Documentation: avoid poor-man's small caps GIT
[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
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347keeps branches which track each of those remote branches, called
348remote-tracking branches, which you
5162e697 349can view using the "-r" option to linkgit:git-branch[1]:
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350
351------------------------------------------------
352$ git branch -r
353 origin/HEAD
354 origin/html
355 origin/maint
356 origin/man
357 origin/master
358 origin/next
359 origin/pu
360 origin/todo
361------------------------------------------------
362
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363In this example, "origin" is called a remote repository, or "remote"
364for short. The branches of this repository are called "remote
365branches" from our point of view. The remote-tracking branches listed
366above were created based on the remote branches at clone time and will
367be updated by "git fetch" (hence "git pull") and "git push". See
368<<Updating-a-repository-With-git-fetch>> for details.
369
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370You might want to build on one of these remote-tracking branches
371on a branch of your own, just as you would for a tag:
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372
373------------------------------------------------
374$ git checkout -b my-todo-copy origin/todo
375------------------------------------------------
376
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377You can also check out "origin/todo" directly to examine it or
378write a one-off patch. See <<detached-head,detached head>>.
379
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380Note that the name "origin" is just the name that git uses by default
381to refer to the repository that you cloned from.
382
383[[how-git-stores-references]]
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384Naming branches, tags, and other references
385-------------------------------------------
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386
387Branches, remote-tracking branches, and tags are all references to
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388commits. All references are named with a slash-separated path name
389starting with "refs"; the names we've been using so far are actually
390shorthand:
d19fbc3c 391
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392 - The branch "test" is short for "refs/heads/test".
393 - The tag "v2.6.18" is short for "refs/tags/v2.6.18".
394 - "origin/master" is short for "refs/remotes/origin/master".
d19fbc3c 395
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396The full name is occasionally useful if, for example, there ever
397exists a tag and a branch with the same name.
d19fbc3c 398
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399(Newly created refs are actually stored in the .git/refs directory,
400under the path given by their name. However, for efficiency reasons
401they may also be packed together in a single file; see
5162e697 402linkgit:git-pack-refs[1]).
fc74ecc1 403
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404As another useful shortcut, the "HEAD" of a repository can be referred
405to just using the name of that repository. So, for example, "origin"
406is usually a shortcut for the HEAD branch in the repository "origin".
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407
408For the complete list of paths which git checks for references, and
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409the order it uses to decide which to choose when there are multiple
410references with the same shorthand name, see the "SPECIFYING
9d83e382 411REVISIONS" section of linkgit:gitrevisions[7].
d19fbc3c 412
aa971cb9 413[[Updating-a-repository-With-git-fetch]]
6127c086 414Updating a repository with git fetch
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415------------------------------------
416
417Eventually the developer cloned from will do additional work in her
418repository, creating new commits and advancing the branches to point
419at the new commits.
420
421The command "git fetch", with no arguments, will update all of the
422remote-tracking branches to the latest version found in her
423repository. It will not touch any of your own branches--not even the
424"master" branch that was created for you on clone.
425
e34caace 426[[fetching-branches]]
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427Fetching branches from other repositories
428-----------------------------------------
429
430You can also track branches from repositories other than the one you
5162e697 431cloned from, using linkgit:git-remote[1]:
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432
433-------------------------------------------------
434$ git remote add linux-nfs git://linux-nfs.org/pub/nfs-2.6.git
04483524 435$ git fetch linux-nfs
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436* refs/remotes/linux-nfs/master: storing branch 'master' ...
437 commit: bf81b46
438-------------------------------------------------
439
440New remote-tracking branches will be stored under the shorthand name
6127c086 441that you gave "git remote add", in this case linux-nfs:
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442
443-------------------------------------------------
444$ git branch -r
445linux-nfs/master
446origin/master
447-------------------------------------------------
448
8b3f3f84 449If you run "git fetch <remote>" later, the remote-tracking branches for the
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450named <remote> will be updated.
451
452If you examine the file .git/config, you will see that git has added
453a new stanza:
454
455-------------------------------------------------
456$ cat .git/config
457...
458[remote "linux-nfs"]
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459 url = git://linux-nfs.org/pub/nfs-2.6.git
460 fetch = +refs/heads/*:refs/remotes/linux-nfs/*
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461...
462-------------------------------------------------
463
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464This is what causes git to track the remote's branches; you may modify
465or delete these configuration options by editing .git/config with a
466text editor. (See the "CONFIGURATION FILE" section of
5162e697 467linkgit:git-config[1] for details.)
d5cd5de4 468
e34caace 469[[exploring-git-history]]
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470Exploring git history
471=====================
472
473Git is best thought of as a tool for storing the history of a
474collection of files. It does this by storing compressed snapshots of
1130845b 475the contents of a file hierarchy, together with "commits" which show
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476the relationships between these snapshots.
477
478Git provides extremely flexible and fast tools for exploring the
479history of a project.
480
aacd404e 481We start with one specialized tool that is useful for finding the
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482commit that introduced a bug into a project.
483
e34caace 484[[using-bisect]]
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485How to use bisect to find a regression
486--------------------------------------
487
488Suppose version 2.6.18 of your project worked, but the version at
489"master" crashes. Sometimes the best way to find the cause of such a
490regression is to perform a brute-force search through the project's
491history to find the particular commit that caused the problem. The
5162e697 492linkgit:git-bisect[1] command can help you do this:
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493
494-------------------------------------------------
495$ git bisect start
496$ git bisect good v2.6.18
497$ git bisect bad master
498Bisecting: 3537 revisions left to test after this
499[65934a9a028b88e83e2b0f8b36618fe503349f8e] BLOCK: Make USB storage depend on SCSI rather than selecting it [try #6]
500-------------------------------------------------
501
502If you run "git branch" at this point, you'll see that git has
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503temporarily moved you in "(no branch)". HEAD is now detached from any
504branch and points directly to a commit (with commit id 65934...) that
505is reachable from "master" but not from v2.6.18. Compile and test it,
506and see whether it crashes. Assume it does crash. Then:
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507
508-------------------------------------------------
509$ git bisect bad
510Bisecting: 1769 revisions left to test after this
511[7eff82c8b1511017ae605f0c99ac275a7e21b867] i2c-core: Drop useless bitmaskings
512-------------------------------------------------
513
514checks out an older version. Continue like this, telling git at each
515stage whether the version it gives you is good or bad, and notice
516that the number of revisions left to test is cut approximately in
517half each time.
