user-manual: the name of the hash function is SHA-1, not sha1
[git/git.git] / Documentation / user-manual.txt
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0eb4f7cd 1Git User's Manual (for version 1.5.3 or newer)
71f4b183 2______________________________________________
d19fbc3c 3
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4
5Git is a fast distributed revision control system.
6
02783075 7This manual is designed to be readable by someone with basic UNIX
79c96c57 8command-line skills, but no previous knowledge of git.
d19fbc3c 9
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10<<repositories-and-branches>> and <<exploring-git-history>> explain how
11to fetch and study a project using git--read these chapters to learn how
12to build and test a particular version of a software project, search for
13regressions, and so on.
ef89f701 14
2624d9a5 15People needing to do actual development will also want to read
aa971cb9 16<<Developing-With-git>> and <<sharing-development>>.
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17
18Further chapters cover more specialized topics.
19
d19fbc3c 20Comprehensive reference documentation is available through the man
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21pages, or linkgit:git-help[1] command. For example, for the command
22"git clone <repo>", you can either use:
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23
24------------------------------------------------
25$ man git-clone
26------------------------------------------------
27
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28or:
29
30------------------------------------------------
31$ git help clone
32------------------------------------------------
33
34With the latter, you can use the manual viewer of your choice; see
35linkgit:git-help[1] for more information.
36
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37See also <<git-quick-start>> for a brief overview of git commands,
38without any explanation.
b181d57f 39
99f171bb 40Finally, see <<todo>> for ways that you can help make this manual more
2624d9a5 41complete.
b181d57f 42
b181d57f 43
e34caace 44[[repositories-and-branches]]
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45Repositories and Branches
46=========================
47
e34caace 48[[how-to-get-a-git-repository]]
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49How to get a git repository
50---------------------------
51
52It will be useful to have a git repository to experiment with as you
53read this manual.
54
5162e697 55The best way to get one is by using the linkgit:git-clone[1] command to
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56download a copy of an existing repository. If you don't already have a
57project in mind, here are some interesting examples:
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58
59------------------------------------------------
60 # git itself (approx. 10MB download):
61$ git clone git://git.kernel.org/pub/scm/git/git.git
c7719fbe 62 # the Linux kernel (approx. 150MB download):
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63$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git
64------------------------------------------------
65
66The initial clone may be time-consuming for a large project, but you
67will only need to clone once.
68
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69The clone command creates a new directory named after the project ("git"
70or "linux-2.6" in the examples above). After you cd into this
d19fbc3c 71directory, you will see that it contains a copy of the project files,
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72called the <<def_working_tree,working tree>>, together with a special
73top-level directory named ".git", which contains all the information
74about the history of the project.
d19fbc3c 75
e34caace 76[[how-to-check-out]]
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77How to check out a different version of a project
78-------------------------------------------------
79
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80Git is best thought of as a tool for storing the history of a collection
81of files. It stores the history as a compressed collection of
82interrelated snapshots of the project's contents. In git each such
83version is called a <<def_commit,commit>>.
d19fbc3c 84
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85Those snapshots aren't necessarily all arranged in a single line from
86oldest to newest; instead, work may simultaneously proceed along
57283291 87parallel lines of development, called <<def_branch,branches>>, which may
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88merge and diverge.
89
90A single git repository can track development on multiple branches. It
91does this by keeping a list of <<def_head,heads>> which reference the
5162e697 92latest commit on each branch; the linkgit:git-branch[1] command shows
81b6c950 93you the list of branch heads:
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94
95------------------------------------------------
96$ git branch
97* master
98------------------------------------------------
99
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100A freshly cloned repository contains a single branch head, by default
101named "master", with the working directory initialized to the state of
102the project referred to by that branch head.
d19fbc3c 103
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104Most projects also use <<def_tag,tags>>. Tags, like heads, are
105references into the project's history, and can be listed using the
5162e697 106linkgit:git-tag[1] command:
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107
108------------------------------------------------
109$ git tag -l
110v2.6.11
111v2.6.11-tree
112v2.6.12
113v2.6.12-rc2
114v2.6.12-rc3
115v2.6.12-rc4
116v2.6.12-rc5
117v2.6.12-rc6
118v2.6.13
119...
120------------------------------------------------
121
fe4b3e59 122Tags are expected to always point at the same version of a project,
81b6c950 123while heads are expected to advance as development progresses.
fe4b3e59 124
81b6c950 125Create a new branch head pointing to one of these versions and check it
5162e697 126out using linkgit:git-checkout[1]:
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127
128------------------------------------------------
129$ git checkout -b new v2.6.13
130------------------------------------------------
131
132The working directory then reflects the contents that the project had
5162e697 133when it was tagged v2.6.13, and linkgit:git-branch[1] shows two
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134branches, with an asterisk marking the currently checked-out branch:
135
136------------------------------------------------
137$ git branch
138 master
139* new
140------------------------------------------------
141
142If you decide that you'd rather see version 2.6.17, you can modify
143the current branch to point at v2.6.17 instead, with
144
145------------------------------------------------
146$ git reset --hard v2.6.17
147------------------------------------------------
148
81b6c950 149Note that if the current branch head was your only reference to a
d19fbc3c 150particular point in history, then resetting that branch may leave you
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151with no way to find the history it used to point to; so use this command
152carefully.
d19fbc3c 153
e34caace 154[[understanding-commits]]
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155Understanding History: Commits
156------------------------------
157
158Every change in the history of a project is represented by a commit.
5162e697 159The linkgit:git-show[1] command shows the most recent commit on the
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160current branch:
161
162------------------------------------------------
163$ git show
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164commit 17cf781661e6d38f737f15f53ab552f1e95960d7
165Author: Linus Torvalds <torvalds@ppc970.osdl.org.(none)>
166Date: Tue Apr 19 14:11:06 2005 -0700
167
168 Remove duplicate getenv(DB_ENVIRONMENT) call
169
170 Noted by Tony Luck.
171
172diff --git a/init-db.c b/init-db.c
173index 65898fa..b002dc6 100644
174--- a/init-db.c
175+++ b/init-db.c
176@@ -7,7 +7,7 @@
d19fbc3c 177
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178 int main(int argc, char **argv)
179 {
180- char *sha1_dir = getenv(DB_ENVIRONMENT), *path;
181+ char *sha1_dir, *path;
182 int len, i;
183
184 if (mkdir(".git", 0755) < 0) {
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185------------------------------------------------
186
187As you can see, a commit shows who made the latest change, what they
188did, and why.
189
35121930 190Every commit has a 40-hexdigit id, sometimes called the "object name" or the
a6e5ef7d 191"SHA-1 id", shown on the first line of the "git show" output. You can usually
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192refer to a commit by a shorter name, such as a tag or a branch name, but this
193longer name can also be useful. Most importantly, it is a globally unique
194name for this commit: so if you tell somebody else the object name (for
195example in email), then you are guaranteed that name will refer to the same
196commit in their repository that it does in yours (assuming their repository
197has that commit at all). Since the object name is computed as a hash over the
198contents of the commit, you are guaranteed that the commit can never change
199without its name also changing.
200
036f8199 201In fact, in <<git-concepts>> we shall see that everything stored in git
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202history, including file data and directory contents, is stored in an object
203with a name that is a hash of its contents.
d19fbc3c 204
e34caace 205[[understanding-reachability]]
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206Understanding history: commits, parents, and reachability
207~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
208
209Every commit (except the very first commit in a project) also has a
210parent commit which shows what happened before this commit.
211Following the chain of parents will eventually take you back to the
212beginning of the project.
213
214However, the commits do not form a simple list; git allows lines of
215development to diverge and then reconverge, and the point where two
216lines of development reconverge is called a "merge". The commit
217representing a merge can therefore have more than one parent, with
218each parent representing the most recent commit on one of the lines
219of development leading to that point.
220
5162e697 221The best way to see how this works is using the linkgit:gitk[1]
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222command; running gitk now on a git repository and looking for merge
223commits will help understand how the git organizes history.
224
225In the following, we say that commit X is "reachable" from commit Y
226if commit X is an ancestor of commit Y. Equivalently, you could say
02783075 227that Y is a descendant of X, or that there is a chain of parents
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228leading from commit Y to commit X.
229
e34caace 230[[history-diagrams]]
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231Understanding history: History diagrams
232~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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233
234We will sometimes represent git history using diagrams like the one
235below. Commits are shown as "o", and the links between them with
236lines drawn with - / and \. Time goes left to right:
237
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238
239................................................
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240 o--o--o <-- Branch A
241 /
242 o--o--o <-- master
243 \
244 o--o--o <-- Branch B
1dc71a91 245................................................
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246
247If we need to talk about a particular commit, the character "o" may
248be replaced with another letter or number.
249
e34caace 250[[what-is-a-branch]]
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251Understanding history: What is a branch?
252~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
253
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254When we need to be precise, we will use the word "branch" to mean a line
255of development, and "branch head" (or just "head") to mean a reference
256to the most recent commit on a branch. In the example above, the branch
257head named "A" is a pointer to one particular commit, but we refer to
258the line of three commits leading up to that point as all being part of
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259"branch A".
260
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261However, when no confusion will result, we often just use the term
262"branch" both for branches and for branch heads.
d19fbc3c 263
e34caace 264[[manipulating-branches]]
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265Manipulating branches
266---------------------
267
268Creating, deleting, and modifying branches is quick and easy; here's
269a summary of the commands:
270
271git branch::
272 list all branches
273git branch <branch>::
274 create a new branch named <branch>, referencing the same
275 point in history as the current branch
276git branch <branch> <start-point>::
277 create a new branch named <branch>, referencing
278 <start-point>, which may be specified any way you like,
279 including using a branch name or a tag name
280git branch -d <branch>::
281 delete the branch <branch>; if the branch you are deleting
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282 points to a commit which is not reachable from the current
283 branch, this command will fail with a warning.
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284git branch -D <branch>::
285 even if the branch points to a commit not reachable
286 from the current branch, you may know that that commit
287 is still reachable from some other branch or tag. In that
288 case it is safe to use this command to force git to delete
289 the branch.
290git checkout <branch>::
291 make the current branch <branch>, updating the working
292 directory to reflect the version referenced by <branch>
293git checkout -b <new> <start-point>::
294 create a new branch <new> referencing <start-point>, and
295 check it out.
296
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297The special symbol "HEAD" can always be used to refer to the current
298branch. In fact, git uses a file named "HEAD" in the .git directory to
299remember which branch is current:
300
301------------------------------------------------
302$ cat .git/HEAD
303ref: refs/heads/master
304------------------------------------------------
305
25d9f3fa 306[[detached-head]]
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307Examining an old version without creating a new branch
308------------------------------------------------------
309
6127c086 310The `git checkout` command normally expects a branch head, but will also
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311accept an arbitrary commit; for example, you can check out the commit
312referenced by a tag:
313
314------------------------------------------------
315$ git checkout v2.6.17
316Note: moving to "v2.6.17" which isn't a local branch
317If you want to create a new branch from this checkout, you may do so
318(now or later) by using -b with the checkout command again. Example:
319 git checkout -b <new_branch_name>
320HEAD is now at 427abfa... Linux v2.6.17
321------------------------------------------------
322
a6e5ef7d 323The HEAD then refers to the SHA-1 of the commit instead of to a branch,
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324and git branch shows that you are no longer on a branch:
325
326------------------------------------------------
327$ cat .git/HEAD
328427abfa28afedffadfca9dd8b067eb6d36bac53f
953f3d6f 329$ git branch
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330* (no branch)
331 master
332------------------------------------------------
333
334In this case we say that the HEAD is "detached".
335
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336This is an easy way to check out a particular version without having to
337make up a name for the new branch. You can still create a new branch
338(or tag) for this version later if you decide to.
d19fbc3c 339
e34caace 340[[examining-remote-branches]]
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341Examining branches from a remote repository
342-------------------------------------------
343
344The "master" branch that was created at the time you cloned is a copy
345of the HEAD in the repository that you cloned from. That repository
346may also have had other branches, though, and your local repository
347keeps branches which track each of those remote branches, which you
5162e697 348can view using the "-r" option to linkgit:git-branch[1]:
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349
350------------------------------------------------
351$ git branch -r
352 origin/HEAD
353 origin/html
354 origin/maint
355 origin/man
356 origin/master
357 origin/next
358 origin/pu
359 origin/todo
360------------------------------------------------
361
362You cannot check out these remote-tracking branches, but you can
363examine them on a branch of your own, just as you would a tag:
364
365------------------------------------------------
366$ git checkout -b my-todo-copy origin/todo
367------------------------------------------------
368
369Note that the name "origin" is just the name that git uses by default
370to refer to the repository that you cloned from.
371
372[[how-git-stores-references]]
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373Naming branches, tags, and other references
374-------------------------------------------
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375
376Branches, remote-tracking branches, and tags are all references to
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377commits. All references are named with a slash-separated path name
378starting with "refs"; the names we've been using so far are actually
379shorthand:
d19fbc3c 380
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381 - The branch "test" is short for "refs/heads/test".
382 - The tag "v2.6.18" is short for "refs/tags/v2.6.18".
383 - "origin/master" is short for "refs/remotes/origin/master".
d19fbc3c 384
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385The full name is occasionally useful if, for example, there ever
386exists a tag and a branch with the same name.
d19fbc3c 387
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388(Newly created refs are actually stored in the .git/refs directory,
389under the path given by their name. However, for efficiency reasons
390they may also be packed together in a single file; see
5162e697 391linkgit:git-pack-refs[1]).
fc74ecc1 392
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393As another useful shortcut, the "HEAD" of a repository can be referred
394to just using the name of that repository. So, for example, "origin"
395is usually a shortcut for the HEAD branch in the repository "origin".
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396
397For the complete list of paths which git checks for references, and
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398the order it uses to decide which to choose when there are multiple
399references with the same shorthand name, see the "SPECIFYING
5162e697 400REVISIONS" section of linkgit:git-rev-parse[1].
d19fbc3c 401
aa971cb9 402[[Updating-a-repository-With-git-fetch]]
6127c086 403Updating a repository with git fetch
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404------------------------------------
405
406Eventually the developer cloned from will do additional work in her
407repository, creating new commits and advancing the branches to point
408at the new commits.
409
410The command "git fetch", with no arguments, will update all of the
411remote-tracking branches to the latest version found in her
412repository. It will not touch any of your own branches--not even the
413"master" branch that was created for you on clone.
414
e34caace 415[[fetching-branches]]
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416Fetching branches from other repositories
417-----------------------------------------
418
419You can also track branches from repositories other than the one you
5162e697 420cloned from, using linkgit:git-remote[1]:
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421
422-------------------------------------------------
423$ git remote add linux-nfs git://linux-nfs.org/pub/nfs-2.6.git
04483524 424$ git fetch linux-nfs
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425* refs/remotes/linux-nfs/master: storing branch 'master' ...
426 commit: bf81b46
427-------------------------------------------------
428
429New remote-tracking branches will be stored under the shorthand name
6127c086 430that you gave "git remote add", in this case linux-nfs:
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431
432-------------------------------------------------
433$ git branch -r
434linux-nfs/master
435origin/master
436-------------------------------------------------
437
438If you run "git fetch <remote>" later, the tracking branches for the
439named <remote> will be updated.
440
441If you examine the file .git/config, you will see that git has added
442a new stanza:
443
444-------------------------------------------------
445$ cat .git/config
446...
447[remote "linux-nfs"]
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448 url = git://linux-nfs.org/pub/nfs-2.6.git
449 fetch = +refs/heads/*:refs/remotes/linux-nfs/*
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450...
