other objects verified. This is generally done with
2228 the `git-fsck` program, which generates a full dependency graph
2229 of all objects, and verifies their internal consistency (in addition
2230 to just verifying their superficial consistency through the hash).
2231
2232 The object types in some more detail:
2233
2234 Blob Object
2235 -----------
2236
2237 A "blob" object is nothing but a binary blob of data, and doesn't
2238 refer to anything else. There is no signature or any other
2239 verification of the data, so while the object is consistent (it 'is'
2240 indexed by its sha1 hash, so the data itself is certainly correct), it
2241 has absolutely no other attributes. No name associations, no
2242 permissions. It is purely a blob of data (i.e. normally "file
2243 contents").
2244
2245 In particular, since the blob is entirely defined by its data, if two
2246 files in a directory tree (or in multiple different versions of the
2247 repository) have the same contents, they will share the same blob
2248 object. The object is totally independent of its location in the
2249 directory tree, and renaming a file does not change the object that
2250 file is associated with in any way.
2251
2252 A blob is typically created when gitlink:git-update-index[1]
2253 is run, and its data can be accessed by gitlink:git-cat-file[1].
2254
2255 Tree Object
2256 -----------
2257
2258 The next hierarchical object type is the "tree" object. A tree object
2259 is a list of mode/name/blob data, sorted by name. Alternatively, the
2260 mode data may specify a directory mode, in which case instead of
2261 naming a blob, that name is associated with another TREE object.
2262
2263 Like the "blob" object, a tree object is uniquely determined by the
2264 set contents, and so two separate but identical trees will always
2265 share the exact same object. This is true at all levels, i.e. it's
2266 true for a "leaf" tree (which does not refer to any other trees, only
2267 blobs) as well as for a whole subdirectory.
2268
2269 For that reason a "tree" object is just a pure data abstraction: it
2270 has no history, no signatures, no verification of validity, except
2271 that since the contents are again protected by the hash itself, we can
2272 trust that the tree is immutable and its contents never change.
2273
2274 So you can trust the contents of a tree to be valid, the same way you
2275 can trust the contents of a blob, but you don't know where those
2276 contents 'came' from.
2277
2278 Side note on trees: since a "tree" object is a sorted list of
2279 "filename+content", you can create a diff between two trees without
2280 actually having to unpack two trees. Just ignore all common parts,
2281 and your diff will look right. In other words, you can effectively
2282 (and efficiently) tell the difference between any two random trees by
2283 O(n) where "n" is the size of the difference, rather than the size of
2284 the tree.
2285
2286 Side note 2 on trees: since the name of a "blob" depends entirely and
2287 exclusively on its contents (i.e. there are no names or permissions
2288 involved), you can see trivial renames or permission changes by
2289 noticing that the blob stayed the same. However, renames with data
2290 changes need a smarter "diff" implementation.
2291
2292 A tree is created with gitlink:git-write-tree[1] and
2293 its data can be accessed by gitlink:git-ls-tree[1].
2294 Two trees can be compared with gitlink:git-diff-tree[1].
2295
2296 Commit Object
2297 -------------
2298
2299 The "commit" object is an object that introduces the notion of
2300 history into the picture. In contrast to the other objects, it
2301 doesn't just describe the physical state of a tree, it describes how
2302 we got there, and why.
2303
2304 A "commit" is defined by the tree-object that it results in, the
2305 parent commits (zero, one or more) that led up to that point, and a
2306 comment on what happened. Again, a commit is not trusted per se:
2307 the contents are well-defined and "safe" due to the cryptographically
2308 strong signatures at all levels, but there is no reason to believe
2309 that the tree is "good" or that the merge information makes sense.
2310 The parents do not have to actually have any relationship with the
2311 result, for example.
2312
2313 Note on commits: unlike real SCM's, commits do not contain
2314 rename information or file mode change information. All of that is
2315 implicit in the trees involved (the result tree, and the result trees
2316 of the parents), and describing that makes no sense in this idiotic
2317 file manager.
2318
2319 A commit is created with gitlink:git-commit-tree[1] and
2320 its data can be accessed by gitlink:git-cat-file[1].