518
519After about 13 tests (in this case), it will output the commit id of
520the guilty commit. You can then examine the commit with
5162e697 521linkgit:git-show[1], find out who wrote it, and mail them your bug
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522report with the commit id. Finally, run
523
524-------------------------------------------------
525$ git bisect reset
526-------------------------------------------------
527
0e25790f 528to return you to the branch you were on before.
d19fbc3c 529
6127c086 530Note that the version which `git bisect` checks out for you at each
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531point is just a suggestion, and you're free to try a different
532version if you think it would be a good idea. For example,
533occasionally you may land on a commit that broke something unrelated;
534run
535
536-------------------------------------------------
04483524 537$ git bisect visualize
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538-------------------------------------------------
539
540which will run gitk and label the commit it chose with a marker that
843c81dc 541says "bisect". Choose a safe-looking commit nearby, note its commit
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542id, and check it out with:
543
544-------------------------------------------------
545$ git reset --hard fb47ddb2db...
546-------------------------------------------------
547
548then test, run "bisect good" or "bisect bad" as appropriate, and
549continue.
550
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551Instead of "git bisect visualize" and then "git reset --hard
552fb47ddb2db...", you might just want to tell git that you want to skip
553the current commit:
554
555-------------------------------------------------
556$ git bisect skip
557-------------------------------------------------
558
559In this case, though, git may not eventually be able to tell the first
a0178ae2 560bad one between some first skipped commits and a later bad commit.
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561
562There are also ways to automate the bisecting process if you have a
563test script that can tell a good from a bad commit. See
564linkgit:git-bisect[1] for more information about this and other "git
565bisect" features.
566
e34caace 567[[naming-commits]]
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568Naming commits
569--------------
570
571We have seen several ways of naming commits already:
572
d55ae921 573 - 40-hexdigit object name
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574 - branch name: refers to the commit at the head of the given
575 branch
576 - tag name: refers to the commit pointed to by the given tag
577 (we've seen branches and tags are special cases of
578 <<how-git-stores-references,references>>).
579 - HEAD: refers to the head of the current branch
580
eb6ae7f4 581There are many more; see the "SPECIFYING REVISIONS" section of the
9d83e382 582linkgit:gitrevisions[7] man page for the complete list of ways to
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583name revisions. Some examples:
584
585-------------------------------------------------
d55ae921 586$ git show fb47ddb2 # the first few characters of the object name
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587 # are usually enough to specify it uniquely
588$ git show HEAD^ # the parent of the HEAD commit
589$ git show HEAD^^ # the grandparent
590$ git show HEAD~4 # the great-great-grandparent
591-------------------------------------------------
592
593Recall that merge commits may have more than one parent; by default,
594^ and ~ follow the first parent listed in the commit, but you can
595also choose:
596
597-------------------------------------------------
598$ git show HEAD^1 # show the first parent of HEAD
599$ git show HEAD^2 # show the second parent of HEAD
600-------------------------------------------------
601
602In addition to HEAD, there are several other special names for
603commits:
604
605Merges (to be discussed later), as well as operations such as
6127c086 606`git reset`, which change the currently checked-out commit, generally
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607set ORIG_HEAD to the value HEAD had before the current operation.
608
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609The `git fetch` operation always stores the head of the last fetched
610branch in FETCH_HEAD. For example, if you run `git fetch` without
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611specifying a local branch as the target of the operation
612
613-------------------------------------------------
614$ git fetch git://example.com/proj.git theirbranch
615-------------------------------------------------
616
617the fetched commits will still be available from FETCH_HEAD.
618
619When we discuss merges we'll also see the special name MERGE_HEAD,
620which refers to the other branch that we're merging in to the current
621branch.
622
5162e697 623The linkgit:git-rev-parse[1] command is a low-level command that is
d55ae921
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624occasionally useful for translating some name for a commit to the object
625name for that commit:
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626
627-------------------------------------------------
628$ git rev-parse origin
629e05db0fd4f31dde7005f075a84f96b360d05984b
630-------------------------------------------------
631
e34caace 632[[creating-tags]]
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633Creating tags
634-------------
635
636We can also create a tag to refer to a particular commit; after
637running
638
639-------------------------------------------------
04483524 640$ git tag stable-1 1b2e1d63ff
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641-------------------------------------------------
642
643You can use stable-1 to refer to the commit 1b2e1d63ff.