451-------------------------------------------------
452
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453This is what causes git to track the remote's branches; you may modify
454or delete these configuration options by editing .git/config with a
455text editor. (See the "CONFIGURATION FILE" section of
5162e697 456linkgit:git-config[1] for details.)
d5cd5de4 457
e34caace 458[[exploring-git-history]]
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459Exploring git history
460=====================
461
462Git is best thought of as a tool for storing the history of a
463collection of files. It does this by storing compressed snapshots of
1130845b 464the contents of a file hierarchy, together with "commits" which show
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465the relationships between these snapshots.
466
467Git provides extremely flexible and fast tools for exploring the
468history of a project.
469
aacd404e 470We start with one specialized tool that is useful for finding the
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471commit that introduced a bug into a project.
472
e34caace 473[[using-bisect]]
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474How to use bisect to find a regression
475--------------------------------------
476
477Suppose version 2.6.18 of your project worked, but the version at
478"master" crashes. Sometimes the best way to find the cause of such a
479regression is to perform a brute-force search through the project's
480history to find the particular commit that caused the problem. The
5162e697 481linkgit:git-bisect[1] command can help you do this:
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482
483-------------------------------------------------
484$ git bisect start
485$ git bisect good v2.6.18
486$ git bisect bad master
487Bisecting: 3537 revisions left to test after this
488[65934a9a028b88e83e2b0f8b36618fe503349f8e] BLOCK: Make USB storage depend on SCSI rather than selecting it [try #6]
489-------------------------------------------------
490
491If you run "git branch" at this point, you'll see that git has
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492temporarily moved you in "(no branch)". HEAD is now detached from any
493branch and points directly to a commit (with commit id 65934...) that
494is reachable from "master" but not from v2.6.18. Compile and test it,
495and see whether it crashes. Assume it does crash. Then:
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496
497-------------------------------------------------
498$ git bisect bad
499Bisecting: 1769 revisions left to test after this
500[7eff82c8b1511017ae605f0c99ac275a7e21b867] i2c-core: Drop useless bitmaskings
501-------------------------------------------------
502
503checks out an older version. Continue like this, telling git at each
504stage whether the version it gives you is good or bad, and notice
505that the number of revisions left to test is cut approximately in
506half each time.
507
508After about 13 tests (in this case), it will output the commit id of
509the guilty commit. You can then examine the commit with
5162e697 510linkgit:git-show[1], find out who wrote it, and mail them your bug
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511report with the commit id. Finally, run
512
513-------------------------------------------------
514$ git bisect reset
515-------------------------------------------------
516
0e25790f 517to return you to the branch you were on before.
d19fbc3c 518
6127c086 519Note that the version which `git bisect` checks out for you at each
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520point is just a suggestion, and you're free to try a different
521version if you think it would be a good idea. For example,
522occasionally you may land on a commit that broke something unrelated;
523run
524
525-------------------------------------------------
04483524 526$ git bisect visualize
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527-------------------------------------------------
528
529which will run gitk and label the commit it chose with a marker that
843c81dc 530says "bisect". Choose a safe-looking commit nearby, note its commit
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531id, and check it out with:
532
533-------------------------------------------------
534$ git reset --hard fb47ddb2db...
535-------------------------------------------------
536
537then test, run "bisect good" or "bisect bad" as appropriate, and
538continue.
539
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540Instead of "git bisect visualize" and then "git reset --hard
541fb47ddb2db...", you might just want to tell git that you want to skip
542the current commit:
543
544-------------------------------------------------
545$ git bisect skip
546-------------------------------------------------
547
548In this case, though, git may not eventually be able to tell the first
a0178ae2 549bad one between some first skipped commits and a later bad commit.
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550
551There are also ways to automate the bisecting process if you have a
552test script that can tell a good from a bad commit. See
553linkgit:git-bisect[1] for more information about this and other "git
554bisect" features.
555
e34caace 556[[naming-commits]]
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557Naming commits
558--------------
559
560We have seen several ways of naming commits already:
561
d55ae921 562 - 40-hexdigit object name
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563 - branch name: refers to the commit at the head of the given
564 branch
565 - tag name: refers to the commit pointed to by the given tag
566 (we've seen branches and tags are special cases of
567 <<how-git-stores-references,references>>).
568 - HEAD: refers to the head of the current branch
569
eb6ae7f4 570There are many more; see the "SPECIFYING REVISIONS" section of the
5162e697 571linkgit:git-rev-parse[1] man page for the complete list of ways to
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572name revisions. Some examples:
573
574-------------------------------------------------
d55ae921 575$ git show fb47ddb2 # the first few characters of the object name
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576 # are usually enough to specify it uniquely
577$ git show HEAD^ # the parent of the HEAD commit
578$ git show HEAD^^ # the grandparent
579$ git show HEAD~4 # the great-great-grandparent
580-------------------------------------------------
581
582Recall that merge commits may have more than one parent; by default,
583^ and ~ follow the first parent listed in the commit, but you can
584also choose:
585
586-------------------------------------------------
587$ git show HEAD^1 # show the first parent of HEAD
588$ git show HEAD^2 # show the second parent of HEAD
589-------------------------------------------------
590
591In addition to HEAD, there are several other special names for
592commits:
593
594Merges (to be discussed later), as well as operations such as
6127c086 595`git reset`, which change the currently checked-out commit, generally
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596set ORIG_HEAD to the value HEAD had before the current operation.
597
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598The `git fetch` operation always stores the head of the last fetched
599branch in FETCH_HEAD. For example, if you run `git fetch` without
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600specifying a local branch as the target of the operation
601
602-------------------------------------------------
603$ git fetch git://example.com/proj.git theirbranch
604-------------------------------------------------
605
606the fetched commits will still be available from FETCH_HEAD.
607
608When we discuss merges we'll also see the special name MERGE_HEAD,
609which refers to the other branch that we're merging in to the current
610branch.
611
5162e697 612The linkgit:git-rev-parse[1] command is a low-level command that is
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613occasionally useful for translating some name for a commit to the object
614name for that commit:
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615
616-------------------------------------------------
617$ git rev-parse origin
618e05db0fd4f31dde7005f075a84f96b360d05984b
619-------------------------------------------------
620
e34caace 621[[creating-tags]]
d19fbc3c
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622Creating tags
623-------------
624
625We can also create a tag to refer to a particular commit; after
626running
627
628-------------------------------------------------
04483524 629$ git tag stable-1 1b2e1d63ff
d19fbc3c
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630-------------------------------------------------
631
632You can use stable-1 to refer to the commit 1b2e1d63ff.
633
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634This creates a "lightweight" tag. If you would also like to include a
635comment with the tag, and possibly sign it cryptographically, then you
5162e697 636should create a tag object instead; see the linkgit:git-tag[1] man page
c64415e2 637for details.
d19fbc3c 638
e34caace 639[[browsing-revisions]]
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640Browsing revisions
641------------------
642
5162e697 643The linkgit:git-log[1] command can show lists of commits. On its
d19fbc3c
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644own, it shows all commits reachable from the parent commit; but you
645can also make more specific requests:
646
647-------------------------------------------------
648$ git log v2.5.. # commits since (not reachable from) v2.5
649$ git log test..master # commits reachable from master but not test
650$ git log master..test # ...reachable from test but not master
651$ git log master...test # ...reachable from either test or master,
652 # but not both
653$ git log --since="2 weeks ago" # commits from the last 2 weeks
654$ git log Makefile # commits which modify Makefile
655$ git log fs/ # ... which modify any file under fs/
656$ git log -S'foo()' # commits which add or remove any file data
657 # matching the string 'foo()'
658-------------------------------------------------
659
660And of course you can combine all of these; the following finds
661commits since v2.5 which touch the Makefile or any file under fs:
662
663-------------------------------------------------
664$ git log v2.5.. Makefile fs/
665-------------------------------------------------
666
667You can also ask git log to show patches:
668
669-------------------------------------------------
670$ git log -p
671-------------------------------------------------
672
5162e697 673See the "--pretty" option in the linkgit:git-log[1] man page for more
d19fbc3c
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674display options.
675
676Note that git log starts with the most recent commit and works
677backwards through the parents; however, since git history can contain
3dff5379 678multiple independent lines of development, the particular order that
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679commits are listed in may be somewhat arbitrary.
680
e34caace 681[[generating-diffs]]
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682Generating diffs
683----------------
684
685You can generate diffs between any two versions using
5162e697 686linkgit:git-diff[1]:
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687
688-------------------------------------------------
689$ git diff master..test
690-------------------------------------------------
691
5b98d9bc
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692That will produce the diff between the tips of the two branches. If
693you'd prefer to find the diff from their common ancestor to test, you
694can use three dots instead of two:
695
696-------------------------------------------------
697$ git diff master...test
698-------------------------------------------------
699
700Sometimes what you want instead is a set of patches; for this you can
5162e697 701use linkgit:git-format-patch[1]:
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702
703-------------------------------------------------
704$ git format-patch master..test
705-------------------------------------------------
706
707will generate a file with a patch for each commit reachable from test
5b98d9bc 708but not from master.
d19fbc3c 709
e34caace 710[[viewing-old-file-versions]]
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711Viewing old file versions
712-------------------------
713
714You can always view an old version of a file by just checking out the
715correct revision first. But sometimes it is more convenient to be
716able to view an old version of a single file without checking
717anything out; this command does that:
718
719-------------------------------------------------
720$ git show v2.5:fs/locks.c
721-------------------------------------------------
722
723Before the colon may be anything that names a commit, and after it
724may be any path to a file tracked by git.
725
e34caace 726[[history-examples]]
aec053bb
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727Examples
728--------
729
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730[[counting-commits-on-a-branch]]
731Counting the number of commits on a branch
732~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
733
734Suppose you want to know how many commits you've made on "mybranch"
735since it diverged from "origin":
736
737-------------------------------------------------
738$ git log --pretty=oneline origin..mybranch | wc -l
739-------------------------------------------------
740
741Alternatively, you may often see this sort of thing done with the
a6e5ef7d 742lower-level command linkgit:git-rev-list[1], which just lists the SHA-1's
46acd3fa
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743of all the given commits:
744
745-------------------------------------------------
746$ git rev-list origin..mybranch | wc -l
747-------------------------------------------------
748
e34caace 749[[checking-for-equal-branches]]
aec053bb 750Check whether two branches point at the same history
2f99710c 751~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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752
753Suppose you want to check whether two branches point at the same point
754in history.
755
756-------------------------------------------------
757$ git diff origin..master
758-------------------------------------------------
759
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760will tell you whether the contents of the project are the same at the
761two branches; in theory, however, it's possible that the same project
762contents could have been arrived at by two different historical
d55ae921 763routes. You could compare the object names:
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764
765-------------------------------------------------
766$ git rev-list origin
767e05db0fd4f31dde7005f075a84f96b360d05984b
768$ git rev-list master
769e05db0fd4f31dde7005f075a84f96b360d05984b
770-------------------------------------------------
771
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772Or you could recall that the ... operator selects all commits
773contained reachable from either one reference or the other but not
774both: so
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775
776-------------------------------------------------
777$ git log origin...master
778-------------------------------------------------
779
780will return no commits when the two branches are equal.
781
e34caace 782[[finding-tagged-descendants]]
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783Find first tagged version including a given fix
784~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
aec053bb 785
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786Suppose you know that the commit e05db0fd fixed a certain problem.
787You'd like to find the earliest tagged release that contains that
788fix.
789
790Of course, there may be more than one answer--if the history branched
791after commit e05db0fd, then there could be multiple "earliest" tagged
792releases.
793
794You could just visually inspect the commits since e05db0fd:
795
796-------------------------------------------------
797$ gitk e05db0fd..
798-------------------------------------------------
799
5162e697 800Or you can use linkgit:git-name-rev[1], which will give the commit a
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801name based on any tag it finds pointing to one of the commit's
802descendants:
803
804-------------------------------------------------
04483524 805$ git name-rev --tags e05db0fd
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806e05db0fd tags/v1.5.0-rc1^0~23
807-------------------------------------------------
808
5162e697 809The linkgit:git-describe[1] command does the opposite, naming the
b181d57f
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810revision using a tag on which the given commit is based:
811
812-------------------------------------------------
813$ git describe e05db0fd
04483524 814v1.5.0-rc0-260-ge05db0f
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815-------------------------------------------------
816
817but that may sometimes help you guess which tags might come after the
818given commit.
819
820If you just want to verify whether a given tagged version contains a
5162e697 821given commit, you could use linkgit:git-merge-base[1]:
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822
823-------------------------------------------------
824$ git merge-base e05db0fd v1.5.0-rc1
825e05db0fd4f31dde7005f075a84f96b360d05984b
826-------------------------------------------------
827
828The merge-base command finds a common ancestor of the given commits,
829and always returns one or the other in the case where one is a
830descendant of the other; so the above output shows that e05db0fd
831actually is an ancestor of v1.5.0-rc1.
832
833Alternatively, note that
834
835-------------------------------------------------
4a7979ca 836$ git log v1.5.0-rc1..e05db0fd
b181d57f
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837-------------------------------------------------
838
4a7979ca 839will produce empty output if and only if v1.5.0-rc1 includes e05db0fd,
b181d57f 840because it outputs only commits that are not reachable from v1.5.0-rc1.
aec053bb 841
5162e697 842As yet another alternative, the linkgit:git-show-branch[1] command lists
4a7979ca
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843the commits reachable from its arguments with a display on the left-hand
844side that indicates which arguments that commit is reachable from. So,
845you can run something like
846
847-------------------------------------------------
848$ git show-branch e05db0fd v1.5.0-rc0 v1.5.0-rc1 v1.5.0-rc2
849! [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
850available
851 ! [v1.5.0-rc0] GIT v1.5.0 preview
852 ! [v1.5.0-rc1] GIT v1.5.0-rc1
853 ! [v1.5.0-rc2] GIT v1.5.0-rc2
854...
855-------------------------------------------------
856
857then search for a line that looks like
858
859-------------------------------------------------
860+ ++ [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
861available
862-------------------------------------------------
863
864Which shows that e05db0fd is reachable from itself, from v1.5.0-rc1, and
865from v1.5.0-rc2, but not from v1.5.0-rc0.
866
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867[[showing-commits-unique-to-a-branch]]
868Showing commits unique to a given branch
869~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4a7979ca 870
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871Suppose you would like to see all the commits reachable from the branch
872head named "master" but not from any other head in your repository.
d19fbc3c 873
629d9f78 874We can list all the heads in this repository with
5162e697 875linkgit:git-show-ref[1]:
d19fbc3c 876
629d9f78
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877-------------------------------------------------
878$ git show-ref --heads
879bf62196b5e363d73353a9dcf094c59595f3153b7 refs/heads/core-tutorial
880db768d5504c1bb46f63ee9d6e1772bd047e05bf9 refs/heads/maint
881a07157ac624b2524a059a3414e99f6f44bebc1e7 refs/heads/master
88224dbc180ea14dc1aebe09f14c8ecf32010690627 refs/heads/tutorial-2
8831e87486ae06626c2f31eaa63d26fc0fd646c8af2 refs/heads/tutorial-fixes
884-------------------------------------------------
d19fbc3c 885
629d9f78
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886We can get just the branch-head names, and remove "master", with
887the help of the standard utilities cut and grep:
888
889-------------------------------------------------
890$ git show-ref --heads | cut -d' ' -f2 | grep -v '^refs/heads/master'
891refs/heads/core-tutorial
892refs/heads/maint
893refs/heads/tutorial-2
894refs/heads/tutorial-fixes
895-------------------------------------------------
896
897And then we can ask to see all the commits reachable from master
898but not from these other heads:
899
900-------------------------------------------------
901$ gitk master --not $( git show-ref --heads | cut -d' ' -f2 |
902 grep -v '^refs/heads/master' )
903-------------------------------------------------
904
905Obviously, endless variations are possible; for example, to see all
906commits reachable from some head but not from any tag in the repository:
907
908-------------------------------------------------
c78974f7 909$ gitk $( git show-ref --heads ) --not $( git show-ref --tags )
629d9f78
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910-------------------------------------------------
911
5162e697 912(See linkgit:git-rev-parse[1] for explanations of commit-selecting
629d9f78
BF
913syntax such as `--not`.)