2321
2322 Trust
2323 -----
2324
2325 An aside on the notion of "trust". Trust is really outside the scope
2326 of "git", but it's worth noting a few things. First off, since
2327 everything is hashed with SHA1, you 'can' trust that an object is
2328 intact and has not been messed with by external sources. So the name
2329 of an object uniquely identifies a known state - just not a state that
2330 you may want to trust.
2331
2332 Furthermore, since the SHA1 signature of a commit refers to the
2333 SHA1 signatures of the tree it is associated with and the signatures
2334 of the parent, a single named commit specifies uniquely a whole set
2335 of history, with full contents. You can't later fake any step of the
2336 way once you have the name of a commit.
2337
2338 So to introduce some real trust in the system, the only thing you need
2339 to do is to digitally sign just 'one' special note, which includes the
2340 name of a top-level commit. Your digital signature shows others
2341 that you trust that commit, and the immutability of the history of
2342 commits tells others that they can trust the whole history.
2343
2344 In other words, you can easily validate a whole archive by just
2345 sending out a single email that tells the people the name (SHA1 hash)
2346 of the top commit, and digitally sign that email using something
2347 like GPG/PGP.
2348
2349 To assist in this, git also provides the tag object...
2350
2351 Tag Object
2352 ----------
2353
2354 Git provides the "tag" object to simplify creating, managing and
2355 exchanging symbolic and signed tokens. The "tag" object at its
2356 simplest simply symbolically identifies another object by containing
2357 the sha1, type and symbolic name.
2358
2359 However it can optionally contain additional signature information
2360 (which git doesn't care about as long as there's less than 8k of
2361 it). This can then be verified externally to git.
2362
2363 Note that despite the tag features, "git" itself only handles content
2364 integrity; the trust framework (and signature provision and
2365 verification) has to come from outside.
2366
2367 A tag is created with gitlink:git-mktag[1],
2368 its data can be accessed by gitlink:git-cat-file[1],
2369 and the signature can be verified by
2370 gitlink:git-verify-tag[1].
2371
2372
2373 The "index" aka "Current Directory Cache"
2374 -----------------------------------------
2375
2376 The index is a simple binary file, which contains an efficient
2377 representation of a virtual directory content at some random time. It
2378 does so by a simple array that associates a set of names, dates,
2379 permissions and content (aka "blob") objects together. The cache is
2380 always kept ordered by name, and names are unique (with a few very
2381 specific rules) at any point in time, but the cache has no long-term
2382 meaning, and can be partially updated at any time.
2383
2384 In particular, the index certainly does not need to be consistent with
2385 the current directory contents (in fact, most operations will depend on
2386 different ways to make the index 'not' be consistent with the directory
2387 hierarchy), but it has three very important attributes:
2388
2389 '(a) it can re-generate the full state it caches (not just the
2390 directory structure: it contains pointers to the "blob" objects so
2391 that it can regenerate the data too)'
2392
2393 As a special case, there is a clear and unambiguous one-way mapping
2394 from a current directory cache to a "tree object", which can be
2395 efficiently created from just the current directory cache without
2396 actually looking at any other data. So a directory cache at any one
2397 time uniquely specifies one and only one "tree" object (but has
2398 additional data to make it easy to match up that tree object with what
2399 has happened in the directory)
2400
2401 '(b) it has efficient methods for finding inconsistencies between that
2402 cached state ("tree object waiting to be instantiated") and the
2403 current state.'
2404
2405 '(c) it can additionally efficiently represent information about merge
2406 conflicts between different tree objects, allowing each pathname to be
2407 associated with sufficient information about the trees involved that
2408 you can create a three-way merge between them.'
2409
2410 Those are the three ONLY things that the directory cache does. It's a
2411 cache, and the normal operation is to re-generate it completely from a
2412 known tree object, or update/compare it with a live tree that is being
2413 developed. If you blow the directory cache away entirely, you generally
2414 haven't lost any information as long as you have the name of the tree
2415 that it described.