644
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645This creates a "lightweight" tag. If you would also like to include a
646comment with the tag, and possibly sign it cryptographically, then you
5162e697 647should create a tag object instead; see the linkgit:git-tag[1] man page
c64415e2 648for details.
d19fbc3c 649
e34caace 650[[browsing-revisions]]
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651Browsing revisions
652------------------
653
5162e697 654The linkgit:git-log[1] command can show lists of commits. On its
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655own, it shows all commits reachable from the parent commit; but you
656can also make more specific requests:
657
658-------------------------------------------------
659$ git log v2.5.. # commits since (not reachable from) v2.5
660$ git log test..master # commits reachable from master but not test
661$ git log master..test # ...reachable from test but not master
662$ git log master...test # ...reachable from either test or master,
663 # but not both
664$ git log --since="2 weeks ago" # commits from the last 2 weeks
665$ git log Makefile # commits which modify Makefile
666$ git log fs/ # ... which modify any file under fs/
667$ git log -S'foo()' # commits which add or remove any file data
668 # matching the string 'foo()'
669-------------------------------------------------
670
671And of course you can combine all of these; the following finds
672commits since v2.5 which touch the Makefile or any file under fs:
673
674-------------------------------------------------
675$ git log v2.5.. Makefile fs/
676-------------------------------------------------
677
678You can also ask git log to show patches:
679
680-------------------------------------------------
681$ git log -p
682-------------------------------------------------
683
5162e697 684See the "--pretty" option in the linkgit:git-log[1] man page for more
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685display options.
686
687Note that git log starts with the most recent commit and works
688backwards through the parents; however, since git history can contain
3dff5379 689multiple independent lines of development, the particular order that
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690commits are listed in may be somewhat arbitrary.
691
e34caace 692[[generating-diffs]]
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693Generating diffs
694----------------
695
696You can generate diffs between any two versions using
5162e697 697linkgit:git-diff[1]:
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698
699-------------------------------------------------
700$ git diff master..test
701-------------------------------------------------
702
5b98d9bc
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703That will produce the diff between the tips of the two branches. If
704you'd prefer to find the diff from their common ancestor to test, you
705can use three dots instead of two:
706
707-------------------------------------------------
708$ git diff master...test
709-------------------------------------------------
710
711Sometimes what you want instead is a set of patches; for this you can
5162e697 712use linkgit:git-format-patch[1]:
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713
714-------------------------------------------------
715$ git format-patch master..test
716-------------------------------------------------
717
718will generate a file with a patch for each commit reachable from test
5b98d9bc 719but not from master.
d19fbc3c 720
e34caace 721[[viewing-old-file-versions]]
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722Viewing old file versions
723-------------------------
724
725You can always view an old version of a file by just checking out the
726correct revision first. But sometimes it is more convenient to be
727able to view an old version of a single file without checking
728anything out; this command does that:
729
730-------------------------------------------------
731$ git show v2.5:fs/locks.c
732-------------------------------------------------
733
734Before the colon may be anything that names a commit, and after it
735may be any path to a file tracked by git.
736
e34caace 737[[history-examples]]
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738Examples
739--------
740
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741[[counting-commits-on-a-branch]]
742Counting the number of commits on a branch
743~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
744
745Suppose you want to know how many commits you've made on "mybranch"
746since it diverged from "origin":
747
748-------------------------------------------------
749$ git log --pretty=oneline origin..mybranch | wc -l
750-------------------------------------------------
751
752Alternatively, you may often see this sort of thing done with the
a6e5ef7d 753lower-level command linkgit:git-rev-list[1], which just lists the SHA-1's
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754of all the given commits:
755
756-------------------------------------------------
757$ git rev-list origin..mybranch | wc -l
758-------------------------------------------------
759
e34caace 760[[checking-for-equal-branches]]
aec053bb 761Check whether two branches point at the same history
2f99710c 762~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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763
764Suppose you want to check whether two branches point at the same point
765in history.
766
767-------------------------------------------------
768$ git diff origin..master
769-------------------------------------------------
770
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771will tell you whether the contents of the project are the same at the
772two branches; in theory, however, it's possible that the same project
773contents could have been arrived at by two different historical
d55ae921 774routes. You could compare the object names:
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775
776-------------------------------------------------
777$ git rev-list origin
778e05db0fd4f31dde7005f075a84f96b360d05984b
779$ git rev-list master
780e05db0fd4f31dde7005f075a84f96b360d05984b
781-------------------------------------------------
782
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783Or you could recall that the ... operator selects all commits
784contained reachable from either one reference or the other but not
785both: so
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786
787-------------------------------------------------
788$ git log origin...master
789-------------------------------------------------
790
791will return no commits when the two branches are equal.
792
e34caace 793[[finding-tagged-descendants]]
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794Find first tagged version including a given fix
795~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
aec053bb 796
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797Suppose you know that the commit e05db0fd fixed a certain problem.
798You'd like to find the earliest tagged release that contains that
799fix.
800
801Of course, there may be more than one answer--if the history branched
802after commit e05db0fd, then there could be multiple "earliest" tagged
803releases.
804
805You could just visually inspect the commits since e05db0fd:
806
807-------------------------------------------------
808$ gitk e05db0fd..
809-------------------------------------------------
810
5162e697 811Or you can use linkgit:git-name-rev[1], which will give the commit a
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812name based on any tag it finds pointing to one of the commit's
813descendants:
814
815-------------------------------------------------
04483524 816$ git name-rev --tags e05db0fd
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817e05db0fd tags/v1.5.0-rc1^0~23
818-------------------------------------------------
819
5162e697 820The linkgit:git-describe[1] command does the opposite, naming the
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821revision using a tag on which the given commit is based:
822
823-------------------------------------------------
824$ git describe e05db0fd
04483524 825v1.5.0-rc0-260-ge05db0f
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826-------------------------------------------------
827
828but that may sometimes help you guess which tags might come after the
829given commit.
830
831If you just want to verify whether a given tagged version contains a
5162e697 832given commit, you could use linkgit:git-merge-base[1]:
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833
834-------------------------------------------------
835$ git merge-base e05db0fd v1.5.0-rc1
836e05db0fd4f31dde7005f075a84f96b360d05984b
837-------------------------------------------------
838
839The merge-base command finds a common ancestor of the given commits,
840and always returns one or the other in the case where one is a
841descendant of the other; so the above output shows that e05db0fd
842actually is an ancestor of v1.5.0-rc1.