914
82c8bf28
BF
915[[making-a-release]]
916Creating a changelog and tarball for a software release
917~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
918
5162e697 919The linkgit:git-archive[1] command can create a tar or zip archive from
82c8bf28
BF
920any version of a project; for example:
921
922-------------------------------------------------
923$ git archive --format=tar --prefix=project/ HEAD | gzip >latest.tar.gz
924-------------------------------------------------
925
926will use HEAD to produce a tar archive in which each filename is
ccd71866 927preceded by "project/".
82c8bf28
BF
928
929If you're releasing a new version of a software project, you may want
930to simultaneously make a changelog to include in the release
931announcement.
932
933Linus Torvalds, for example, makes new kernel releases by tagging them,
934then running:
935
936-------------------------------------------------
937$ release-script 2.6.12 2.6.13-rc6 2.6.13-rc7
938-------------------------------------------------
939
940where release-script is a shell script that looks like:
941
942-------------------------------------------------
943#!/bin/sh
944stable="$1"
945last="$2"
946new="$3"
947echo "# git tag v$new"
948echo "git archive --prefix=linux-$new/ v$new | gzip -9 > ../linux-$new.tar.gz"
949echo "git diff v$stable v$new | gzip -9 > ../patch-$new.gz"
950echo "git log --no-merges v$new ^v$last > ../ChangeLog-$new"
951echo "git shortlog --no-merges v$new ^v$last > ../ShortLog"
952echo "git diff --stat --summary -M v$last v$new > ../diffstat-$new"
953-------------------------------------------------
954
955and then he just cut-and-pastes the output commands after verifying that
956they look OK.
4a7979ca 957
aa971cb9 958[[Finding-comments-With-given-Content]]
187b0d80 959Finding commits referencing a file with given content
d5821de2 960~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
187b0d80
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961
962Somebody hands you a copy of a file, and asks which commits modified a
963file such that it contained the given content either before or after the
964commit. You can find out with this:
965
966-------------------------------------------------
477ff5b7 967$ git log --raw --abbrev=40 --pretty=oneline |
187b0d80
BF
968 grep -B 1 `git hash-object filename`
969-------------------------------------------------
970
971Figuring out why this works is left as an exercise to the (advanced)
5162e697
DM
972student. The linkgit:git-log[1], linkgit:git-diff-tree[1], and
973linkgit:git-hash-object[1] man pages may prove helpful.
187b0d80 974
aa971cb9 975[[Developing-With-git]]
d19fbc3c
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976Developing with git
977===================
978
e34caace 979[[telling-git-your-name]]
d19fbc3c
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980Telling git your name
981---------------------
982
983Before creating any commits, you should introduce yourself to git. The
58c19d1f
BF
984easiest way to do so is to make sure the following lines appear in a
985file named .gitconfig in your home directory:
d19fbc3c
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986
987------------------------------------------------
d19fbc3c
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988[user]
989 name = Your Name Comes Here
990 email = you@yourdomain.example.com
d19fbc3c
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991------------------------------------------------
992
5162e697 993(See the "CONFIGURATION FILE" section of linkgit:git-config[1] for
fc90c536
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994details on the configuration file.)
995
d19fbc3c 996
e34caace 997[[creating-a-new-repository]]
d19fbc3c
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998Creating a new repository
999-------------------------
1000
1001Creating a new repository from scratch is very easy:
1002
1003-------------------------------------------------
1004$ mkdir project
1005$ cd project
f1d2b477 1006$ git init
d19fbc3c
BF
1007-------------------------------------------------
1008
1009If you have some initial content (say, a tarball):
1010
1011-------------------------------------------------
0ddd93b2 1012$ tar xzvf project.tar.gz
d19fbc3c 1013$ cd project
f1d2b477 1014$ git init
d19fbc3c
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1015$ git add . # include everything below ./ in the first commit:
1016$ git commit
1017-------------------------------------------------
1018
1019[[how-to-make-a-commit]]
ae25c67a 1020How to make a commit
d19fbc3c
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1021--------------------
1022
1023Creating a new commit takes three steps:
1024
1025 1. Making some changes to the working directory using your
1026 favorite editor.
1027 2. Telling git about your changes.
1028 3. Creating the commit using the content you told git about
1029 in step 2.
1030
1031In practice, you can interleave and repeat steps 1 and 2 as many
1032times as you want: in order to keep track of what you want committed
1033at step 3, git maintains a snapshot of the tree's contents in a
1034special staging area called "the index."
1035
01997b4a
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1036At the beginning, the content of the index will be identical to
1037that of the HEAD. The command "git diff --cached", which shows
1038the difference between the HEAD and the index, should therefore
1039produce no output at that point.
eb6ae7f4 1040
d19fbc3c
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1041Modifying the index is easy:
1042
1043To update the index with the new contents of a modified file, use
1044
1045-------------------------------------------------
1046$ git add path/to/file
1047-------------------------------------------------
1048
1049To add the contents of a new file to the index, use
1050
1051-------------------------------------------------
1052$ git add path/to/file
1053-------------------------------------------------
1054
eb6ae7f4 1055To remove a file from the index and from the working tree,
d19fbc3c
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1056
1057-------------------------------------------------
1058$ git rm path/to/file
1059-------------------------------------------------
1060
1061After each step you can verify that
1062
1063-------------------------------------------------
1064$ git diff --cached
1065-------------------------------------------------
1066
1067always shows the difference between the HEAD and the index file--this
1068is what you'd commit if you created the commit now--and that
1069
1070-------------------------------------------------
1071$ git diff
1072-------------------------------------------------
1073
1074shows the difference between the working tree and the index file.
1075
6127c086 1076Note that "git add" always adds just the current contents of a file
d19fbc3c 1077to the index; further changes to the same file will be ignored unless
6127c086 1078you run `git add` on the file again.
d19fbc3c
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1079
1080When you're ready, just run
1081
1082-------------------------------------------------
1083$ git commit
1084-------------------------------------------------
1085
1086and git will prompt you for a commit message and then create the new
3dff5379 1087commit. Check to make sure it looks like what you expected with
d19fbc3c
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1088
1089-------------------------------------------------
1090$ git show
1091-------------------------------------------------
1092
1093As a special shortcut,
a6080a0a 1094
d19fbc3c
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1095-------------------------------------------------
1096$ git commit -a
1097-------------------------------------------------
1098
1099will update the index with any files that you've modified or removed
1100and create a commit, all in one step.
1101
1102A number of commands are useful for keeping track of what you're
1103about to commit:
1104
1105-------------------------------------------------
1106$ git diff --cached # difference between HEAD and the index; what
1130845b 1107 # would be committed if you ran "commit" now.
d19fbc3c
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1108$ git diff # difference between the index file and your
1109 # working directory; changes that would not
1110 # be included if you ran "commit" now.
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1111$ git diff HEAD # difference between HEAD and working tree; what
1112 # would be committed if you ran "commit -a" now.
d19fbc3c
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1113$ git status # a brief per-file summary of the above.
1114-------------------------------------------------
1115
5162e697 1116You can also use linkgit:git-gui[1] to create commits, view changes in
407c0c87
BF
1117the index and the working tree files, and individually select diff hunks
1118for inclusion in the index (by right-clicking on the diff hunk and
1119choosing "Stage Hunk For Commit").
1120
e34caace 1121[[creating-good-commit-messages]]
ae25c67a 1122Creating good commit messages
d19fbc3c
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1123-----------------------------
1124
1125Though not required, it's a good idea to begin the commit message
1126with a single short (less than 50 character) line summarizing the
1127change, followed by a blank line and then a more thorough
1128description. Tools that turn commits into email, for example, use
1129the first line on the Subject line and the rest of the commit in the
1130body.
1131
2dc53617
JH
1132[[ignoring-files]]
1133Ignoring files
1134--------------
1135
1136A project will often generate files that you do 'not' want to track with git.
1137This typically includes files generated by a build process or temporary
1138backup files made by your editor. Of course, 'not' tracking files with git
6127c086 1139is just a matter of 'not' calling `git add` on them. But it quickly becomes
2dc53617 1140annoying to have these untracked files lying around; e.g. they make
dcb11263
CJ
1141`git add .` practically useless, and they keep showing up in the output of
1142`git status`.
2dc53617 1143
464a8a7a
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1144You can tell git to ignore certain files by creating a file called .gitignore
1145in the top level of your working directory, with contents such as:
2dc53617
JH
1146
1147-------------------------------------------------
1148# Lines starting with '#' are considered comments.
464a8a7a 1149# Ignore any file named foo.txt.
2dc53617
JH
1150foo.txt
1151# Ignore (generated) html files,
1152*.html
1153# except foo.html which is maintained by hand.
1154!foo.html
1155# Ignore objects and archives.
1156*.[oa]
1157-------------------------------------------------
1158
5162e697 1159See linkgit:gitignore[5] for a detailed explanation of the syntax. You can
464a8a7a
BF
1160also place .gitignore files in other directories in your working tree, and they
1161will apply to those directories and their subdirectories. The `.gitignore`
1162files can be added to your repository like any other files (just run `git add
1163.gitignore` and `git commit`, as usual), which is convenient when the exclude
1164patterns (such as patterns matching build output files) would also make sense
1165for other users who clone your repository.
1166
1167If you wish the exclude patterns to affect only certain repositories
1168(instead of every repository for a given project), you may instead put
1169them in a file in your repository named .git/info/exclude, or in any file
1170specified by the `core.excludesfile` configuration variable. Some git
1171commands can also take exclude patterns directly on the command line.
5162e697 1172See linkgit:gitignore[5] for the details.
2dc53617 1173
e34caace 1174[[how-to-merge]]
ae25c67a 1175How to merge
d19fbc3c
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1176------------
1177
1178You can rejoin two diverging branches of development using
5162e697 1179linkgit:git-merge[1]:
d19fbc3c
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1180
1181-------------------------------------------------
1182$ git merge branchname
1183-------------------------------------------------
1184
1185merges the development in the branch "branchname" into the current
1186branch. If there are conflicts--for example, if the same file is
1187modified in two different ways in the remote branch and the local
1188branch--then you are warned; the output may look something like this:
1189
1190-------------------------------------------------
fabbd8f6
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1191$ git merge next
1192 100% (4/4) done
1193Auto-merged file.txt
d19fbc3c
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1194CONFLICT (content): Merge conflict in file.txt
1195Automatic merge failed; fix conflicts and then commit the result.
1196-------------------------------------------------
1197
1198Conflict markers are left in the problematic files, and after
1199you resolve the conflicts manually, you can update the index
1200with the contents and run git commit, as you normally would when
1201creating a new file.
1202
1203If you examine the resulting commit using gitk, you will see that it
1204has two parents, one pointing to the top of the current branch, and
1205one to the top of the other branch.
1206
d19fbc3c
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1207[[resolving-a-merge]]
1208Resolving a merge
1209-----------------
1210
1211When a merge isn't resolved automatically, git leaves the index and
1212the working tree in a special state that gives you all the
1213information you need to help resolve the merge.
1214
1215Files with conflicts are marked specially in the index, so until you
5162e697 1216resolve the problem and update the index, linkgit:git-commit[1] will
ef561ac7 1217fail:
d19fbc3c
BF
1218
1219-------------------------------------------------
1220$ git commit
1221file.txt: needs merge
1222-------------------------------------------------
1223
5162e697 1224Also, linkgit:git-status[1] will list those files as "unmerged", and the
ef561ac7
BF
1225files with conflicts will have conflict markers added, like this:
1226
1227-------------------------------------------------
1228<<<<<<< HEAD:file.txt
1229Hello world
1230=======
1231Goodbye
1232>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
1233-------------------------------------------------
1234
1235All you need to do is edit the files to resolve the conflicts, and then
1236
1237-------------------------------------------------
1238$ git add file.txt
1239$ git commit
1240-------------------------------------------------
1241
1242Note that the commit message will already be filled in for you with
1243some information about the merge. Normally you can just use this
1244default message unchanged, but you may add additional commentary of
1245your own if desired.
1246
1247The above is all you need to know to resolve a simple merge. But git
1248also provides more information to help resolve conflicts:
1249
e34caace 1250[[conflict-resolution]]
ef561ac7
BF
1251Getting conflict-resolution help during a merge
1252~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d19fbc3c
BF
1253
1254All of the changes that git was able to merge automatically are
5162e697 1255already added to the index file, so linkgit:git-diff[1] shows only
ef561ac7 1256the conflicts. It uses an unusual syntax:
d19fbc3c
BF
1257
1258-------------------------------------------------
1259$ git diff
1260diff --cc file.txt
1261index 802992c,2b60207..0000000
1262--- a/file.txt
1263+++ b/file.txt
1264@@@ -1,1 -1,1 +1,5 @@@
1265++<<<<<<< HEAD:file.txt
1266 +Hello world
1267++=======
1268+ Goodbye
1269++>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
1270-------------------------------------------------
1271
1130845b 1272Recall that the commit which will be committed after we resolve this
d19fbc3c
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1273conflict will have two parents instead of the usual one: one parent
1274will be HEAD, the tip of the current branch; the other will be the
1275tip of the other branch, which is stored temporarily in MERGE_HEAD.
1276
ef561ac7
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1277During the merge, the index holds three versions of each file. Each of
1278these three "file stages" represents a different version of the file:
1279
1280-------------------------------------------------
1281$ git show :1:file.txt # the file in a common ancestor of both branches
4209752d
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1282$ git show :2:file.txt # the version from HEAD.
1283$ git show :3:file.txt # the version from MERGE_HEAD.
ef561ac7
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1284-------------------------------------------------
1285
4209752d
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1286When you ask linkgit:git-diff[1] to show the conflicts, it runs a
1287three-way diff between the conflicted merge results in the work tree with
1288stages 2 and 3 to show only hunks whose contents come from both sides,
1289mixed (in other words, when a hunk's merge results come only from stage 2,
1290that part is not conflicting and is not shown. Same for stage 3).
ef561ac7
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1291
1292The diff above shows the differences between the working-tree version of
1293file.txt and the stage 2 and stage 3 versions. So instead of preceding
1294each line by a single "+" or "-", it now uses two columns: the first
1295column is used for differences between the first parent and the working
1296directory copy, and the second for differences between the second parent
1297and the working directory copy. (See the "COMBINED DIFF FORMAT" section
5162e697 1298of linkgit:git-diff-files[1] for a details of the format.)
ef561ac7
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1299
1300After resolving the conflict in the obvious way (but before updating the
1301index), the diff will look like:
d19fbc3c
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1302
1303-------------------------------------------------
1304$ git diff
1305diff --cc file.txt
1306index 802992c,2b60207..0000000
1307--- a/file.txt
1308+++ b/file.txt
1309@@@ -1,1 -1,1 +1,1 @@@
1310- Hello world
1311 -Goodbye
1312++Goodbye world
1313-------------------------------------------------
1314
1315This shows that our resolved version deleted "Hello world" from the
1316first parent, deleted "Goodbye" from the second parent, and added
1317"Goodbye world", which was previously absent from both.
1318
ef561ac7
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1319Some special diff options allow diffing the working directory against
1320any of these stages:
1321
1322-------------------------------------------------
1323$ git diff -1 file.txt # diff against stage 1
1324$ git diff --base file.txt # same as the above
1325$ git diff -2 file.txt # diff against stage 2
1326$ git diff --ours file.txt # same as the above
1327$ git diff -3 file.txt # diff against stage 3
1328$ git diff --theirs file.txt # same as the above.