2416
2417 At the same time, the index is at the same time also the
2418 staging area for creating new trees, and creating a new tree always
2419 involves a controlled modification of the index file. In particular,
2420 the index file can have the representation of an intermediate tree that
2421 has not yet been instantiated. So the index can be thought of as a
2422 write-back cache, which can contain dirty information that has not yet
2423 been written back to the backing store.
2424
2425
2426
2427 The Workflow
2428 ------------
2429
2430 Generally, all "git" operations work on the index file. Some operations
2431 work *purely* on the index file (showing the current state of the
2432 index), but most operations move data to and from the index file. Either
2433 from the database or from the working directory. Thus there are four
2434 main combinations:
2435
2436 working directory -> index
2437 ~~~~~~~~~~~~~~~~~~~~~~~~~~
2438
2439 You update the index with information from the working directory with
2440 the gitlink:git-update-index[1] command. You
2441 generally update the index information by just specifying the filename
2442 you want to update, like so:
2443
2444 -------------------------------------------------
2445 $ git-update-index filename
2446 -------------------------------------------------
2447
2448 but to avoid common mistakes with filename globbing etc, the command
2449 will not normally add totally new entries or remove old entries,
2450 i.e. it will normally just update existing cache entries.
2451
2452 To tell git that yes, you really do realize that certain files no
2453 longer exist, or that new files should be added, you
2454 should use the `--remove` and `--add` flags respectively.
2455
2456 NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
2457 necessarily be removed: if the files still exist in your directory
2458 structure, the index will be updated with their new status, not
2459 removed. The only thing `--remove` means is that update-cache will be
2460 considering a removed file to be a valid thing, and if the file really
2461 does not exist any more, it will update the index accordingly.
2462
2463 As a special case, you can also do `git-update-index --refresh`, which
2464 will refresh the "stat" information of each index to match the current
2465 stat information. It will 'not' update the object status itself, and
2466 it will only update the fields that are used to quickly test whether
2467 an object still matches its old backing store object.
2468
2469 index -> object database
2470 ~~~~~~~~~~~~~~~~~~~~~~~~
2471
2472 You write your current index file to a "tree" object with the program
2473
2474 -------------------------------------------------
2475 $ git-write-tree
2476 -------------------------------------------------
2477
2478 that doesn't come with any options - it will just write out the
2479 current index into the set of tree objects that describe that state,
2480 and it will return the name of the resulting top-level tree. You can
2481 use that tree to re-generate the index at any time by going in the
2482 other direction:
2483
2484 object database -> index
2485 ~~~~~~~~~~~~~~~~~~~~~~~~
2486
2487 You read a "tree" file from the object database, and use that to
2488 populate (and overwrite - don't do this if your index contains any
2489 unsaved state that you might want to restore later!) your current
2490 index. Normal operation is just
2491
2492 -------------------------------------------------
2493 $ git-read-tree <sha1 of tree>
2494 -------------------------------------------------
2495
2496 and your index file will now be equivalent to the tree that you saved
2497 earlier. However, that is only your 'index' file: your working
2498 directory contents have not been modified.
2499
2500 index -> working directory
2501 ~~~~~~~~~~~~~~~~~~~~~~~~~~
2502
2503 You update your working directory from the index by "checking out"
2504 files. This is not a very common operation, since normally you'd just
2505 keep your files updated, and rather than write to your working
2506 directory, you'd tell the index files about the changes in your
2507 working directory (i.e. `git-update-index`).
2508
2509 However, if you decide to jump to a new version, or check out somebody
2510 else's version, or just restore a previous tree, you'd populate your
2511 index file with read-tree, and then you need to check out the result
2512 with
2513
2514 -------------------------------------------------
2515 $ git-checkout-index filename
2516 -------------------------------------------------
2517
2518 or, if you want to check out all of the index, use `-a`.
2519
2520 NOTE! git-checkout-index normally refuses to overwrite old files, so
2521 if you have an old version of the tree already checked out, you will
2522 need to use the "-f" flag ('before' the "-a" flag or the filename) to
2523 'force' the checkout.