843
844Alternatively, note that
845
846-------------------------------------------------
4a7979ca 847$ git log v1.5.0-rc1..e05db0fd
b181d57f
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848-------------------------------------------------
849
4a7979ca 850will produce empty output if and only if v1.5.0-rc1 includes e05db0fd,
b181d57f 851because it outputs only commits that are not reachable from v1.5.0-rc1.
aec053bb 852
5162e697 853As yet another alternative, the linkgit:git-show-branch[1] command lists
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854the commits reachable from its arguments with a display on the left-hand
855side that indicates which arguments that commit is reachable from. So,
856you can run something like
857
858-------------------------------------------------
859$ git show-branch e05db0fd v1.5.0-rc0 v1.5.0-rc1 v1.5.0-rc2
860! [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
861available
862 ! [v1.5.0-rc0] GIT v1.5.0 preview
863 ! [v1.5.0-rc1] GIT v1.5.0-rc1
864 ! [v1.5.0-rc2] GIT v1.5.0-rc2
865...
866-------------------------------------------------
867
868then search for a line that looks like
869
870-------------------------------------------------
871+ ++ [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
872available
873-------------------------------------------------
874
875Which shows that e05db0fd is reachable from itself, from v1.5.0-rc1, and
876from v1.5.0-rc2, but not from v1.5.0-rc0.
877
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878[[showing-commits-unique-to-a-branch]]
879Showing commits unique to a given branch
880~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4a7979ca 881
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882Suppose you would like to see all the commits reachable from the branch
883head named "master" but not from any other head in your repository.
d19fbc3c 884
629d9f78 885We can list all the heads in this repository with
5162e697 886linkgit:git-show-ref[1]:
d19fbc3c 887
629d9f78
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888-------------------------------------------------
889$ git show-ref --heads
890bf62196b5e363d73353a9dcf094c59595f3153b7 refs/heads/core-tutorial
891db768d5504c1bb46f63ee9d6e1772bd047e05bf9 refs/heads/maint
892a07157ac624b2524a059a3414e99f6f44bebc1e7 refs/heads/master
89324dbc180ea14dc1aebe09f14c8ecf32010690627 refs/heads/tutorial-2
8941e87486ae06626c2f31eaa63d26fc0fd646c8af2 refs/heads/tutorial-fixes
895-------------------------------------------------
d19fbc3c 896
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897We can get just the branch-head names, and remove "master", with
898the help of the standard utilities cut and grep:
899
900-------------------------------------------------
901$ git show-ref --heads | cut -d' ' -f2 | grep -v '^refs/heads/master'
902refs/heads/core-tutorial
903refs/heads/maint
904refs/heads/tutorial-2
905refs/heads/tutorial-fixes
906-------------------------------------------------
907
908And then we can ask to see all the commits reachable from master
909but not from these other heads:
910
911-------------------------------------------------
912$ gitk master --not $( git show-ref --heads | cut -d' ' -f2 |
913 grep -v '^refs/heads/master' )
914-------------------------------------------------
915
916Obviously, endless variations are possible; for example, to see all
917commits reachable from some head but not from any tag in the repository:
918
919-------------------------------------------------
c78974f7 920$ gitk $( git show-ref --heads ) --not $( git show-ref --tags )
629d9f78
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921-------------------------------------------------
922
9d83e382 923(See linkgit:gitrevisions[7] for explanations of commit-selecting
629d9f78
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924syntax such as `--not`.)
925
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926[[making-a-release]]
927Creating a changelog and tarball for a software release
928~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
929
5162e697 930The linkgit:git-archive[1] command can create a tar or zip archive from
82c8bf28
BF
931any version of a project; for example:
932
933-------------------------------------------------
934$ git archive --format=tar --prefix=project/ HEAD | gzip >latest.tar.gz
935-------------------------------------------------
936
937will use HEAD to produce a tar archive in which each filename is
ccd71866 938preceded by "project/".
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939
940If you're releasing a new version of a software project, you may want
941to simultaneously make a changelog to include in the release
942announcement.
943
944Linus Torvalds, for example, makes new kernel releases by tagging them,
945then running:
946
947-------------------------------------------------
948$ release-script 2.6.12 2.6.13-rc6 2.6.13-rc7
949-------------------------------------------------
950
951where release-script is a shell script that looks like:
952
953-------------------------------------------------
954#!/bin/sh
955stable="$1"
956last="$2"
957new="$3"
958echo "# git tag v$new"
959echo "git archive --prefix=linux-$new/ v$new | gzip -9 > ../linux-$new.tar.gz"
960echo "git diff v$stable v$new | gzip -9 > ../patch-$new.gz"
961echo "git log --no-merges v$new ^v$last > ../ChangeLog-$new"
962echo "git shortlog --no-merges v$new ^v$last > ../ShortLog"
963echo "git diff --stat --summary -M v$last v$new > ../diffstat-$new"
964-------------------------------------------------
965
966and then he just cut-and-pastes the output commands after verifying that
967they look OK.
4a7979ca 968
e1ba4c32 969[[Finding-commits-With-given-Content]]
187b0d80 970Finding commits referencing a file with given content
d5821de2 971~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
187b0d80
BF
972
973Somebody hands you a copy of a file, and asks which commits modified a
974file such that it contained the given content either before or after the
975commit. You can find out with this:
976
977-------------------------------------------------
477ff5b7 978$ git log --raw --abbrev=40 --pretty=oneline |
187b0d80
BF
979 grep -B 1 `git hash-object filename`
980-------------------------------------------------
981
982Figuring out why this works is left as an exercise to the (advanced)
5162e697
DM
983student. The linkgit:git-log[1], linkgit:git-diff-tree[1], and
984linkgit:git-hash-object[1] man pages may prove helpful.