1329-------------------------------------------------
1330
0cafe944 1331The linkgit:git-log[1] and linkgit:gitk[1] commands also provide special help
ef561ac7 1332for merges:
d19fbc3c
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1333
1334-------------------------------------------------
1335$ git log --merge
ef561ac7 1336$ gitk --merge
d19fbc3c
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1337-------------------------------------------------
1338
ef561ac7
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1339These will display all commits which exist only on HEAD or on
1340MERGE_HEAD, and which touch an unmerged file.
d19fbc3c 1341
5162e697 1342You may also use linkgit:git-mergetool[1], which lets you merge the
c7719fbe 1343unmerged files using external tools such as Emacs or kdiff3.
c64415e2 1344
ef561ac7 1345Each time you resolve the conflicts in a file and update the index:
d19fbc3c
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1346
1347-------------------------------------------------
1348$ git add file.txt
d19fbc3c
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1349-------------------------------------------------
1350
ef561ac7 1351the different stages of that file will be "collapsed", after which
6127c086 1352`git diff` will (by default) no longer show diffs for that file.
d19fbc3c
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1353
1354[[undoing-a-merge]]
ae25c67a 1355Undoing a merge
d19fbc3c
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1356---------------
1357
1358If you get stuck and decide to just give up and throw the whole mess
1359away, you can always return to the pre-merge state with
1360
1361-------------------------------------------------
1362$ git reset --hard HEAD
1363-------------------------------------------------
1364
1130845b 1365Or, if you've already committed the merge that you want to throw away,
d19fbc3c
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1366
1367-------------------------------------------------
1c73bb0e 1368$ git reset --hard ORIG_HEAD
d19fbc3c
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1369-------------------------------------------------
1370
1371However, this last command can be dangerous in some cases--never
1372throw away a commit you have already committed if that commit may
1373itself have been merged into another branch, as doing so may confuse
1374further merges.
1375
e34caace 1376[[fast-forwards]]
d19fbc3c
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1377Fast-forward merges
1378-------------------
1379
1380There is one special case not mentioned above, which is treated
1381differently. Normally, a merge results in a merge commit, with two
1382parents, one pointing at each of the two lines of development that
1383were merged.
1384
59723040
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1385However, if the current branch is a descendant of the other--so every
1386commit present in the one is already contained in the other--then git
1387just performs a "fast forward"; the head of the current branch is moved
1388forward to point at the head of the merged-in branch, without any new
1389commits being created.
d19fbc3c 1390
e34caace 1391[[fixing-mistakes]]
b684f830
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1392Fixing mistakes
1393---------------
1394
1395If you've messed up the working tree, but haven't yet committed your
1396mistake, you can return the entire working tree to the last committed
1397state with
1398
1399-------------------------------------------------
1400$ git reset --hard HEAD
1401-------------------------------------------------
1402
1403If you make a commit that you later wish you hadn't, there are two
1404fundamentally different ways to fix the problem:
1405
1406 1. You can create a new commit that undoes whatever was done
93cbbd71 1407 by the old commit. This is the correct thing if your
b684f830
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1408 mistake has already been made public.
1409
1410 2. You can go back and modify the old commit. You should
1411 never do this if you have already made the history public;
1412 git does not normally expect the "history" of a project to
1413 change, and cannot correctly perform repeated merges from
1414 a branch that has had its history changed.
1415
e34caace 1416[[reverting-a-commit]]
b684f830
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1417Fixing a mistake with a new commit
1418~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1419
1420Creating a new commit that reverts an earlier change is very easy;
5162e697 1421just pass the linkgit:git-revert[1] command a reference to the bad
b684f830
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1422commit; for example, to revert the most recent commit:
1423
1424-------------------------------------------------
1425$ git revert HEAD
1426-------------------------------------------------
1427
1428This will create a new commit which undoes the change in HEAD. You
1429will be given a chance to edit the commit message for the new commit.
1430
1431You can also revert an earlier change, for example, the next-to-last:
1432
1433-------------------------------------------------
1434$ git revert HEAD^
1435-------------------------------------------------
1436
1437In this case git will attempt to undo the old change while leaving
1438intact any changes made since then. If more recent changes overlap
1439with the changes to be reverted, then you will be asked to fix
1440conflicts manually, just as in the case of <<resolving-a-merge,
1441resolving a merge>>.
1442
7cb192ea
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1443[[fixing-a-mistake-by-rewriting-history]]
1444Fixing a mistake by rewriting history
b684f830
BF
1445~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1446
1447If the problematic commit is the most recent commit, and you have not
1448yet made that commit public, then you may just
6127c086 1449<<undoing-a-merge,destroy it using `git reset`>>.
b684f830
BF
1450
1451Alternatively, you
1452can edit the working directory and update the index to fix your
1453mistake, just as if you were going to <<how-to-make-a-commit,create a
1454new commit>>, then run
1455
1456-------------------------------------------------
1457$ git commit --amend
1458-------------------------------------------------
1459
1460which will replace the old commit by a new commit incorporating your
1461changes, giving you a chance to edit the old commit message first.
1462
1463Again, you should never do this to a commit that may already have
5162e697 1464been merged into another branch; use linkgit:git-revert[1] instead in
b684f830
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1465that case.
1466
7cb192ea 1467It is also possible to replace commits further back in the history, but
b684f830
BF
1468this is an advanced topic to be left for
1469<<cleaning-up-history,another chapter>>.
1470
e34caace 1471[[checkout-of-path]]
b684f830
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1472Checking out an old version of a file
1473~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1474
1475In the process of undoing a previous bad change, you may find it
1476useful to check out an older version of a particular file using
6127c086 1477linkgit:git-checkout[1]. We've used `git checkout` before to switch
b684f830
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1478branches, but it has quite different behavior if it is given a path
1479name: the command
1480
1481-------------------------------------------------
1482$ git checkout HEAD^ path/to/file
1483-------------------------------------------------
1484
1485replaces path/to/file by the contents it had in the commit HEAD^, and
1486also updates the index to match. It does not change branches.
1487
1488If you just want to look at an old version of the file, without
1489modifying the working directory, you can do that with
5162e697 1490linkgit:git-show[1]:
b684f830
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1491
1492-------------------------------------------------
ed4eb0d8 1493$ git show HEAD^:path/to/file
b684f830
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1494-------------------------------------------------
1495
1496which will display the given version of the file.
1497
7a7cc594
JH
1498[[interrupted-work]]
1499Temporarily setting aside work in progress
1500~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1501
1502While you are in the middle of working on something complicated, you
1503find an unrelated but obvious and trivial bug. You would like to fix it
5162e697 1504before continuing. You can use linkgit:git-stash[1] to save the current
7a7cc594
JH
1505state of your work, and after fixing the bug (or, optionally after doing
1506so on a different branch and then coming back), unstash the
1507work-in-progress changes.
1508
1509------------------------------------------------
7a85f6ae 1510$ git stash save "work in progress for foo feature"
7a7cc594
JH
1511------------------------------------------------
1512
1513This command will save your changes away to the `stash`, and
1514reset your working tree and the index to match the tip of your
1515current branch. Then you can make your fix as usual.
1516
1517------------------------------------------------
1518... edit and test ...
1519$ git commit -a -m "blorpl: typofix"
1520------------------------------------------------
1521
1522After that, you can go back to what you were working on with
1523`git stash apply`:
1524
1525------------------------------------------------
1526$ git stash apply
1527------------------------------------------------
1528
1529
e34caace 1530[[ensuring-good-performance]]
d19fbc3c
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1531Ensuring good performance
1532-------------------------
1533
1534On large repositories, git depends on compression to keep the history
06ada152 1535information from taking up too much space on disk or in memory.
d19fbc3c
BF
1536
1537This compression is not performed automatically. Therefore you
5162e697 1538should occasionally run linkgit:git-gc[1]:
d19fbc3c
BF
1539
1540-------------------------------------------------
1541$ git gc
1542-------------------------------------------------
1543
17217090 1544to recompress the archive. This can be very time-consuming, so
6127c086 1545you may prefer to run `git gc` when you are not doing other work.
d19fbc3c 1546
e34caace
BF
1547
1548[[ensuring-reliability]]
11e016a3
BF
1549Ensuring reliability
1550--------------------
1551
e34caace 1552[[checking-for-corruption]]
11e016a3
BF
1553Checking the repository for corruption
1554~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1555
5162e697 1556The linkgit:git-fsck[1] command runs a number of self-consistency checks
1191ee18 1557on the repository, and reports on any problems. This may take some
21dcb3b7
BF
1558time. The most common warning by far is about "dangling" objects:
1559
1560-------------------------------------------------
04e50e94 1561$ git fsck
21dcb3b7
BF
1562dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
1563dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
1564dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
1565dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb
1566dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f
1567dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e
1568dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085
1569dangling tree b24c2473f1fd3d91352a624795be026d64c8841f
1570...
1571-------------------------------------------------
1572
59723040 1573Dangling objects are not a problem. At worst they may take up a little
54782859 1574extra disk space. They can sometimes provide a last-resort method for
208641cf 1575recovering lost work--see <<dangling-objects>> for details.
1cdade2c 1576
e34caace 1577[[recovering-lost-changes]]
11e016a3
BF
1578Recovering lost changes
1579~~~~~~~~~~~~~~~~~~~~~~~
1580
e34caace 1581[[reflogs]]
559e4d7a
BF
1582Reflogs
1583^^^^^^^
1584
5162e697 1585Say you modify a branch with `linkgit:git-reset[1] --hard`, and then
559e4d7a
BF
1586realize that the branch was the only reference you had to that point in
1587history.
1588
1589Fortunately, git also keeps a log, called a "reflog", of all the
1590previous values of each branch. So in this case you can still find the
a6080a0a 1591old history using, for example,
559e4d7a
BF
1592
1593-------------------------------------------------
1594$ git log master@{1}
1595-------------------------------------------------
1596
e502c2c3
SO
1597This lists the commits reachable from the previous version of the
1598"master" branch head. This syntax can be used with any git command
1599that accepts a commit, not just with git log. Some other examples:
559e4d7a
BF
1600
1601-------------------------------------------------
1602$ git show master@{2} # See where the branch pointed 2,
1603$ git show master@{3} # 3, ... changes ago.
1604$ gitk master@{yesterday} # See where it pointed yesterday,
1605$ gitk master@{"1 week ago"} # ... or last week
953f3d6f
BF
1606$ git log --walk-reflogs master # show reflog entries for master
1607-------------------------------------------------
1608
1609A separate reflog is kept for the HEAD, so
1610
1611-------------------------------------------------
1612$ git show HEAD@{"1 week ago"}
559e4d7a
BF
1613-------------------------------------------------
1614
953f3d6f
BF
1615will show what HEAD pointed to one week ago, not what the current branch
1616pointed to one week ago. This allows you to see the history of what
1617you've checked out.
1618
559e4d7a 1619The reflogs are kept by default for 30 days, after which they may be
5162e697 1620pruned. See linkgit:git-reflog[1] and linkgit:git-gc[1] to learn
559e4d7a 1621how to control this pruning, and see the "SPECIFYING REVISIONS"
5162e697 1622section of linkgit:git-rev-parse[1] for details.
559e4d7a
BF
1623
1624Note that the reflog history is very different from normal git history.
1625While normal history is shared by every repository that works on the
1626same project, the reflog history is not shared: it tells you only about
1627how the branches in your local repository have changed over time.
1628
59723040 1629[[dangling-object-recovery]]
559e4d7a
BF
1630Examining dangling objects
1631^^^^^^^^^^^^^^^^^^^^^^^^^^
1632
59723040
BF
1633In some situations the reflog may not be able to save you. For example,
1634suppose you delete a branch, then realize you need the history it
1635contained. The reflog is also deleted; however, if you have not yet
1636pruned the repository, then you may still be able to find the lost
6127c086 1637commits in the dangling objects that `git fsck` reports. See
59723040 1638<<dangling-objects>> for the details.
559e4d7a
BF
1639
1640-------------------------------------------------
1641$ git fsck
1642dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
1643dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
1644dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
1645...
1646-------------------------------------------------
1647
aacd404e 1648You can examine
559e4d7a
BF
1649one of those dangling commits with, for example,
1650
1651------------------------------------------------
1652$ gitk 7281251ddd --not --all
1653------------------------------------------------
1654
1655which does what it sounds like: it says that you want to see the commit
1656history that is described by the dangling commit(s), but not the
1657history that is described by all your existing branches and tags. Thus
1658you get exactly the history reachable from that commit that is lost.
1659(And notice that it might not be just one commit: we only report the
1660"tip of the line" as being dangling, but there might be a whole deep
79c96c57 1661and complex commit history that was dropped.)
559e4d7a
BF
1662
1663If you decide you want the history back, you can always create a new
1664reference pointing to it, for example, a new branch:
1665
1666------------------------------------------------
a6080a0a 1667$ git branch recovered-branch 7281251ddd
559e4d7a
BF
1668------------------------------------------------
1669
59723040
BF
1670Other types of dangling objects (blobs and trees) are also possible, and
1671dangling objects can arise in other situations.
1672
11e016a3 1673
e34caace 1674[[sharing-development]]
d19fbc3c 1675Sharing development with others
b684f830 1676===============================
d19fbc3c 1677
aa971cb9 1678[[getting-updates-With-git-pull]]
6127c086 1679Getting updates with git pull
b684f830 1680-----------------------------
d19fbc3c
BF
1681
1682After you clone a repository and make a few changes of your own, you
1683may wish to check the original repository for updates and merge them
1684into your own work.
1685
aa971cb9 1686We have already seen <<Updating-a-repository-With-git-fetch,how to
5162e697 1687keep remote tracking branches up to date>> with linkgit:git-fetch[1],
d19fbc3c
BF
1688and how to merge two branches. So you can merge in changes from the
1689original repository's master branch with:
1690
1691-------------------------------------------------
1692$ git fetch
1693$ git merge origin/master
1694-------------------------------------------------
1695
5162e697 1696However, the linkgit:git-pull[1] command provides a way to do this in
d19fbc3c
BF
1697one step:
1698
1699-------------------------------------------------
1700$ git pull origin master
1701-------------------------------------------------
1702
0eb4f7cd
BF
1703In fact, if you have "master" checked out, then by default "git pull"
1704merges from the HEAD branch of the origin repository. So often you can
1705accomplish the above with just a simple
d19fbc3c
BF
1706
1707-------------------------------------------------
1708$ git pull
1709-------------------------------------------------
1710
0eb4f7cd
BF
1711More generally, a branch that is created from a remote branch will pull
1712by default from that branch. See the descriptions of the
1713branch.<name>.remote and branch.<name>.merge options in
5162e697
DM
1714linkgit:git-config[1], and the discussion of the `--track` option in
1715linkgit:git-checkout[1], to learn how to control these defaults.
d19fbc3c
BF
1716
1717In addition to saving you keystrokes, "git pull" also helps you by
1718producing a default commit message documenting the branch and
1719repository that you pulled from.
1720
1721(But note that no such commit will be created in the case of a
1722<<fast-forwards,fast forward>>; instead, your branch will just be
79c96c57 1723updated to point to the latest commit from the upstream branch.)
d19fbc3c 1724
6127c086 1725The `git pull` command can also be given "." as the "remote" repository,
1191ee18 1726in which case it just merges in a branch from the current repository; so
4c63ff45
BF
1727the commands
1728
1729-------------------------------------------------
1730$ git pull . branch
1731$ git merge branch
1732-------------------------------------------------
1733
1734are roughly equivalent. The former is actually very commonly used.