2524
2525
2526 Finally, there are a few odds and ends which are not purely moving
2527 from one representation to the other:
2528
2529 Tying it all together
2530 ~~~~~~~~~~~~~~~~~~~~~
2531
2532 To commit a tree you have instantiated with "git-write-tree", you'd
2533 create a "commit" object that refers to that tree and the history
2534 behind it - most notably the "parent" commits that preceded it in
2535 history.
2536
2537 Normally a "commit" has one parent: the previous state of the tree
2538 before a certain change was made. However, sometimes it can have two
2539 or more parent commits, in which case we call it a "merge", due to the
2540 fact that such a commit brings together ("merges") two or more
2541 previous states represented by other commits.
2542
2543 In other words, while a "tree" represents a particular directory state
2544 of a working directory, a "commit" represents that state in "time",
2545 and explains how we got there.
2546
2547 You create a commit object by giving it the tree that describes the
2548 state at the time of the commit, and a list of parents:
2549
2550 -------------------------------------------------
2551 $ git-commit-tree <tree> -p <parent> [-p <parent2> ..]
2552 -------------------------------------------------
2553
2554 and then giving the reason for the commit on stdin (either through
2555 redirection from a pipe or file, or by just typing it at the tty).
2556
2557 git-commit-tree will return the name of the object that represents
2558 that commit, and you should save it away for later use. Normally,
2559 you'd commit a new `HEAD` state, and while git doesn't care where you
2560 save the note about that state, in practice we tend to just write the
2561 result to the file pointed at by `.git/HEAD`, so that we can always see
2562 what the last committed state was.
2563
2564 Here is an ASCII art by Jon Loeliger that illustrates how
2565 various pieces fit together.
2566
2567 ------------
2568
2569 commit-tree
2570 commit obj
2571 +----+
2572 | |
2573 | |
2574 V V
2575 +-----------+
2576 | Object DB |
2577 | Backing |
2578 | Store |
2579 +-----------+
2580 ^
2581 write-tree | |
2582 tree obj | |
2583 | | read-tree
2584 | | tree obj
2585 V
2586 +-----------+
2587 | Index |
2588 | "cache" |
2589 +-----------+
2590 update-index ^
2591 blob obj | |
2592 | |
2593 checkout-index -u | | checkout-index
2594 stat | | blob obj
2595 V
2596 +-----------+
2597 | Working |
2598 | Directory |
2599 +-----------+
2600
2601 ------------
2602
2603
2604 Examining the data
2605 ------------------
2606
2607 You can examine the data represented in the object database and the
2608 index with various helper tools. For every object, you can use
2609 gitlink:git-cat-file[1] to examine details about the
2610 object:
2611
2612 -------------------------------------------------
2613 $ git-cat-file -t <objectname>
2614 -------------------------------------------------
2615
2616 shows the type of the object, and once you have the type (which is
2617 usually implicit in where you find the object), you can use
2618
2619 -------------------------------------------------
2620 $ git-cat-file blob|tree|commit|tag <objectname>
2621 -------------------------------------------------
2622
2623 to show its contents. NOTE! Trees have binary content, and as a result
2624 there is a special helper for showing that content, called
2625 `git-ls-tree`, which turns the binary content into a more easily
2626 readable form.
2627
2628 It's especially instructive to look at "commit" objects, since those
2629 tend to be small and fairly self-explanatory. In particular, if you
2630 follow the convention of having the top commit name in `.git/HEAD`,
2631 you can do
2632
2633 -------------------------------------------------
2634 $ git-cat-file commit HEAD
2635 -------------------------------------------------
2636
2637 to see what the top commit was.
2638
2639 Merging multiple trees
2640 ----------------------
2641
2642 Git helps you do a three-way merge, which you can expand to n-way by
2643 repeating the merge procedure arbitrary times until you finally
2644 "commit" the state. The normal situation is that you'd only do one
2645 three-way merge (two parents), and commit it, but if you like to, you
2646 can do multiple parents in one go.
2647
2648 To do a three-way merge, you need the two sets of "commit" objects
2649 that you want to merge, use those to find the closest common parent (a
2650 third "commit" object), and then use those commit objects to find the
2651 state of the directory ("tree" object) at these points.