187b0d80 985
aa971cb9 986[[Developing-With-git]]
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987Developing with git
988===================
989
e34caace 990[[telling-git-your-name]]
d19fbc3c
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991Telling git your name
992---------------------
993
994Before creating any commits, you should introduce yourself to git. The
58c19d1f
BF
995easiest way to do so is to make sure the following lines appear in a
996file named .gitconfig in your home directory:
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997
998------------------------------------------------
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999[user]
1000 name = Your Name Comes Here
1001 email = you@yourdomain.example.com
d19fbc3c
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1002------------------------------------------------
1003
5162e697 1004(See the "CONFIGURATION FILE" section of linkgit:git-config[1] for
fc90c536
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1005details on the configuration file.)
1006
d19fbc3c 1007
e34caace 1008[[creating-a-new-repository]]
d19fbc3c
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1009Creating a new repository
1010-------------------------
1011
1012Creating a new repository from scratch is very easy:
1013
1014-------------------------------------------------
1015$ mkdir project
1016$ cd project
f1d2b477 1017$ git init
d19fbc3c
BF
1018-------------------------------------------------
1019
1020If you have some initial content (say, a tarball):
1021
1022-------------------------------------------------
0ddd93b2 1023$ tar xzvf project.tar.gz
d19fbc3c 1024$ cd project
f1d2b477 1025$ git init
d19fbc3c
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1026$ git add . # include everything below ./ in the first commit:
1027$ git commit
1028-------------------------------------------------
1029
1030[[how-to-make-a-commit]]
ae25c67a 1031How to make a commit
d19fbc3c
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1032--------------------
1033
1034Creating a new commit takes three steps:
1035
1036 1. Making some changes to the working directory using your
1037 favorite editor.
1038 2. Telling git about your changes.
1039 3. Creating the commit using the content you told git about
1040 in step 2.
1041
1042In practice, you can interleave and repeat steps 1 and 2 as many
1043times as you want: in order to keep track of what you want committed
1044at step 3, git maintains a snapshot of the tree's contents in a
1045special staging area called "the index."
1046
01997b4a
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1047At the beginning, the content of the index will be identical to
1048that of the HEAD. The command "git diff --cached", which shows
1049the difference between the HEAD and the index, should therefore
1050produce no output at that point.
eb6ae7f4 1051
d19fbc3c
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1052Modifying the index is easy:
1053
1054To update the index with the new contents of a modified file, use
1055
1056-------------------------------------------------
1057$ git add path/to/file
1058-------------------------------------------------
1059
1060To add the contents of a new file to the index, use
1061
1062-------------------------------------------------
1063$ git add path/to/file
1064-------------------------------------------------
1065
eb6ae7f4 1066To remove a file from the index and from the working tree,
d19fbc3c
BF
1067
1068-------------------------------------------------
1069$ git rm path/to/file
1070-------------------------------------------------
1071
1072After each step you can verify that
1073
1074-------------------------------------------------
1075$ git diff --cached
1076-------------------------------------------------
1077
1078always shows the difference between the HEAD and the index file--this
1079is what you'd commit if you created the commit now--and that
1080
1081-------------------------------------------------
1082$ git diff
1083-------------------------------------------------
1084
1085shows the difference between the working tree and the index file.
1086
6127c086 1087Note that "git add" always adds just the current contents of a file
d19fbc3c 1088to the index; further changes to the same file will be ignored unless
6127c086 1089you run `git add` on the file again.
d19fbc3c
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1090
1091When you're ready, just run
1092
1093-------------------------------------------------
1094$ git commit
1095-------------------------------------------------
1096
1097and git will prompt you for a commit message and then create the new
3dff5379 1098commit. Check to make sure it looks like what you expected with
d19fbc3c
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1099
1100-------------------------------------------------
1101$ git show
1102-------------------------------------------------
1103
1104As a special shortcut,
a6080a0a 1105
d19fbc3c
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1106-------------------------------------------------
1107$ git commit -a
1108-------------------------------------------------
1109
1110will update the index with any files that you've modified or removed
1111and create a commit, all in one step.
1112
1113A number of commands are useful for keeping track of what you're
1114about to commit:
1115
1116-------------------------------------------------
1117$ git diff --cached # difference between HEAD and the index; what
1130845b 1118 # would be committed if you ran "commit" now.
d19fbc3c
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1119$ git diff # difference between the index file and your
1120 # working directory; changes that would not
1121 # be included if you ran "commit" now.
c64415e2
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1122$ git diff HEAD # difference between HEAD and working tree; what
1123 # would be committed if you ran "commit -a" now.
d19fbc3c
BF
1124$ git status # a brief per-file summary of the above.
1125-------------------------------------------------
1126
5162e697 1127You can also use linkgit:git-gui[1] to create commits, view changes in
407c0c87
BF
1128the index and the working tree files, and individually select diff hunks
1129for inclusion in the index (by right-clicking on the diff hunk and
1130choosing "Stage Hunk For Commit").
1131
e34caace 1132[[creating-good-commit-messages]]
ae25c67a 1133Creating good commit messages
d19fbc3c
BF
1134-----------------------------
1135
1136Though not required, it's a good idea to begin the commit message
1137with a single short (less than 50 character) line summarizing the
1138change, followed by a blank line and then a more thorough
52ffe995
JW
1139description. The text up to the first blank line in a commit
1140message is treated as the commit title, and that title is used
1141throughout git. For example, linkgit:git-format-patch[1] turns a
1142commit into email, and it uses the title on the Subject line and the
1143rest of the commit in the body.
1144
d19fbc3c 1145
2dc53617
JH
1146[[ignoring-files]]
1147Ignoring files
1148--------------
1149
1150A project will often generate files that you do 'not' want to track with git.