1735
e34caace 1736[[submitting-patches]]
d19fbc3c 1737Submitting patches to a project
b684f830 1738-------------------------------
d19fbc3c
BF
1739
1740If you just have a few changes, the simplest way to submit them may
1741just be to send them as patches in email:
1742
5162e697 1743First, use linkgit:git-format-patch[1]; for example:
d19fbc3c
BF
1744
1745-------------------------------------------------
eb6ae7f4 1746$ git format-patch origin
d19fbc3c
BF
1747-------------------------------------------------
1748
1749will produce a numbered series of files in the current directory, one
1750for each patch in the current branch but not in origin/HEAD.
1751
1752You can then import these into your mail client and send them by
1753hand. However, if you have a lot to send at once, you may prefer to
5162e697 1754use the linkgit:git-send-email[1] script to automate the process.
d19fbc3c
BF
1755Consult the mailing list for your project first to determine how they
1756prefer such patches be handled.
1757
e34caace 1758[[importing-patches]]
d19fbc3c 1759Importing patches to a project
b684f830 1760------------------------------
d19fbc3c 1761
5162e697 1762Git also provides a tool called linkgit:git-am[1] (am stands for
d19fbc3c
BF
1763"apply mailbox"), for importing such an emailed series of patches.
1764Just save all of the patch-containing messages, in order, into a
1765single mailbox file, say "patches.mbox", then run
1766
1767-------------------------------------------------
eb6ae7f4 1768$ git am -3 patches.mbox
d19fbc3c
BF
1769-------------------------------------------------
1770
1771Git will apply each patch in order; if any conflicts are found, it
1772will stop, and you can fix the conflicts as described in
01997b4a
BF
1773"<<resolving-a-merge,Resolving a merge>>". (The "-3" option tells
1774git to perform a merge; if you would prefer it just to abort and
1775leave your tree and index untouched, you may omit that option.)
1776
1777Once the index is updated with the results of the conflict
1778resolution, instead of creating a new commit, just run
d19fbc3c
BF
1779
1780-------------------------------------------------
1781$ git am --resolved
1782-------------------------------------------------
1783
1784and git will create the commit for you and continue applying the
1785remaining patches from the mailbox.
1786
1787The final result will be a series of commits, one for each patch in
1788the original mailbox, with authorship and commit log message each
1789taken from the message containing each patch.
1790
eda69449
BF
1791[[public-repositories]]
1792Public git repositories
1793-----------------------
d19fbc3c 1794
6e30fb0c
DK
1795Another way to submit changes to a project is to tell the maintainer
1796of that project to pull the changes from your repository using
aa971cb9 1797linkgit:git-pull[1]. In the section "<<getting-updates-With-git-pull,
6127c086 1798Getting updates with `git pull`>>" we described this as a way to get
6e30fb0c
DK
1799updates from the "main" repository, but it works just as well in the
1800other direction.
d19fbc3c 1801
eda69449
BF
1802If you and the maintainer both have accounts on the same machine, then
1803you can just pull changes from each other's repositories directly;
11d51533 1804commands that accept repository URLs as arguments will also accept a
eda69449 1805local directory name:
d19fbc3c
BF
1806
1807-------------------------------------------------
1808$ git clone /path/to/repository
1809$ git pull /path/to/other/repository
1810-------------------------------------------------
1811
c9016158 1812or an ssh URL:
11d51533
BF
1813
1814-------------------------------------------------
1815$ git clone ssh://yourhost/~you/repository
1816-------------------------------------------------
1817
1818For projects with few developers, or for synchronizing a few private
1819repositories, this may be all you need.
1820
eda69449
BF
1821However, the more common way to do this is to maintain a separate public
1822repository (usually on a different host) for others to pull changes
1823from. This is usually more convenient, and allows you to cleanly
1824separate private work in progress from publicly visible work.
d19fbc3c
BF
1825
1826You will continue to do your day-to-day work in your personal
1827repository, but periodically "push" changes from your personal
1828repository into your public repository, allowing other developers to
1829pull from that repository. So the flow of changes, in a situation
1830where there is one other developer with a public repository, looks
1831like this:
1832
1833 you push
1834 your personal repo ------------------> your public repo
a6080a0a 1835 ^ |
d19fbc3c
BF
1836 | |
1837 | you pull | they pull
1838 | |
1839 | |
1840 | they push V
1841 their public repo <------------------- their repo
1842
11d51533
BF
1843We explain how to do this in the following sections.
1844
eda69449
BF
1845[[setting-up-a-public-repository]]
1846Setting up a public repository
1847~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1848
1849Assume your personal repository is in the directory ~/proj. We
6127c086 1850first create a new clone of the repository and tell `git daemon` that it
eda69449 1851is meant to be public:
d19fbc3c
BF
1852
1853-------------------------------------------------
52c80037 1854$ git clone --bare ~/proj proj.git
eda69449 1855$ touch proj.git/git-daemon-export-ok
d19fbc3c
BF
1856-------------------------------------------------
1857
52c80037 1858The resulting directory proj.git contains a "bare" git repository--it is
eda69449
BF
1859just the contents of the ".git" directory, without any files checked out
1860around it.
d19fbc3c 1861
c64415e2 1862Next, copy proj.git to the server where you plan to host the
d19fbc3c
BF
1863public repository. You can use scp, rsync, or whatever is most
1864convenient.
1865
eda69449
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1866[[exporting-via-git]]
1867Exporting a git repository via the git protocol
1868~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1869
1870This is the preferred method.
1871
1872If someone else administers the server, they should tell you what
c9016158 1873directory to put the repository in, and what git:// URL it will appear
eda69449 1874at. You can then skip to the section
d19fbc3c
BF
1875"<<pushing-changes-to-a-public-repository,Pushing changes to a public
1876repository>>", below.
1877
5162e697 1878Otherwise, all you need to do is start linkgit:git-daemon[1]; it will
eda69449
BF
1879listen on port 9418. By default, it will allow access to any directory
1880that looks like a git directory and contains the magic file
6127c086 1881git-daemon-export-ok. Passing some directory paths as `git daemon`
eda69449
BF
1882arguments will further restrict the exports to those paths.
1883
6127c086 1884You can also run `git daemon` as an inetd service; see the
5162e697 1885linkgit:git-daemon[1] man page for details. (See especially the
eda69449 1886examples section.)
d19fbc3c
BF
1887
1888[[exporting-via-http]]
1889Exporting a git repository via http
eda69449 1890~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d19fbc3c
BF
1891
1892The git protocol gives better performance and reliability, but on a
1893host with a web server set up, http exports may be simpler to set up.
1894
1895All you need to do is place the newly created bare git repository in
1896a directory that is exported by the web server, and make some
1897adjustments to give web clients some extra information they need:
1898
1899-------------------------------------------------
1900$ mv proj.git /home/you/public_html/proj.git
1901$ cd proj.git
c64415e2 1902$ git --bare update-server-info
7dce9918 1903$ mv hooks/post-update.sample hooks/post-update
d19fbc3c
BF
1904-------------------------------------------------
1905
1906(For an explanation of the last two lines, see
6998e4db 1907linkgit:git-update-server-info[1] and linkgit:githooks[5].)
d19fbc3c 1908
c9016158
RW
1909Advertise the URL of proj.git. Anybody else should then be able to
1910clone or pull from that URL, for example with a command line like:
d19fbc3c
BF
1911
1912-------------------------------------------------
1913$ git clone http://yourserver.com/~you/proj.git
1914-------------------------------------------------
1915
1916(See also
1917link:howto/setup-git-server-over-http.txt[setup-git-server-over-http]
1918for a slightly more sophisticated setup using WebDAV which also
1919allows pushing over http.)
1920
d19fbc3c
BF
1921[[pushing-changes-to-a-public-repository]]
1922Pushing changes to a public repository
eda69449 1923~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d19fbc3c 1924
eda69449 1925Note that the two techniques outlined above (exporting via
d19fbc3c
BF
1926<<exporting-via-http,http>> or <<exporting-via-git,git>>) allow other
1927maintainers to fetch your latest changes, but they do not allow write
1928access, which you will need to update the public repository with the
1929latest changes created in your private repository.
1930
5162e697 1931The simplest way to do this is using linkgit:git-push[1] and ssh; to
d19fbc3c
BF
1932update the remote branch named "master" with the latest state of your
1933branch named "master", run
1934
1935-------------------------------------------------
1936$ git push ssh://yourserver.com/~you/proj.git master:master
1937-------------------------------------------------
1938
1939or just
1940
1941-------------------------------------------------
1942$ git push ssh://yourserver.com/~you/proj.git master
1943-------------------------------------------------
1944
6127c086 1945As with `git fetch`, `git push` will complain if this does not result in a
81eb417a
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1946<<fast-forwards,fast forward>>; see the following section for details on
1947handling this case.
d19fbc3c 1948
11d51533
BF
1949Note that the target of a "push" is normally a
1950<<def_bare_repository,bare>> repository. You can also push to a
1951repository that has a checked-out working tree, but the working tree
1952will not be updated by the push. This may lead to unexpected results if
1953the branch you push to is the currently checked-out branch!
1954
6127c086 1955As with `git fetch`, you may also set up configuration options to
d19fbc3c
BF
1956save typing; so, for example, after
1957
1958-------------------------------------------------
c64415e2 1959$ cat >>.git/config <<EOF
d19fbc3c
BF
1960[remote "public-repo"]
1961 url = ssh://yourserver.com/~you/proj.git
1962EOF
1963-------------------------------------------------
1964
1965you should be able to perform the above push with just
1966
1967-------------------------------------------------
1968$ git push public-repo master
1969-------------------------------------------------
1970
1971See the explanations of the remote.<name>.url, branch.<name>.remote,
5162e697 1972and remote.<name>.push options in linkgit:git-config[1] for
d19fbc3c
BF
1973details.
1974
81eb417a
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1975[[forcing-push]]
1976What to do when a push fails
1977~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1978
1979If a push would not result in a <<fast-forwards,fast forward>> of the
1980remote branch, then it will fail with an error like:
1981
1982-------------------------------------------------
1983error: remote 'refs/heads/master' is not an ancestor of
1984 local 'refs/heads/master'.
1985 Maybe you are not up-to-date and need to pull first?
1986error: failed to push to 'ssh://yourserver.com/~you/proj.git'
1987-------------------------------------------------
1988
1989This can happen, for example, if you:
1990
6127c086
FC
1991 - use `git reset --hard` to remove already-published commits, or
1992 - use `git commit --amend` to replace already-published commits
7cb192ea 1993 (as in <<fixing-a-mistake-by-rewriting-history>>), or
6127c086 1994 - use `git rebase` to rebase any already-published commits (as
81eb417a
BF
1995 in <<using-git-rebase>>).
1996
6127c086 1997You may force `git push` to perform the update anyway by preceding the
81eb417a
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1998branch name with a plus sign:
1999
2000-------------------------------------------------
2001$ git push ssh://yourserver.com/~you/proj.git +master
2002-------------------------------------------------
2003
2004Normally whenever a branch head in a public repository is modified, it
9e5d87d4 2005is modified to point to a descendant of the commit that it pointed to
81eb417a 2006before. By forcing a push in this situation, you break that convention.
aa971cb9 2007(See <<problems-With-rewriting-history>>.)
81eb417a
BF
2008
2009Nevertheless, this is a common practice for people that need a simple
2010way to publish a work-in-progress patch series, and it is an acceptable
2011compromise as long as you warn other developers that this is how you
2012intend to manage the branch.
2013
2014It's also possible for a push to fail in this way when other people have
2015the right to push to the same repository. In that case, the correct
843c81dc
EH
2016solution is to retry the push after first updating your work: either by a
2017pull, or by a fetch followed by a rebase; see the
81eb417a 2018<<setting-up-a-shared-repository,next section>> and
6998e4db 2019linkgit:gitcvs-migration[7] for more.
81eb417a 2020
e34caace 2021[[setting-up-a-shared-repository]]
d19fbc3c 2022Setting up a shared repository
eda69449 2023~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d19fbc3c
BF
2024
2025Another way to collaborate is by using a model similar to that
2026commonly used in CVS, where several developers with special rights
2027all push to and pull from a single shared repository. See
6998e4db 2028linkgit:gitcvs-migration[7] for instructions on how to
d19fbc3c
BF
2029set this up.
2030
8fae2225
BF
2031However, while there is nothing wrong with git's support for shared
2032repositories, this mode of operation is not generally recommended,
2033simply because the mode of collaboration that git supports--by
2034exchanging patches and pulling from public repositories--has so many
2035advantages over the central shared repository:
2036
2037 - Git's ability to quickly import and merge patches allows a
2038 single maintainer to process incoming changes even at very
6127c086 2039 high rates. And when that becomes too much, `git pull` provides
8fae2225
BF
2040 an easy way for that maintainer to delegate this job to other
2041 maintainers while still allowing optional review of incoming
2042 changes.
2043 - Since every developer's repository has the same complete copy
2044 of the project history, no repository is special, and it is
2045 trivial for another developer to take over maintenance of a
2046 project, either by mutual agreement, or because a maintainer
2047 becomes unresponsive or difficult to work with.
2048 - The lack of a central group of "committers" means there is
2049 less need for formal decisions about who is "in" and who is
2050 "out".
2051
e34caace 2052[[setting-up-gitweb]]
eda69449
BF
2053Allowing web browsing of a repository
2054~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d19fbc3c 2055
a8cd1402
BF
2056The gitweb cgi script provides users an easy way to browse your
2057project's files and history without having to install git; see the file
04483524 2058gitweb/INSTALL in the git source tree for instructions on setting it up.
d19fbc3c 2059
e34caace 2060[[sharing-development-examples]]
b684f830
BF
2061Examples
2062--------
d19fbc3c 2063
9e2163ea
BF
2064[[maintaining-topic-branches]]
2065Maintaining topic branches for a Linux subsystem maintainer
2066~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2067
2068This describes how Tony Luck uses git in his role as maintainer of the
2069IA64 architecture for the Linux kernel.
2070
2071He uses two public branches:
2072
2073 - A "test" tree into which patches are initially placed so that they
2074 can get some exposure when integrated with other ongoing development.
2075 This tree is available to Andrew for pulling into -mm whenever he
2076 wants.
2077
2078 - A "release" tree into which tested patches are moved for final sanity
2079 checking, and as a vehicle to send them upstream to Linus (by sending
2080 him a "please pull" request.)
2081
2082He also uses a set of temporary branches ("topic branches"), each
2083containing a logical grouping of patches.
2084
2085To set this up, first create your work tree by cloning Linus's public
2086tree:
2087
2088-------------------------------------------------
2089$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git work
2090$ cd work
2091-------------------------------------------------
2092
2093Linus's tree will be stored in the remote branch named origin/master,
5162e697
DM
2094and can be updated using linkgit:git-fetch[1]; you can track other
2095public trees using linkgit:git-remote[1] to set up a "remote" and
2096linkgit:git-fetch[1] to keep them up-to-date; see
6e30fb0c 2097<<repositories-and-branches>>.
9e2163ea
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2098
2099Now create the branches in which you are going to work; these start out
2100at the current tip of origin/master branch, and should be set up (using
5162e697 2101the --track option to linkgit:git-branch[1]) to merge changes in from
9e2163ea
BF
2102Linus by default.
2103
2104-------------------------------------------------
2105$ git branch --track test origin/master
2106$ git branch --track release origin/master
2107-------------------------------------------------
2108
5162e697 2109These can be easily kept up to date using linkgit:git-pull[1].
9e2163ea
BF
2110
2111-------------------------------------------------
2112$ git checkout test && git pull
2113$ git checkout release && git pull
2114-------------------------------------------------
2115
2116Important note! If you have any local changes in these branches, then
2117this merge will create a commit object in the history (with no local
2118changes git will simply do a "Fast forward" merge). Many people dislike
2119the "noise" that this creates in the Linux history, so you should avoid
2120doing this capriciously in the "release" branch, as these noisy commits
2121will become part of the permanent history when you ask Linus to pull
2122from the release branch.