2652
2653 To get the "base" for the merge, you first look up the common parent
2654 of two commits with
2655
2656 -------------------------------------------------
2657 $ git-merge-base <commit1> <commit2>
2658 -------------------------------------------------
2659
2660 which will return you the commit they are both based on. You should
2661 now look up the "tree" objects of those commits, which you can easily
2662 do with (for example)
2663
2664 -------------------------------------------------
2665 $ git-cat-file commit <commitname> | head -1
2666 -------------------------------------------------
2667
2668 since the tree object information is always the first line in a commit
2669 object.
2670
2671 Once you know the three trees you are going to merge (the one "original"
2672 tree, aka the common case, and the two "result" trees, aka the branches
2673 you want to merge), you do a "merge" read into the index. This will
2674 complain if it has to throw away your old index contents, so you should
2675 make sure that you've committed those - in fact you would normally
2676 always do a merge against your last commit (which should thus match what
2677 you have in your current index anyway).
2678
2679 To do the merge, do
2680
2681 -------------------------------------------------
2682 $ git-read-tree -m -u <origtree> <yourtree> <targettree>
2683 -------------------------------------------------
2684
2685 which will do all trivial merge operations for you directly in the
2686 index file, and you can just write the result out with
2687 `git-write-tree`.
2688
2689
2690 Merging multiple trees, continued
2691 ---------------------------------
2692
2693 Sadly, many merges aren't trivial. If there are files that have
2694 been added.moved or removed, or if both branches have modified the
2695 same file, you will be left with an index tree that contains "merge
2696 entries" in it. Such an index tree can 'NOT' be written out to a tree
2697 object, and you will have to resolve any such merge clashes using
2698 other tools before you can write out the result.
2699
2700 You can examine such index state with `git-ls-files --unmerged`
2701 command. An example:
2702
2703 ------------------------------------------------
2704 $ git-read-tree -m $orig HEAD $target
2705 $ git-ls-files --unmerged
2706 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello.c
2707 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello.c
2708 100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello.c
2709 ------------------------------------------------
2710
2711 Each line of the `git-ls-files --unmerged` output begins with
2712 the blob mode bits, blob SHA1, 'stage number', and the
2713 filename. The 'stage number' is git's way to say which tree it
2714 came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD`
2715 tree, and stage3 `$target` tree.
2716
2717 Earlier we said that trivial merges are done inside
2718 `git-read-tree -m`. For example, if the file did not change
2719 from `$orig` to `HEAD` nor `$target`, or if the file changed
2720 from `$orig` to `HEAD` and `$orig` to `$target` the same way,
2721 obviously the final outcome is what is in `HEAD`. What the
2722 above example shows is that file `hello.c` was changed from
2723 `$orig` to `HEAD` and `$orig` to `$target` in a different way.
2724 You could resolve this by running your favorite 3-way merge
2725 program, e.g. `diff3` or `merge`, on the blob objects from
2726 these three stages yourself, like this:
2727
2728 ------------------------------------------------
2729 $ git-cat-file blob 263414f... >hello.c~1
2730 $ git-cat-file blob 06fa6a2... >hello.c~2
2731 $ git-cat-file blob cc44c73... >hello.c~3
2732 $ merge hello.c~2 hello.c~1 hello.c~3
2733 ------------------------------------------------
2734
2735 This would leave the merge result in `hello.c~2` file, along
2736 with conflict markers if there are conflicts. After verifying
2737 the merge result makes sense, you can tell git what the final
2738 merge result for this file is by:
2739
2740 -------------------------------------------------
2741 $ mv -f hello.c~2 hello.c
2742 $ git-update-index hello.c
2743 -------------------------------------------------
2744
2745 When a path is in unmerged state, running `git-update-index` for
2746 that path tells git to mark the path resolved.
2747
2748 The above is the description of a git merge at the lowest level,
2749 to help you understand what conceptually happens under the hood.