1151This typically includes files generated by a build process or temporary
1152backup files made by your editor. Of course, 'not' tracking files with git
6127c086 1153is just a matter of 'not' calling `git add` on them. But it quickly becomes
2dc53617 1154annoying to have these untracked files lying around; e.g. they make
dcb11263
CJ
1155`git add .` practically useless, and they keep showing up in the output of
1156`git status`.
2dc53617 1157
464a8a7a
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1158You can tell git to ignore certain files by creating a file called .gitignore
1159in the top level of your working directory, with contents such as:
2dc53617
JH
1160
1161-------------------------------------------------
1162# Lines starting with '#' are considered comments.
464a8a7a 1163# Ignore any file named foo.txt.
2dc53617
JH
1164foo.txt
1165# Ignore (generated) html files,
1166*.html
1167# except foo.html which is maintained by hand.
1168!foo.html
1169# Ignore objects and archives.
1170*.[oa]
1171-------------------------------------------------
1172
5162e697 1173See linkgit:gitignore[5] for a detailed explanation of the syntax. You can
464a8a7a
BF
1174also place .gitignore files in other directories in your working tree, and they
1175will apply to those directories and their subdirectories. The `.gitignore`
1176files can be added to your repository like any other files (just run `git add
1177.gitignore` and `git commit`, as usual), which is convenient when the exclude
1178patterns (such as patterns matching build output files) would also make sense
1179for other users who clone your repository.
1180
1181If you wish the exclude patterns to affect only certain repositories
1182(instead of every repository for a given project), you may instead put
1183them in a file in your repository named .git/info/exclude, or in any file
1184specified by the `core.excludesfile` configuration variable. Some git
1185commands can also take exclude patterns directly on the command line.
5162e697 1186See linkgit:gitignore[5] for the details.
2dc53617 1187
e34caace 1188[[how-to-merge]]
ae25c67a 1189How to merge
d19fbc3c
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1190------------
1191
1192You can rejoin two diverging branches of development using
5162e697 1193linkgit:git-merge[1]:
d19fbc3c
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1194
1195-------------------------------------------------
1196$ git merge branchname
1197-------------------------------------------------
1198
1199merges the development in the branch "branchname" into the current
e63ec003
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1200branch.
1201
1202A merge is made by combining the changes made in "branchname" and the
1203changes made up to the latest commit in your current branch since
1204their histories forked. The work tree is overwritten by the result of
1205the merge when this combining is done cleanly, or overwritten by a
1206half-merged results when this combining results in conflicts.
1207Therefore, if you have uncommitted changes touching the same files as
1208the ones impacted by the merge, Git will refuse to proceed. Most of
1209the time, you will want to commit your changes before you can merge,
1210and if you don't, then linkgit:git-stash[1] can take these changes
1211away while you're doing the merge, and reapply them afterwards.
1212
6a5d0b0a 1213If the changes are independent enough, Git will automatically complete
e63ec003
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1214the merge and commit the result (or reuse an existing commit in case
1215of <<fast-forwards,fast-forward>>, see below). On the other hand,
1216if there are conflicts--for example, if the same file is
d19fbc3c
BF
1217modified in two different ways in the remote branch and the local
1218branch--then you are warned; the output may look something like this:
1219
1220-------------------------------------------------
fabbd8f6
BF
1221$ git merge next
1222 100% (4/4) done
1223Auto-merged file.txt
d19fbc3c
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1224CONFLICT (content): Merge conflict in file.txt
1225Automatic merge failed; fix conflicts and then commit the result.
1226-------------------------------------------------
1227
1228Conflict markers are left in the problematic files, and after
1229you resolve the conflicts manually, you can update the index
1230with the contents and run git commit, as you normally would when
1231creating a new file.
1232
1233If you examine the resulting commit using gitk, you will see that it
1234has two parents, one pointing to the top of the current branch, and
1235one to the top of the other branch.
1236
d19fbc3c
BF
1237[[resolving-a-merge]]
1238Resolving a merge
1239-----------------
1240
1241When a merge isn't resolved automatically, git leaves the index and
1242the working tree in a special state that gives you all the
1243information you need to help resolve the merge.
1244
1245Files with conflicts are marked specially in the index, so until you
5162e697 1246resolve the problem and update the index, linkgit:git-commit[1] will
ef561ac7 1247fail:
d19fbc3c
BF
1248
1249-------------------------------------------------
1250$ git commit
1251file.txt: needs merge
1252-------------------------------------------------
1253
5162e697 1254Also, linkgit:git-status[1] will list those files as "unmerged", and the
ef561ac7
BF
1255files with conflicts will have conflict markers added, like this:
1256
1257-------------------------------------------------
1258<<<<<<< HEAD:file.txt
1259Hello world
1260=======
1261Goodbye
1262>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
1263-------------------------------------------------
1264
1265All you need to do is edit the files to resolve the conflicts, and then
1266
1267-------------------------------------------------
1268$ git add file.txt
1269$ git commit
1270-------------------------------------------------
1271
1272Note that the commit message will already be filled in for you with
1273some information about the merge. Normally you can just use this
1274default message unchanged, but you may add additional commentary of
1275your own if desired.
1276
1277The above is all you need to know to resolve a simple merge. But git
1278also provides more information to help resolve conflicts:
1279
e34caace 1280[[conflict-resolution]]
ef561ac7
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1281Getting conflict-resolution help during a merge
1282~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d19fbc3c
BF
1283
1284All of the changes that git was able to merge automatically are
5162e697 1285already added to the index file, so linkgit:git-diff[1] shows only
ef561ac7 1286the conflicts. It uses an unusual syntax:
d19fbc3c
BF
1287
1288-------------------------------------------------
1289$ git diff
1290diff --cc file.txt
1291index 802992c,2b60207..0000000
1292--- a/file.txt
1293+++ b/file.txt
1294@@@ -1,1 -1,1 +1,5 @@@
1295++<<<<<<< HEAD:file.txt
1296 +Hello world
1297++=======
1298+ Goodbye
1299++>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
1300-------------------------------------------------
1301
1130845b 1302Recall that the commit which will be committed after we resolve this
d19fbc3c
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1303conflict will have two parents instead of the usual one: one parent
1304will be HEAD, the tip of the current branch; the other will be the
1305tip of the other branch, which is stored temporarily in MERGE_HEAD.