2123
5162e697 2124A few configuration variables (see linkgit:git-config[1]) can
9e2163ea
BF
2125make it easy to push both branches to your public tree. (See
2126<<setting-up-a-public-repository>>.)
2127
2128-------------------------------------------------
2129$ cat >> .git/config <<EOF
2130[remote "mytree"]
2131 url = master.kernel.org:/pub/scm/linux/kernel/git/aegl/linux-2.6.git
2132 push = release
2133 push = test
2134EOF
2135-------------------------------------------------
2136
2137Then you can push both the test and release trees using
5162e697 2138linkgit:git-push[1]:
9e2163ea
BF
2139
2140-------------------------------------------------
2141$ git push mytree
2142-------------------------------------------------
2143
2144or push just one of the test and release branches using:
2145
2146-------------------------------------------------
2147$ git push mytree test
2148-------------------------------------------------
2149
2150or
2151
2152-------------------------------------------------
2153$ git push mytree release
2154-------------------------------------------------
2155
2156Now to apply some patches from the community. Think of a short
2157snappy name for a branch to hold this patch (or related group of
2158patches), and create a new branch from the current tip of Linus's
2159branch:
2160
2161-------------------------------------------------
2162$ git checkout -b speed-up-spinlocks origin
2163-------------------------------------------------
2164
2165Now you apply the patch(es), run some tests, and commit the change(s). If
2166the patch is a multi-part series, then you should apply each as a separate
2167commit to this branch.
2168
2169-------------------------------------------------
2170$ ... patch ... test ... commit [ ... patch ... test ... commit ]*
2171-------------------------------------------------
2172
2173When you are happy with the state of this change, you can pull it into the
2174"test" branch in preparation to make it public:
2175
2176-------------------------------------------------
2177$ git checkout test && git pull . speed-up-spinlocks
2178-------------------------------------------------
2179
2180It is unlikely that you would have any conflicts here ... but you might if you
2181spent a while on this step and had also pulled new versions from upstream.
2182
2183Some time later when enough time has passed and testing done, you can pull the
2184same branch into the "release" tree ready to go upstream. This is where you
2185see the value of keeping each patch (or patch series) in its own branch. It
2186means that the patches can be moved into the "release" tree in any order.
2187
2188-------------------------------------------------
2189$ git checkout release && git pull . speed-up-spinlocks
2190-------------------------------------------------
2191
2192After a while, you will have a number of branches, and despite the
2193well chosen names you picked for each of them, you may forget what
2194they are for, or what status they are in. To get a reminder of what
2195changes are in a specific branch, use:
2196
2197-------------------------------------------------
467c0197 2198$ git log linux..branchname | git shortlog
9e2163ea
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2199-------------------------------------------------
2200
06ada152 2201To see whether it has already been merged into the test or release branches,
9e2163ea
BF
2202use:
2203
2204-------------------------------------------------
2205$ git log test..branchname
2206-------------------------------------------------
2207
2208or
2209
2210-------------------------------------------------
2211$ git log release..branchname
2212-------------------------------------------------
2213
06ada152 2214(If this branch has not yet been merged, you will see some log entries.
9e2163ea
BF
2215If it has been merged, then there will be no output.)
2216
2217Once a patch completes the great cycle (moving from test to release,
2218then pulled by Linus, and finally coming back into your local
06ada152 2219"origin/master" branch), the branch for this change is no longer needed.
9e2163ea
BF
2220You detect this when the output from:
2221
2222-------------------------------------------------
2223$ git log origin..branchname
2224-------------------------------------------------
2225
2226is empty. At this point the branch can be deleted:
2227
2228-------------------------------------------------
2229$ git branch -d branchname
2230-------------------------------------------------
2231
2232Some changes are so trivial that it is not necessary to create a separate
2233branch and then merge into each of the test and release branches. For
2234these changes, just apply directly to the "release" branch, and then
2235merge that into the "test" branch.
2236
2237To create diffstat and shortlog summaries of changes to include in a "please
2238pull" request to Linus you can use:
2239
2240-------------------------------------------------
2241$ git diff --stat origin..release
2242-------------------------------------------------
2243
2244and
2245
2246-------------------------------------------------
2247$ git log -p origin..release | git shortlog
2248-------------------------------------------------
2249
2250Here are some of the scripts that simplify all this even further.
2251
2252-------------------------------------------------
2253==== update script ====
2254# Update a branch in my GIT tree. If the branch to be updated
2255# is origin, then pull from kernel.org. Otherwise merge
2256# origin/master branch into test|release branch
2257
2258case "$1" in
2259test|release)
2260 git checkout $1 && git pull . origin
2261 ;;
2262origin)
fc74ecc1 2263 before=$(git rev-parse refs/remotes/origin/master)
9e2163ea 2264 git fetch origin
fc74ecc1 2265 after=$(git rev-parse refs/remotes/origin/master)
9e2163ea
BF
2266 if [ $before != $after ]
2267 then
2268 git log $before..$after | git shortlog
2269 fi
2270 ;;
2271*)
2272 echo "Usage: $0 origin|test|release" 1>&2
2273 exit 1
2274 ;;
2275esac
2276-------------------------------------------------
2277
2278-------------------------------------------------
2279==== merge script ====
2280# Merge a branch into either the test or release branch
2281
2282pname=$0
2283
2284usage()
2285{
2286 echo "Usage: $pname branch test|release" 1>&2
2287 exit 1
2288}
2289
fc74ecc1 2290git show-ref -q --verify -- refs/heads/"$1" || {
9e2163ea
BF
2291 echo "Can't see branch <$1>" 1>&2
2292 usage
fc74ecc1 2293}
9e2163ea
BF
2294
2295case "$2" in
2296test|release)
2297 if [ $(git log $2..$1 | wc -c) -eq 0 ]
2298 then
2299 echo $1 already merged into $2 1>&2
2300 exit 1
2301 fi
2302 git checkout $2 && git pull . $1
2303 ;;
2304*)
2305 usage
2306 ;;
2307esac
2308-------------------------------------------------
2309
2310-------------------------------------------------
2311==== status script ====
2312# report on status of my ia64 GIT tree
2313
2314gb=$(tput setab 2)
2315rb=$(tput setab 1)
2316restore=$(tput setab 9)
2317
2318if [ `git rev-list test..release | wc -c` -gt 0 ]
2319then
2320 echo $rb Warning: commits in release that are not in test $restore
2321 git log test..release
2322fi
2323
fc74ecc1 2324for branch in `git show-ref --heads | sed 's|^.*/||'`
9e2163ea
BF
2325do
2326 if [ $branch = test -o $branch = release ]
2327 then
2328 continue
2329 fi
2330
2331 echo -n $gb ======= $branch ====== $restore " "
2332 status=
2333 for ref in test release origin/master
2334 do
2335 if [ `git rev-list $ref..$branch | wc -c` -gt 0 ]
2336 then
2337 status=$status${ref:0:1}
2338 fi
2339 done
2340 case $status in
2341 trl)
2342 echo $rb Need to pull into test $restore
2343 ;;
2344 rl)
2345 echo "In test"
2346 ;;
2347 l)
2348 echo "Waiting for linus"
2349 ;;
2350 "")
2351 echo $rb All done $restore
2352 ;;
2353 *)
2354 echo $rb "<$status>" $restore
2355 ;;
2356 esac
2357 git log origin/master..$branch | git shortlog
2358done
2359-------------------------------------------------
d19fbc3c 2360
d19fbc3c 2361
d19fbc3c 2362[[cleaning-up-history]]
4c63ff45
BF
2363Rewriting history and maintaining patch series
2364==============================================
2365
2366Normally commits are only added to a project, never taken away or
2367replaced. Git is designed with this assumption, and violating it will
2368cause git's merge machinery (for example) to do the wrong thing.
2369
2370However, there is a situation in which it can be useful to violate this
2371assumption.
2372
e34caace 2373[[patch-series]]
4c63ff45
BF
2374Creating the perfect patch series
2375---------------------------------
2376
2377Suppose you are a contributor to a large project, and you want to add a
2378complicated feature, and to present it to the other developers in a way
2379that makes it easy for them to read your changes, verify that they are
2380correct, and understand why you made each change.
2381
b181d57f 2382If you present all of your changes as a single patch (or commit), they
79c96c57 2383may find that it is too much to digest all at once.
4c63ff45
BF
2384
2385If you present them with the entire history of your work, complete with
2386mistakes, corrections, and dead ends, they may be overwhelmed.
2387
2388So the ideal is usually to produce a series of patches such that:
2389
2390 1. Each patch can be applied in order.
2391
2392 2. Each patch includes a single logical change, together with a
2393 message explaining the change.
2394
2395 3. No patch introduces a regression: after applying any initial
2396 part of the series, the resulting project still compiles and
2397 works, and has no bugs that it didn't have before.
2398
2399 4. The complete series produces the same end result as your own
2400 (probably much messier!) development process did.
2401
b181d57f
BF
2402We will introduce some tools that can help you do this, explain how to
2403use them, and then explain some of the problems that can arise because
2404you are rewriting history.
4c63ff45 2405
e34caace 2406[[using-git-rebase]]
6127c086 2407Keeping a patch series up to date using git rebase
4c63ff45
BF
2408--------------------------------------------------
2409
79c96c57
MC
2410Suppose that you create a branch "mywork" on a remote-tracking branch
2411"origin", and create some commits on top of it:
4c63ff45
BF
2412
2413-------------------------------------------------
2414$ git checkout -b mywork origin
2415$ vi file.txt
2416$ git commit
2417$ vi otherfile.txt
2418$ git commit
2419...
2420-------------------------------------------------
2421
2422You have performed no merges into mywork, so it is just a simple linear
2423sequence of patches on top of "origin":
2424
1dc71a91 2425................................................
4c63ff45
BF
2426 o--o--o <-- origin
2427 \
2428 o--o--o <-- mywork
1dc71a91 2429................................................
4c63ff45
BF
2430
2431Some more interesting work has been done in the upstream project, and
2432"origin" has advanced:
2433
1dc71a91 2434................................................
4c63ff45
BF
2435 o--o--O--o--o--o <-- origin
2436 \
2437 a--b--c <-- mywork
1dc71a91 2438................................................
4c63ff45
BF
2439
2440At this point, you could use "pull" to merge your changes back in;
2441the result would create a new merge commit, like this:
2442
1dc71a91 2443................................................
4c63ff45
BF
2444 o--o--O--o--o--o <-- origin
2445 \ \
2446 a--b--c--m <-- mywork
1dc71a91 2447................................................
a6080a0a 2448
4c63ff45
BF
2449However, if you prefer to keep the history in mywork a simple series of
2450commits without any merges, you may instead choose to use
5162e697 2451linkgit:git-rebase[1]:
4c63ff45
BF
2452
2453-------------------------------------------------
2454$ git checkout mywork
2455$ git rebase origin
2456-------------------------------------------------
2457
b181d57f 2458This will remove each of your commits from mywork, temporarily saving
51ef1daa 2459them as patches (in a directory named ".git/rebase-apply"), update mywork to
b181d57f
BF
2460point at the latest version of origin, then apply each of the saved
2461patches to the new mywork. The result will look like:
4c63ff45
BF
2462
2463
1dc71a91 2464................................................
4c63ff45
BF
2465 o--o--O--o--o--o <-- origin
2466 \
2467 a'--b'--c' <-- mywork
1dc71a91 2468................................................
4c63ff45 2469
b181d57f 2470In the process, it may discover conflicts. In that case it will stop
6127c086 2471and allow you to fix the conflicts; after fixing conflicts, use `git add`
7a7d4ef6 2472to update the index with those contents, and then, instead of
6127c086 2473running `git commit`, just run
4c63ff45
BF
2474
2475-------------------------------------------------
2476$ git rebase --continue
2477-------------------------------------------------
2478
2479and git will continue applying the rest of the patches.
2480
b6cbca38 2481At any point you may use the `--abort` option to abort this process and
4c63ff45
BF
2482return mywork to the state it had before you started the rebase:
2483
2484-------------------------------------------------
2485$ git rebase --abort
2486-------------------------------------------------
2487
7cb192ea
BF
2488[[rewriting-one-commit]]
2489Rewriting a single commit
365aa199
BF
2490-------------------------
2491
7cb192ea 2492We saw in <<fixing-a-mistake-by-rewriting-history>> that you can replace the
365aa199
BF
2493most recent commit using
2494
2495-------------------------------------------------
2496$ git commit --amend
2497-------------------------------------------------
2498
2499which will replace the old commit by a new commit incorporating your
2500changes, giving you a chance to edit the old commit message first.
2501
5162e697 2502You can also use a combination of this and linkgit:git-rebase[1] to
7cb192ea
BF
2503replace a commit further back in your history and recreate the
2504intervening changes on top of it. First, tag the problematic commit
2505with
365aa199
BF
2506
2507-------------------------------------------------
2508$ git tag bad mywork~5
2509-------------------------------------------------
2510
6127c086 2511(Either gitk or `git log` may be useful for finding the commit.)
365aa199 2512
25d9f3fa
BF
2513Then check out that commit, edit it, and rebase the rest of the series
2514on top of it (note that we could check out the commit on a temporary
2515branch, but instead we're using a <<detached-head,detached head>>):
365aa199
BF
2516
2517-------------------------------------------------
25d9f3fa 2518$ git checkout bad
365aa199
BF
2519$ # make changes here and update the index
2520$ git commit --amend
25d9f3fa 2521$ git rebase --onto HEAD bad mywork
365aa199
BF
2522-------------------------------------------------
2523
25d9f3fa
BF
2524When you're done, you'll be left with mywork checked out, with the top
2525patches on mywork reapplied on top of your modified commit. You can
365aa199
BF
2526then clean up with
2527
2528-------------------------------------------------
365aa199
BF
2529$ git tag -d bad
2530-------------------------------------------------
2531
2532Note that the immutable nature of git history means that you haven't really
2533"modified" existing commits; instead, you have replaced the old commits with
2534new commits having new object names.
2535
e34caace 2536[[reordering-patch-series]]
4c63ff45
BF
2537Reordering or selecting from a patch series
2538-------------------------------------------
2539
5162e697 2540Given one existing commit, the linkgit:git-cherry-pick[1] command
b181d57f
BF
2541allows you to apply the change introduced by that commit and create a
2542new commit that records it. So, for example, if "mywork" points to a
2543series of patches on top of "origin", you might do something like:
2544
2545-------------------------------------------------
2546$ git checkout -b mywork-new origin
2547$ gitk origin..mywork &
2548-------------------------------------------------
2549
06ada152 2550and browse through the list of patches in the mywork branch using gitk,
b181d57f 2551applying them (possibly in a different order) to mywork-new using
6127c086 2552cherry-pick, and possibly modifying them as you go using `git commit --amend`.
5162e697 2553The linkgit:git-gui[1] command may also help as it allows you to
6e30fb0c
DK
2554individually select diff hunks for inclusion in the index (by
2555right-clicking on the diff hunk and choosing "Stage Hunk for Commit").
b181d57f 2556
6127c086 2557Another technique is to use `git format-patch` to create a series of
b181d57f 2558patches, then reset the state to before the patches:
4c63ff45 2559
b181d57f
BF
2560-------------------------------------------------
2561$ git format-patch origin
2562$ git reset --hard origin
2563-------------------------------------------------
4c63ff45 2564
b181d57f 2565Then modify, reorder, or eliminate patches as preferred before applying
5162e697 2566them again with linkgit:git-am[1].
4c63ff45 2567
e34caace 2568[[patch-series-tools]]
4c63ff45
BF
2569Other tools
2570-----------
2571
02783075 2572There are numerous other tools, such as StGIT, which exist for the
79c96c57 2573purpose of maintaining a patch series. These are outside of the scope of
b181d57f 2574this manual.