2750 In practice, nobody, not even git itself, uses three `git-cat-file`
2751 for this. There is `git-merge-index` program that extracts the
2752 stages to temporary files and calls a "merge" script on it:
2753
2754 -------------------------------------------------
2755 $ git-merge-index git-merge-one-file hello.c
2756 -------------------------------------------------
2757
2758 and that is what higher level `git merge -s resolve` is implemented with.
2759
2760 How git stores objects efficiently: pack files
2761 ----------------------------------------------
2762
2763 We've seen how git stores each object in a file named after the
2764 object's SHA1 hash.
2765
2766 Unfortunately this system becomes inefficient once a project has a
2767 lot of objects. Try this on an old project:
2768
2769 ------------------------------------------------
2770 $ git count-objects
2771 6930 objects, 47620 kilobytes
2772 ------------------------------------------------
2773
2774 The first number is the number of objects which are kept in
2775 individual files. The second is the amount of space taken up by
2776 those "loose" objects.
2777
2778 You can save space and make git faster by moving these loose objects in
2779 to a "pack file", which stores a group of objects in an efficient
2780 compressed format; the details of how pack files are formatted can be
2781 found in link:technical/pack-format.txt[technical/pack-format.txt].
2782
2783 To put the loose objects into a pack, just run git repack:
2784
2785 ------------------------------------------------
2786 $ git repack
2787 Generating pack...
2788 Done counting 6020 objects.
2789 Deltifying 6020 objects.
2790 100% (6020/6020) done
2791 Writing 6020 objects.
2792 100% (6020/6020) done
2793 Total 6020, written 6020 (delta 4070), reused 0 (delta 0)
2794 Pack pack-3e54ad29d5b2e05838c75df582c65257b8d08e1c created.
2795 ------------------------------------------------
2796
2797 You can then run
2798
2799 ------------------------------------------------
2800 $ git prune
2801 ------------------------------------------------
2802
2803 to remove any of the "loose" objects that are now contained in the
2804 pack. This will also remove any unreferenced objects (which may be
2805 created when, for example, you use "git reset" to remove a commit).
2806 You can verify that the loose objects are gone by looking at the
2807 .git/objects directory or by running
2808
2809 ------------------------------------------------
2810 $ git count-objects
2811 0 objects, 0 kilobytes
2812 ------------------------------------------------
2813
2814 Although the object files are gone, any commands that refer to those
2815 objects will work exactly as they did before.
2816
2817 The gitlink:git-gc[1] command performs packing, pruning, and more for
2818 you, so is normally the only high-level command you need.
2819
2820 [[dangling-objects]]
2821 Dangling objects
2822 ----------------
2823
2824 The gitlink:git-fsck[1] command will sometimes complain about dangling
2825 objects. They are not a problem.
2826
2827 The most common cause of dangling objects is that you've rebased a
2828 branch, or you have pulled from somebody else who rebased a branch--see
2829 <<cleaning-up-history>>. In that case, the old head of the original
2830 branch still exists, as does obviously everything it pointed to. The
2831 branch pointer itself just doesn't, since you replaced it with another
2832 one.
2833
2834 There are also other situations too that cause dangling objects. For
2835 example, a "dangling blob" may arise because you did a "git add" of a
2836 file, but then, before you actually committed it and made it part of the
2837 bigger picture, you changed something else in that file and committed
2838 that *updated* thing - the old state that you added originally ends up
2839 not being pointed to by any commit or tree, so it's now a dangling blob
2840 object.
2841
2842 Similarly, when the "recursive" merge strategy runs, and finds that
2843 there are criss-cross merges and thus more than one merge base (which is
2844 fairly unusual, but it does happen), it will generate one temporary
2845 midway tree (or possibly even more, if you had lots of criss-crossing
2846 merges and more than two merge bases) as a temporary internal merge
2847 base, and again, those are real objects, but the end result will not end
2848 up pointing to them, so they end up "dangling" in your repository.
2849
2850 Generally, dangling objects aren't anything to worry about. They can
2851 even be very useful: if you screw something up, the dangling objects can
2852 be how you recover your old tree (say, you did a rebase, and realized
2853 that you really didn't want to - you can look at what dangling objects
2854 you have, and decide to reset your head to some old dangling state).