1306
ef561ac7
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1307During the merge, the index holds three versions of each file. Each of
1308these three "file stages" represents a different version of the file:
1309
1310-------------------------------------------------
1311$ git show :1:file.txt # the file in a common ancestor of both branches
4209752d
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1312$ git show :2:file.txt # the version from HEAD.
1313$ git show :3:file.txt # the version from MERGE_HEAD.
ef561ac7
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1314-------------------------------------------------
1315
4209752d
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1316When you ask linkgit:git-diff[1] to show the conflicts, it runs a
1317three-way diff between the conflicted merge results in the work tree with
1318stages 2 and 3 to show only hunks whose contents come from both sides,
1319mixed (in other words, when a hunk's merge results come only from stage 2,
1320that part is not conflicting and is not shown. Same for stage 3).
ef561ac7
BF
1321
1322The diff above shows the differences between the working-tree version of
1323file.txt and the stage 2 and stage 3 versions. So instead of preceding
1324each line by a single "+" or "-", it now uses two columns: the first
1325column is used for differences between the first parent and the working
1326directory copy, and the second for differences between the second parent
1327and the working directory copy. (See the "COMBINED DIFF FORMAT" section
5162e697 1328of linkgit:git-diff-files[1] for a details of the format.)
ef561ac7
BF
1329
1330After resolving the conflict in the obvious way (but before updating the
1331index), the diff will look like:
d19fbc3c
BF
1332
1333-------------------------------------------------
1334$ git diff
1335diff --cc file.txt
1336index 802992c,2b60207..0000000
1337--- a/file.txt
1338+++ b/file.txt
1339@@@ -1,1 -1,1 +1,1 @@@
1340- Hello world
1341 -Goodbye
1342++Goodbye world
1343-------------------------------------------------
1344
1345This shows that our resolved version deleted "Hello world" from the
1346first parent, deleted "Goodbye" from the second parent, and added
1347"Goodbye world", which was previously absent from both.
1348
ef561ac7
BF
1349Some special diff options allow diffing the working directory against
1350any of these stages:
1351
1352-------------------------------------------------
1353$ git diff -1 file.txt # diff against stage 1
1354$ git diff --base file.txt # same as the above
1355$ git diff -2 file.txt # diff against stage 2
1356$ git diff --ours file.txt # same as the above
1357$ git diff -3 file.txt # diff against stage 3
1358$ git diff --theirs file.txt # same as the above.
1359-------------------------------------------------
1360
0cafe944 1361The linkgit:git-log[1] and linkgit:gitk[1] commands also provide special help
ef561ac7 1362for merges:
d19fbc3c
BF
1363
1364-------------------------------------------------
1365$ git log --merge
ef561ac7 1366$ gitk --merge
d19fbc3c
BF
1367-------------------------------------------------
1368
ef561ac7
BF
1369These will display all commits which exist only on HEAD or on
1370MERGE_HEAD, and which touch an unmerged file.
d19fbc3c 1371
5162e697 1372You may also use linkgit:git-mergetool[1], which lets you merge the
c7719fbe 1373unmerged files using external tools such as Emacs or kdiff3.
c64415e2 1374
ef561ac7 1375Each time you resolve the conflicts in a file and update the index:
d19fbc3c
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1376
1377-------------------------------------------------
1378$ git add file.txt
d19fbc3c
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1379-------------------------------------------------
1380
ef561ac7 1381the different stages of that file will be "collapsed", after which
6127c086 1382`git diff` will (by default) no longer show diffs for that file.
d19fbc3c
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1383
1384[[undoing-a-merge]]
ae25c67a 1385Undoing a merge
d19fbc3c
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1386---------------
1387
1388If you get stuck and decide to just give up and throw the whole mess
1389away, you can always return to the pre-merge state with
1390
1391-------------------------------------------------
1392$ git reset --hard HEAD
1393-------------------------------------------------
1394
1130845b 1395Or, if you've already committed the merge that you want to throw away,
d19fbc3c
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1396
1397-------------------------------------------------
1c73bb0e 1398$ git reset --hard ORIG_HEAD
d19fbc3c
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1399-------------------------------------------------
1400
1401However, this last command can be dangerous in some cases--never
1402throw away a commit you have already committed if that commit may
1403itself have been merged into another branch, as doing so may confuse
1404further merges.
1405
e34caace 1406[[fast-forwards]]
d19fbc3c
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1407Fast-forward merges
1408-------------------
1409
1410There is one special case not mentioned above, which is treated
1411differently. Normally, a merge results in a merge commit, with two
1412parents, one pointing at each of the two lines of development that
1413were merged.
1414
59723040
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1415However, if the current branch is a descendant of the other--so every
1416commit present in the one is already contained in the other--then git
a75d7b54 1417just performs a "fast-forward"; the head of the current branch is moved
59723040
BF
1418forward to point at the head of the merged-in branch, without any new
1419commits being created.
d19fbc3c 1420
e34caace 1421[[fixing-mistakes]]
b684f830
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1422Fixing mistakes
1423---------------
1424
1425If you've messed up the working tree, but haven't yet committed your
1426mistake, you can return the entire working tree to the last committed
1427state with
1428
1429-------------------------------------------------
1430$ git reset --hard HEAD
1431-------------------------------------------------
1432
1433If you make a commit that you later wish you hadn't, there are two
1434fundamentally different ways to fix the problem:
1435
1436 1. You can create a new commit that undoes whatever was done
93cbbd71 1437 by the old commit. This is the correct thing if your
b684f830
BF
1438 mistake has already been made public.