4c63ff45 2575
aa971cb9 2576[[problems-With-rewriting-history]]
4c63ff45
BF
2577Problems with rewriting history
2578-------------------------------
2579
b181d57f
BF
2580The primary problem with rewriting the history of a branch has to do
2581with merging. Suppose somebody fetches your branch and merges it into
2582their branch, with a result something like this:
2583
1dc71a91 2584................................................
b181d57f
BF
2585 o--o--O--o--o--o <-- origin
2586 \ \
2587 t--t--t--m <-- their branch:
1dc71a91 2588................................................
b181d57f
BF
2589
2590Then suppose you modify the last three commits:
2591
1dc71a91 2592................................................
b181d57f
BF
2593 o--o--o <-- new head of origin
2594 /
2595 o--o--O--o--o--o <-- old head of origin
1dc71a91 2596................................................
b181d57f
BF
2597
2598If we examined all this history together in one repository, it will
2599look like:
2600
1dc71a91 2601................................................
b181d57f
BF
2602 o--o--o <-- new head of origin
2603 /
2604 o--o--O--o--o--o <-- old head of origin
2605 \ \
2606 t--t--t--m <-- their branch:
1dc71a91 2607................................................
b181d57f
BF
2608
2609Git has no way of knowing that the new head is an updated version of
2610the old head; it treats this situation exactly the same as it would if
2611two developers had independently done the work on the old and new heads
2612in parallel. At this point, if someone attempts to merge the new head
2613in to their branch, git will attempt to merge together the two (old and
2614new) lines of development, instead of trying to replace the old by the
2615new. The results are likely to be unexpected.
2616
2617You may still choose to publish branches whose history is rewritten,
2618and it may be useful for others to be able to fetch those branches in
2619order to examine or test them, but they should not attempt to pull such
2620branches into their own work.
2621
2622For true distributed development that supports proper merging,
2623published branches should never be rewritten.
2624
3fb00282
SP
2625[[bisect-merges]]
2626Why bisecting merge commits can be harder than bisecting linear history
2627-----------------------------------------------------------------------
2628
5162e697 2629The linkgit:git-bisect[1] command correctly handles history that
3fb00282
SP
2630includes merge commits. However, when the commit that it finds is a
2631merge commit, the user may need to work harder than usual to figure out
2632why that commit introduced a problem.
2633
2634Imagine this history:
2635
2636................................................
2637 ---Z---o---X---...---o---A---C---D
2638 \ /
2639 o---o---Y---...---o---B
2640................................................
2641
2642Suppose that on the upper line of development, the meaning of one
2643of the functions that exists at Z is changed at commit X. The
2644commits from Z leading to A change both the function's
2645implementation and all calling sites that exist at Z, as well
2646as new calling sites they add, to be consistent. There is no
2647bug at A.
2648
2649Suppose that in the meantime on the lower line of development somebody
2650adds a new calling site for that function at commit Y. The
2651commits from Z leading to B all assume the old semantics of that
2652function and the callers and the callee are consistent with each
2653other. There is no bug at B, either.
2654
2655Suppose further that the two development lines merge cleanly at C,
2656so no conflict resolution is required.
2657
2658Nevertheless, the code at C is broken, because the callers added
2659on the lower line of development have not been converted to the new
2660semantics introduced on the upper line of development. So if all
2661you know is that D is bad, that Z is good, and that
5162e697 2662linkgit:git-bisect[1] identifies C as the culprit, how will you
3fb00282
SP
2663figure out that the problem is due to this change in semantics?
2664
6127c086 2665When the result of a `git bisect` is a non-merge commit, you should
3fb00282
SP
2666normally be able to discover the problem by examining just that commit.
2667Developers can make this easy by breaking their changes into small
2668self-contained commits. That won't help in the case above, however,
2669because the problem isn't obvious from examination of any single
2670commit; instead, a global view of the development is required. To
2671make matters worse, the change in semantics in the problematic
2672function may be just one small part of the changes in the upper
2673line of development.
2674
2675On the other hand, if instead of merging at C you had rebased the
2676history between Z to B on top of A, you would have gotten this
2677linear history:
2678
2679................................................................
2680 ---Z---o---X--...---o---A---o---o---Y*--...---o---B*--D*
2681................................................................
2682
2683Bisecting between Z and D* would hit a single culprit commit Y*,
2684and understanding why Y* was broken would probably be easier.
2685
2686Partly for this reason, many experienced git users, even when
2687working on an otherwise merge-heavy project, keep the history
2688linear by rebasing against the latest upstream version before
2689publishing.
2690
e34caace 2691[[advanced-branch-management]]
b181d57f
BF
2692Advanced branch management
2693==========================
4c63ff45 2694
e34caace 2695[[fetching-individual-branches]]
b181d57f
BF
2696Fetching individual branches
2697----------------------------
2698
5162e697 2699Instead of using linkgit:git-remote[1], you can also choose just
b181d57f
BF
2700to update one branch at a time, and to store it locally under an
2701arbitrary name:
2702
2703-------------------------------------------------
2704$ git fetch origin todo:my-todo-work
2705-------------------------------------------------
2706
2707The first argument, "origin", just tells git to fetch from the
2708repository you originally cloned from. The second argument tells git
2709to fetch the branch named "todo" from the remote repository, and to
2710store it locally under the name refs/heads/my-todo-work.
2711
2712You can also fetch branches from other repositories; so
2713
2714-------------------------------------------------
2715$ git fetch git://example.com/proj.git master:example-master
2716-------------------------------------------------
2717
2718will create a new branch named "example-master" and store in it the
2719branch named "master" from the repository at the given URL. If you
2720already have a branch named example-master, it will attempt to
59723040
BF
2721<<fast-forwards,fast-forward>> to the commit given by example.com's
2722master branch. In more detail:
b181d57f 2723
59723040
BF
2724[[fetch-fast-forwards]]
2725git fetch and fast-forwards
2726---------------------------
b181d57f 2727
6127c086 2728In the previous example, when updating an existing branch, "git fetch"
7a7d4ef6 2729checks to make sure that the most recent commit on the remote
b181d57f
BF
2730branch is a descendant of the most recent commit on your copy of the
2731branch before updating your copy of the branch to point at the new
59723040 2732commit. Git calls this process a <<fast-forwards,fast forward>>.
b181d57f
BF
2733
2734A fast forward looks something like this:
2735
1dc71a91 2736................................................
b181d57f
BF
2737 o--o--o--o <-- old head of the branch
2738 \
2739 o--o--o <-- new head of the branch
1dc71a91 2740................................................
b181d57f
BF
2741
2742
2743In some cases it is possible that the new head will *not* actually be
2744a descendant of the old head. For example, the developer may have
2745realized she made a serious mistake, and decided to backtrack,
2746resulting in a situation like:
2747
1dc71a91 2748................................................
b181d57f
BF
2749 o--o--o--o--a--b <-- old head of the branch
2750 \
2751 o--o--o <-- new head of the branch
1dc71a91 2752................................................
b181d57f 2753
6127c086 2754In this case, "git fetch" will fail, and print out a warning.
b181d57f
BF
2755
2756In that case, you can still force git to update to the new head, as
2757described in the following section. However, note that in the
2758situation above this may mean losing the commits labeled "a" and "b",
2759unless you've already created a reference of your own pointing to
2760them.
2761
e34caace 2762[[forcing-fetch]]
6127c086 2763Forcing git fetch to do non-fast-forward updates
b181d57f
BF
2764------------------------------------------------
2765
2766If git fetch fails because the new head of a branch is not a
2767descendant of the old head, you may force the update with:
2768
2769-------------------------------------------------
2770$ git fetch git://example.com/proj.git +master:refs/remotes/example/master
2771-------------------------------------------------
2772
c64415e2
BF
2773Note the addition of the "+" sign. Alternatively, you can use the "-f"
2774flag to force updates of all the fetched branches, as in:
2775
2776-------------------------------------------------
2777$ git fetch -f origin
2778-------------------------------------------------
2779
2780Be aware that commits that the old version of example/master pointed at
2781may be lost, as we saw in the previous section.
b181d57f 2782
e34caace 2783[[remote-branch-configuration]]
b181d57f
BF
2784Configuring remote branches
2785---------------------------
2786
2787We saw above that "origin" is just a shortcut to refer to the
79c96c57 2788repository that you originally cloned from. This information is
b181d57f 2789stored in git configuration variables, which you can see using
5162e697 2790linkgit:git-config[1]:
b181d57f
BF
2791
2792-------------------------------------------------
9d13bda3 2793$ git config -l
b181d57f
BF
2794core.repositoryformatversion=0
2795core.filemode=true
2796core.logallrefupdates=true
2797remote.origin.url=git://git.kernel.org/pub/scm/git/git.git
2798remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
2799branch.master.remote=origin
2800branch.master.merge=refs/heads/master
2801-------------------------------------------------
2802
2803If there are other repositories that you also use frequently, you can
2804create similar configuration options to save typing; for example,
2805after
2806
2807-------------------------------------------------
9d13bda3 2808$ git config remote.example.url git://example.com/proj.git
b181d57f
BF
2809-------------------------------------------------
2810
2811then the following two commands will do the same thing:
2812
2813-------------------------------------------------
2814$ git fetch git://example.com/proj.git master:refs/remotes/example/master
2815$ git fetch example master:refs/remotes/example/master
2816-------------------------------------------------
2817
2818Even better, if you add one more option:
2819
2820-------------------------------------------------
9d13bda3 2821$ git config remote.example.fetch master:refs/remotes/example/master
b181d57f
BF
2822-------------------------------------------------
2823
2824then the following commands will all do the same thing:
2825
2826-------------------------------------------------
52c80037
BF
2827$ git fetch git://example.com/proj.git master:refs/remotes/example/master
2828$ git fetch example master:refs/remotes/example/master
b181d57f
BF
2829$ git fetch example
2830-------------------------------------------------
2831
2832You can also add a "+" to force the update each time:
2833
2834-------------------------------------------------
9d13bda3 2835$ git config remote.example.fetch +master:ref/remotes/example/master
b181d57f
BF
2836-------------------------------------------------
2837
2838Don't do this unless you're sure you won't mind "git fetch" possibly
4f80b27d 2839throwing away commits on 'example/master'.
b181d57f
BF
2840
2841Also note that all of the above configuration can be performed by
2842directly editing the file .git/config instead of using
5162e697 2843linkgit:git-config[1].
b181d57f 2844
5162e697 2845See linkgit:git-config[1] for more details on the configuration
b181d57f 2846options mentioned above.
d19fbc3c 2847
d19fbc3c 2848
036f8199
BF
2849[[git-concepts]]
2850Git concepts
2851============
d19fbc3c 2852
036f8199
BF
2853Git is built on a small number of simple but powerful ideas. While it
2854is possible to get things done without understanding them, you will find
2855git much more intuitive if you do.
2856
2857We start with the most important, the <<def_object_database,object
2858database>> and the <<def_index,index>>.
b181d57f 2859
e34caace 2860[[the-object-database]]
b181d57f
BF
2861The Object Database
2862-------------------
2863
1bbf1c79
BF
2864
2865We already saw in <<understanding-commits>> that all commits are stored
2866under a 40-digit "object name". In fact, all the information needed to
2867represent the history of a project is stored in objects with such names.
a6e5ef7d
FC
2868In each case the name is calculated by taking the SHA-1 hash of the
2869contents of the object. The SHA-1 hash is a cryptographic hash function.
1bbf1c79
BF
2870What that means to us is that it is impossible to find two different
2871objects with the same name. This has a number of advantages; among
2872others:
2873
2874- Git can quickly determine whether two objects are identical or not,
2875 just by comparing names.
06ada152 2876- Since object names are computed the same way in every repository, the
1bbf1c79
BF
2877 same content stored in two repositories will always be stored under
2878 the same name.
2879- Git can detect errors when it reads an object, by checking that the
a6e5ef7d 2880 object's name is still the SHA-1 hash of its contents.
1bbf1c79
BF
2881
2882(See <<object-details>> for the details of the object formatting and
a6e5ef7d 2883SHA-1 calculation.)
1bbf1c79
BF
2884
2885There are four different types of objects: "blob", "tree", "commit", and
2886"tag".
2887
2888- A <<def_blob_object,"blob" object>> is used to store file data.
843c81dc 2889- A <<def_tree_object,"tree" object>> ties one or more
1bbf1c79
BF
2890 "blob" objects into a directory structure. In addition, a tree object
2891 can refer to other tree objects, thus creating a directory hierarchy.
2892- A <<def_commit_object,"commit" object>> ties such directory hierarchies
2ef8ac1b 2893 together into a <<def_DAG,directed acyclic graph>> of revisions--each
1bbf1c79
BF
2894 commit contains the object name of exactly one tree designating the
2895 directory hierarchy at the time of the commit. In addition, a commit
2896 refers to "parent" commit objects that describe the history of how we
2897 arrived at that directory hierarchy.
2898- A <<def_tag_object,"tag" object>> symbolically identifies and can be
2899 used to sign other objects. It contains the object name and type of
2900 another object, a symbolic name (of course!) and, optionally, a
2901 signature.
b181d57f 2902
b181d57f
BF
2903The object types in some more detail:
2904
513d419c
BF
2905[[commit-object]]
2906Commit Object
2907~~~~~~~~~~~~~
b181d57f 2908
1bbf1c79
BF
2909The "commit" object links a physical state of a tree with a description
2910of how we got there and why. Use the --pretty=raw option to
5162e697 2911linkgit:git-show[1] or linkgit:git-log[1] to examine your favorite
1bbf1c79
BF
2912commit:
2913
2914------------------------------------------------
2915$ git show -s --pretty=raw 2be7fcb476
2916commit 2be7fcb4764f2dbcee52635b91fedb1b3dcf7ab4
2917tree fb3a8bdd0ceddd019615af4d57a53f43d8cee2bf
2918parent 257a84d9d02e90447b149af58b271c19405edb6a
2919author Dave Watson <dwatson@mimvista.com> 1187576872 -0400
2920committer Junio C Hamano <gitster@pobox.com> 1187591163 -0700
2921
2922 Fix misspelling of 'suppress' in docs
2923
2924 Signed-off-by: Junio C Hamano <gitster@pobox.com>
2925------------------------------------------------
2926
2927As you can see, a commit is defined by:
2928
a6e5ef7d 2929- a tree: The SHA-1 name of a tree object (as defined below), representing
1bbf1c79 2930 the contents of a directory at a certain point in time.
a6e5ef7d 2931- parent(s): The SHA-1 name of some number of commits which represent the
9e5d87d4 2932 immediately previous step(s) in the history of the project. The
1bbf1c79
BF
2933 example above has one parent; merge commits may have more than
2934 one. A commit with no parents is called a "root" commit, and
2935 represents the initial revision of a project. Each project must have
2936 at least one root. A project can also have multiple roots, though
2937 that isn't common (or necessarily a good idea).
2938- an author: The name of the person responsible for this change, together
2939 with its date.
2940- a committer: The name of the person who actually created the commit,
2941 with the date it was done. This may be different from the author, for
2942 example, if the author was someone who wrote a patch and emailed it
2943 to the person who used it to create the commit.
2944- a comment describing this commit.
2945
2946Note that a commit does not itself contain any information about what
2947actually changed; all changes are calculated by comparing the contents
2948of the tree referred to by this commit with the trees associated with
2949its parents. In particular, git does not attempt to record file renames
2950explicitly, though it can identify cases where the existence of the same
2951file data at changing paths suggests a rename. (See, for example, the
5162e697 2952-M option to linkgit:git-diff[1]).