2855
2856 For commits, the most useful thing to do with dangling objects tends to
2857 be to do a simple
2858
2859 ------------------------------------------------
2860 $ gitk <dangling-commit-sha-goes-here> --not --all
2861 ------------------------------------------------
2862
2863 For blobs and trees, you can't do the same, but you can examine them.
2864 You can just do
2865
2866 ------------------------------------------------
2867 $ git show <dangling-blob/tree-sha-goes-here>
2868 ------------------------------------------------
2869
2870 to show what the contents of the blob were (or, for a tree, basically
2871 what the "ls" for that directory was), and that may give you some idea
2872 of what the operation was that left that dangling object.
2873
2874 Usually, dangling blobs and trees aren't very interesting. They're
2875 almost always the result of either being a half-way mergebase (the blob
2876 will often even have the conflict markers from a merge in it, if you
2877 have had conflicting merges that you fixed up by hand), or simply
2878 because you interrupted a "git fetch" with ^C or something like that,
2879 leaving _some_ of the new objects in the object database, but just
2880 dangling and useless.
2881
2882 Anyway, once you are sure that you're not interested in any dangling
2883 state, you can just prune all unreachable objects:
2884
2885 ------------------------------------------------
2886 $ git prune
2887 ------------------------------------------------
2888
2889 and they'll be gone. But you should only run "git prune" on a quiescent
2890 repository - it's kind of like doing a filesystem fsck recovery: you
2891 don't want to do that while the filesystem is mounted.
2892
2893 (The same is true of "git-fsck" itself, btw - but since
2894 git-fsck never actually *changes* the repository, it just reports
2895 on what it found, git-fsck itself is never "dangerous" to run.
2896 Running it while somebody is actually changing the repository can cause
2897 confusing and scary messages, but it won't actually do anything bad. In
2898 contrast, running "git prune" while somebody is actively changing the
2899 repository is a *BAD* idea).
2900
2901 Glossary of git terms
2902 =====================
2903
2904 include::glossary.txt[]
2905
2906 Notes and todo list for this manual
2907 ===================================
2908
2909 This is a work in progress.
2910
2911 The basic requirements:
2912 - It must be readable in order, from beginning to end, by
2913 someone intelligent with a basic grasp of the unix
2914 commandline, but without any special knowledge of git. If
2915 necessary, any other prerequisites should be specifically
2916 mentioned as they arise.
2917 - Whenever possible, section headings should clearly describe
2918 the task they explain how to do, in language that requires
2919 no more knowledge than necessary: for example, "importing
2920 patches into a project" rather than "the git-am command"
2921
2922 Think about how to create a clear chapter dependency graph that will
2923 allow people to get to important topics without necessarily reading
2924 everything in between.
2925
2926 Say something about .gitignore.
2927
2928 Scan Documentation/ for other stuff left out; in particular:
2929 howto's
2930 some of technical/?
2931 hooks
2932 list of commands in gitlink:git[1]
2933
2934 Scan email archives for other stuff left out
2935
2936 Scan man pages to see if any assume more background than this manual
2937 provides.
2938
2939 Simplify beginning by suggesting disconnected head instead of
2940 temporary branch creation?
2941
2942 Explain how to refer to file stages in the "how to resolve a merge"
2943 section: diff -1, -2, -3, --ours, --theirs :1:/path notation. The
2944 "git ls-files --unmerged --stage" thing is sorta useful too,
2945 actually. And note gitk --merge.
2946
2947 Add more good examples. Entire sections of just cookbook examples
2948 might be a good idea; maybe make an "advanced examples" section a
2949 standard end-of-chapter section?
2950
2951 Include cross-references to the glossary, where appropriate.
2952
2953 Document shallow clones? See draft 1.5.0 release notes for some
2954 documentation.
2955
2956 Add a section on working with other version control systems, including
2957 CVS, Subversion, and just imports of series of release tarballs.
2958
2959 More details on gitweb?
2960
2961 Write a chapter on using plumbing and writing scripts.