1bbf1c79 2953
5162e697 2954A commit is usually created by linkgit:git-commit[1], which creates a
1bbf1c79
BF
2955commit whose parent is normally the current HEAD, and whose tree is
2956taken from the content currently stored in the index.
b181d57f 2957
e34caace 2958[[tree-object]]
b181d57f 2959Tree Object
971aa71f 2960~~~~~~~~~~~
b181d57f 2961
5162e697
DM
2962The ever-versatile linkgit:git-show[1] command can also be used to
2963examine tree objects, but linkgit:git-ls-tree[1] will give you more
1bbf1c79
BF
2964details:
2965
2966------------------------------------------------
2967$ git ls-tree fb3a8bdd0ce
2968100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c .gitignore
2969100644 blob 5529b198e8d14decbe4ad99db3f7fb632de0439d .mailmap
2970100644 blob 6ff87c4664981e4397625791c8ea3bbb5f2279a3 COPYING
2971040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745 Documentation
2972100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200 GIT-VERSION-GEN
2973100644 blob 289b046a443c0647624607d471289b2c7dcd470b INSTALL
2974100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1 Makefile
2975100644 blob 548142c327a6790ff8821d67c2ee1eff7a656b52 README
2976...
2977------------------------------------------------
2978
2979As you can see, a tree object contains a list of entries, each with a
a6e5ef7d 2980mode, object type, SHA-1 name, and name, sorted by name. It represents
1bbf1c79
BF
2981the contents of a single directory tree.
2982
2983The object type may be a blob, representing the contents of a file, or
2984another tree, representing the contents of a subdirectory. Since trees
a6e5ef7d
FC
2985and blobs, like all other objects, are named by the SHA-1 hash of their
2986contents, two trees have the same SHA-1 name if and only if their
1bbf1c79
BF
2987contents (including, recursively, the contents of all subdirectories)
2988are identical. This allows git to quickly determine the differences
2989between two related tree objects, since it can ignore any entries with
2990identical object names.
2991
2992(Note: in the presence of submodules, trees may also have commits as
6dd14366 2993entries. See <<submodules>> for documentation.)
1bbf1c79
BF
2994
2995Note that the files all have mode 644 or 755: git actually only pays
2996attention to the executable bit.
b181d57f 2997
513d419c
BF
2998[[blob-object]]
2999Blob Object
3000~~~~~~~~~~~
b181d57f 3001
5162e697 3002You can use linkgit:git-show[1] to examine the contents of a blob; take,
1bbf1c79 3003for example, the blob in the entry for "COPYING" from the tree above:
b181d57f 3004
1bbf1c79
BF
3005------------------------------------------------
3006$ git show 6ff87c4664
3007
3008 Note that the only valid version of the GPL as far as this project
3009 is concerned is _this_ particular version of the license (ie v2, not
3010 v2.2 or v3.x or whatever), unless explicitly otherwise stated.
3011...
3012------------------------------------------------
b181d57f 3013
1bbf1c79
BF
3014A "blob" object is nothing but a binary blob of data. It doesn't refer
3015to anything else or have attributes of any kind.
3016
3017Since the blob is entirely defined by its data, if two files in a
3018directory tree (or in multiple different versions of the repository)
3019have the same contents, they will share the same blob object. The object
3020is totally independent of its location in the directory tree, and
3021renaming a file does not change the object that file is associated with.
3022
3023Note that any tree or blob object can be examined using
5162e697 3024linkgit:git-show[1] with the <revision>:<path> syntax. This can
1bbf1c79
BF
3025sometimes be useful for browsing the contents of a tree that is not
3026currently checked out.
b181d57f 3027
e34caace 3028[[trust]]
b181d57f 3029Trust
971aa71f 3030~~~~~
b181d57f 3031
a6e5ef7d 3032If you receive the SHA-1 name of a blob from one source, and its contents
1bbf1c79 3033from another (possibly untrusted) source, you can still trust that those
a6e5ef7d
FC
3034contents are correct as long as the SHA-1 name agrees. This is because
3035the SHA-1 is designed so that it is infeasible to find different contents
1bbf1c79 3036that produce the same hash.
b181d57f 3037
a6e5ef7d 3038Similarly, you need only trust the SHA-1 name of a top-level tree object
1bbf1c79 3039to trust the contents of the entire directory that it refers to, and if
a6e5ef7d 3040you receive the SHA-1 name of a commit from a trusted source, then you
1bbf1c79
BF
3041can easily verify the entire history of commits reachable through
3042parents of that commit, and all of those contents of the trees referred
3043to by those commits.
b181d57f
BF
3044
3045So to introduce some real trust in the system, the only thing you need
3046to do is to digitally sign just 'one' special note, which includes the
3047name of a top-level commit. Your digital signature shows others
3048that you trust that commit, and the immutability of the history of
3049commits tells others that they can trust the whole history.
3050
3051In other words, you can easily validate a whole archive by just
a6e5ef7d 3052sending out a single email that tells the people the name (SHA-1 hash)
b181d57f
BF
3053of the top commit, and digitally sign that email using something
3054like GPG/PGP.
3055
3056To assist in this, git also provides the tag object...
3057
e34caace 3058[[tag-object]]
b181d57f 3059Tag Object
971aa71f 3060~~~~~~~~~~
b181d57f 3061
1bbf1c79
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3062A tag object contains an object, object type, tag name, the name of the
3063person ("tagger") who created the tag, and a message, which may contain
843c81dc 3064a signature, as can be seen using linkgit:git-cat-file[1]:
b181d57f 3065
1bbf1c79
BF
3066------------------------------------------------
3067$ git cat-file tag v1.5.0
3068object 437b1b20df4b356c9342dac8d38849f24ef44f27
3069type commit
3070tag v1.5.0
3071tagger Junio C Hamano <junkio@cox.net> 1171411200 +0000
3072
3073GIT 1.5.0
3074-----BEGIN PGP SIGNATURE-----
3075Version: GnuPG v1.4.6 (GNU/Linux)
3076
3077iD8DBQBF0lGqwMbZpPMRm5oRAuRiAJ9ohBLd7s2kqjkKlq1qqC57SbnmzQCdG4ui
3078nLE/L9aUXdWeTFPron96DLA=
3079=2E+0
3080-----END PGP SIGNATURE-----
3081------------------------------------------------
b181d57f 3082
5162e697
DM
3083See the linkgit:git-tag[1] command to learn how to create and verify tag
3084objects. (Note that linkgit:git-tag[1] can also be used to create
1bbf1c79 3085"lightweight tags", which are not tag objects at all, but just simple
fc74ecc1 3086references whose names begin with "refs/tags/").
b181d57f 3087
09eff7b0
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3088[[pack-files]]
3089How git stores objects efficiently: pack files
3090~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3091
9644ffdd 3092Newly created objects are initially created in a file named after the
a6e5ef7d 3093object's SHA-1 hash (stored in .git/objects).
09eff7b0
BF
3094
3095Unfortunately this system becomes inefficient once a project has a
3096lot of objects. Try this on an old project:
3097
3098------------------------------------------------
3099$ git count-objects
31006930 objects, 47620 kilobytes
3101------------------------------------------------
3102
3103The first number is the number of objects which are kept in
3104individual files. The second is the amount of space taken up by
3105those "loose" objects.
3106
3107You can save space and make git faster by moving these loose objects in
3108to a "pack file", which stores a group of objects in an efficient
3109compressed format; the details of how pack files are formatted can be
3110found in link:technical/pack-format.txt[technical/pack-format.txt].
3111
3112To put the loose objects into a pack, just run git repack:
3113
3114------------------------------------------------
3115$ git repack
3116Generating pack...
3117Done counting 6020 objects.
3118Deltifying 6020 objects.
3119 100% (6020/6020) done
3120Writing 6020 objects.
3121 100% (6020/6020) done
3122Total 6020, written 6020 (delta 4070), reused 0 (delta 0)
3123Pack pack-3e54ad29d5b2e05838c75df582c65257b8d08e1c created.
3124------------------------------------------------
3125
3126You can then run
3127
3128------------------------------------------------
3129$ git prune
3130------------------------------------------------
3131
3132to remove any of the "loose" objects that are now contained in the
3133pack. This will also remove any unreferenced objects (which may be
6127c086 3134created when, for example, you use "git reset" to remove a commit).
09eff7b0
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3135You can verify that the loose objects are gone by looking at the
3136.git/objects directory or by running
3137
3138------------------------------------------------
3139$ git count-objects
31400 objects, 0 kilobytes
3141------------------------------------------------
3142
3143Although the object files are gone, any commands that refer to those
3144objects will work exactly as they did before.
3145
5162e697 3146The linkgit:git-gc[1] command performs packing, pruning, and more for
09eff7b0
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3147you, so is normally the only high-level command you need.
3148
3149[[dangling-objects]]
3150Dangling objects
3151~~~~~~~~~~~~~~~~
3152
5162e697 3153The linkgit:git-fsck[1] command will sometimes complain about dangling
09eff7b0
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3154objects. They are not a problem.
3155
3156The most common cause of dangling objects is that you've rebased a
3157branch, or you have pulled from somebody else who rebased a branch--see
3158<<cleaning-up-history>>. In that case, the old head of the original
3159branch still exists, as does everything it pointed to. The branch
3160pointer itself just doesn't, since you replaced it with another one.
3161
3162There are also other situations that cause dangling objects. For
6127c086 3163example, a "dangling blob" may arise because you did a "git add" of a
09eff7b0
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3164file, but then, before you actually committed it and made it part of the
3165bigger picture, you changed something else in that file and committed
2ef8ac1b 3166that *updated* thing--the old state that you added originally ends up
09eff7b0
BF
3167not being pointed to by any commit or tree, so it's now a dangling blob
3168object.
3169
3170Similarly, when the "recursive" merge strategy runs, and finds that
3171there are criss-cross merges and thus more than one merge base (which is
3172fairly unusual, but it does happen), it will generate one temporary
3173midway tree (or possibly even more, if you had lots of criss-crossing
3174merges and more than two merge bases) as a temporary internal merge
3175base, and again, those are real objects, but the end result will not end
3176up pointing to them, so they end up "dangling" in your repository.
3177
3178Generally, dangling objects aren't anything to worry about. They can
3179even be very useful: if you screw something up, the dangling objects can
3180be how you recover your old tree (say, you did a rebase, and realized
2ef8ac1b 3181that you really didn't want to--you can look at what dangling objects
09eff7b0
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3182you have, and decide to reset your head to some old dangling state).
3183
3184For commits, you can just use:
3185
3186------------------------------------------------
3187$ gitk <dangling-commit-sha-goes-here> --not --all
3188------------------------------------------------
3189
3190This asks for all the history reachable from the given commit but not
3191from any branch, tag, or other reference. If you decide it's something
3192you want, you can always create a new reference to it, e.g.,
3193
3194------------------------------------------------
3195$ git branch recovered-branch <dangling-commit-sha-goes-here>
3196------------------------------------------------
3197
3198For blobs and trees, you can't do the same, but you can still examine
3199them. You can just do
3200
3201------------------------------------------------
3202$ git show <dangling-blob/tree-sha-goes-here>
3203------------------------------------------------
3204
3205to show what the contents of the blob were (or, for a tree, basically
3206what the "ls" for that directory was), and that may give you some idea
3207of what the operation was that left that dangling object.
3208
3209Usually, dangling blobs and trees aren't very interesting. They're
3210almost always the result of either being a half-way mergebase (the blob
3211will often even have the conflict markers from a merge in it, if you
3212have had conflicting merges that you fixed up by hand), or simply
6127c086 3213because you interrupted a "git fetch" with ^C or something like that,
09eff7b0
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3214leaving _some_ of the new objects in the object database, but just
3215dangling and useless.
3216
3217Anyway, once you are sure that you're not interested in any dangling
3218state, you can just prune all unreachable objects:
3219
3220------------------------------------------------
3221$ git prune
3222------------------------------------------------
3223
3224and they'll be gone. But you should only run "git prune" on a quiescent
2ef8ac1b 3225repository--it's kind of like doing a filesystem fsck recovery: you
09eff7b0
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3226don't want to do that while the filesystem is mounted.
3227
6127c086
FC
3228(The same is true of "git fsck" itself, btw, but since
3229`git fsck` never actually *changes* the repository, it just reports
3230on what it found, `git fsck` itself is never 'dangerous' to run.
09eff7b0
BF
3231Running it while somebody is actually changing the repository can cause
3232confusing and scary messages, but it won't actually do anything bad. In
3233contrast, running "git prune" while somebody is actively changing the
3234repository is a *BAD* idea).
b181d57f 3235
1cdade2c
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3236[[recovering-from-repository-corruption]]
3237Recovering from repository corruption
3238~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3239
3240By design, git treats data trusted to it with caution. However, even in
3241the absence of bugs in git itself, it is still possible that hardware or
3242operating system errors could corrupt data.
3243
3244The first defense against such problems is backups. You can back up a
3245git directory using clone, or just using cp, tar, or any other backup
3246mechanism.
3247
3248As a last resort, you can search for the corrupted objects and attempt
3249to replace them by hand. Back up your repository before attempting this
3250in case you corrupt things even more in the process.
3251
3252We'll assume that the problem is a single missing or corrupted blob,
9e5d87d4 3253which is sometimes a solvable problem. (Recovering missing trees and
1cdade2c
BF
3254especially commits is *much* harder).
3255
3256Before starting, verify that there is corruption, and figure out where
5162e697 3257it is with linkgit:git-fsck[1]; this may be time-consuming.
1cdade2c
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3258
3259Assume the output looks like this:
3260
3261------------------------------------------------
b1889c36 3262$ git fsck --full
1cdade2c
BF
3263broken link from tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
3264 to blob 4b9458b3786228369c63936db65827de3cc06200
3265missing blob 4b9458b3786228369c63936db65827de3cc06200
3266------------------------------------------------
3267
3268(Typically there will be some "dangling object" messages too, but they
3269aren't interesting.)
3270
3271Now you know that blob 4b9458b3 is missing, and that the tree 2d9263c6
3272points to it. If you could find just one copy of that missing blob
3273object, possibly in some other repository, you could move it into
3274.git/objects/4b/9458b3... and be done. Suppose you can't. You can
5162e697 3275still examine the tree that pointed to it with linkgit:git-ls-tree[1],
1cdade2c
BF
3276which might output something like:
3277
3278------------------------------------------------
3279$ git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
3280100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8 .gitignore
3281100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883 .mailmap
3282100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c COPYING
3283...
3284100644 blob 4b9458b3786228369c63936db65827de3cc06200 myfile
3285...
3286------------------------------------------------
3287
3288So now you know that the missing blob was the data for a file named
3289"myfile". And chances are you can also identify the directory--let's
3290say it's in "somedirectory". If you're lucky the missing copy might be
3291the same as the copy you have checked out in your working tree at
3292"somedirectory/myfile"; you can test whether that's right with
5162e697 3293linkgit:git-hash-object[1]:
1cdade2c
BF
3294
3295------------------------------------------------
3296$ git hash-object -w somedirectory/myfile
3297------------------------------------------------
3298
3299which will create and store a blob object with the contents of
a6e5ef7d 3300somedirectory/myfile, and output the SHA-1 of that object. if you're
1cdade2c
BF
3301extremely lucky it might be 4b9458b3786228369c63936db65827de3cc06200, in
3302which case you've guessed right, and the corruption is fixed!
3303
3304Otherwise, you need more information. How do you tell which version of
3305the file has been lost?
3306
3307The easiest way to do this is with:
3308
3309------------------------------------------------
3310$ git log --raw --all --full-history -- somedirectory/myfile
3311------------------------------------------------
3312
3313Because you're asking for raw output, you'll now get something like
3314
3315------------------------------------------------
3316commit abc
3317Author:
3318Date:
3319...
33