1439
1440 2. You can go back and modify the old commit. You should
1441 never do this if you have already made the history public;
1442 git does not normally expect the "history" of a project to
1443 change, and cannot correctly perform repeated merges from
1444 a branch that has had its history changed.
1445
e34caace 1446[[reverting-a-commit]]
b684f830
BF
1447Fixing a mistake with a new commit
1448~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1449
1450Creating a new commit that reverts an earlier change is very easy;
5162e697 1451just pass the linkgit:git-revert[1] command a reference to the bad
b684f830
BF
1452commit; for example, to revert the most recent commit:
1453
1454-------------------------------------------------
1455$ git revert HEAD
1456-------------------------------------------------
1457
1458This will create a new commit which undoes the change in HEAD. You
1459will be given a chance to edit the commit message for the new commit.
1460
1461You can also revert an earlier change, for example, the next-to-last:
1462
1463-------------------------------------------------
1464$ git revert HEAD^
1465-------------------------------------------------
1466
1467In this case git will attempt to undo the old change while leaving
1468intact any changes made since then. If more recent changes overlap
1469with the changes to be reverted, then you will be asked to fix
1470conflicts manually, just as in the case of <<resolving-a-merge,
1471resolving a merge>>.
1472
7cb192ea
BF
1473[[fixing-a-mistake-by-rewriting-history]]
1474Fixing a mistake by rewriting history
b684f830
BF
1475~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1476
1477If the problematic commit is the most recent commit, and you have not
1478yet made that commit public, then you may just
6127c086 1479<<undoing-a-merge,destroy it using `git reset`>>.
b684f830
BF
1480
1481Alternatively, you
1482can edit the working directory and update the index to fix your
1483mistake, just as if you were going to <<how-to-make-a-commit,create a
1484new commit>>, then run
1485
1486-------------------------------------------------
1487$ git commit --amend
1488-------------------------------------------------
1489
1490which will replace the old commit by a new commit incorporating your
1491changes, giving you a chance to edit the old commit message first.
1492
1493Again, you should never do this to a commit that may already have
5162e697 1494been merged into another branch; use linkgit:git-revert[1] instead in
b684f830
BF
1495that case.
1496
7cb192ea 1497It is also possible to replace commits further back in the history, but
b684f830
BF
1498this is an advanced topic to be left for
1499<<cleaning-up-history,another chapter>>.
1500
e34caace 1501[[checkout-of-path]]
b684f830
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1502Checking out an old version of a file
1503~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1504
1505In the process of undoing a previous bad change, you may find it
1506useful to check out an older version of a particular file using
6127c086 1507linkgit:git-checkout[1]. We've used `git checkout` before to switch
b684f830
BF
1508branches, but it has quite different behavior if it is given a path
1509name: the command
1510
1511-------------------------------------------------
1512$ git checkout HEAD^ path/to/file
1513-------------------------------------------------
1514
1515replaces path/to/file by the contents it had in the commit HEAD^, and
1516also updates the index to match. It does not change branches.
1517
1518If you just want to look at an old version of the file, without
1519modifying the working directory, you can do that with
5162e697 1520linkgit:git-show[1]:
b684f830
BF
1521
1522-------------------------------------------------
ed4eb0d8 1523$ git show HEAD^:path/to/file
b684f830
BF
1524-------------------------------------------------
1525
1526which will display the given version of the file.
1527
7a7cc594
JH
1528[[interrupted-work]]
1529Temporarily setting aside work in progress
1530~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1531
1532While you are in the middle of working on something complicated, you
1533find an unrelated but obvious and trivial bug. You would like to fix it
5162e697 1534before continuing. You can use linkgit:git-stash[1] to save the current
7a7cc594
JH
1535state of your work, and after fixing the bug (or, optionally after doing
1536so on a different branch and then coming back), unstash the
1537work-in-progress changes.
1538
1539------------------------------------------------
7a85f6ae 1540$ git stash save "work in progress for foo feature"
7a7cc594
JH
1541------------------------------------------------
1542
1543This command will save your changes away to the `stash`, and
1544reset your working tree and the index to match the tip of your
1545current branch. Then you can make your fix as usual.
1546
1547------------------------------------------------
1548... edit and test ...
1549$ git commit -a -m "blorpl: typofix"
1550------------------------------------------------
1551
1552After that, you can go back to what you were working on with
7b8988e1 1553`git stash pop`:
7a7cc594
JH
1554
1555------------------------------------------------
7b8988e1 1556$ git stash pop
7a7cc594
JH
1557------------------------------------------------
1558
1559
e34caace 1560[[ensuring-good-performance]]
d19fbc3c
BF
1561Ensuring good performance
1562-------------------------
1563
1564On large repositories, git depends on compression to keep the history
06ada152 1565information from taking up too much space on disk or in memory.
d19fbc3c
BF
1566
1567This compression is not performed automatically. Therefore you
5162e697 1568should occasionally run linkgit:git-gc[1]:
d19fbc3c
BF
1569
1570-------------------------------------------------
1571$ git gc
1572-------------------------------------------------
1573
17217090 1574to recompress the archive. This can be very time-consuming, so
6127c086 1575you may prefer to run `git gc` when you are not doing other work.
d19fbc3c 1576
e34caace
BF
1577
1578[[ensuring-reliability]]
11e016a3
BF
1579Ensuring reliability
1580--------------------
1581
e34caace 1582[[checking-for-corruption]]
11e016a3
BF
1583Checking the repository for corruption
1584~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1585
5162e697 1586The linkgit:git-fsck[1] command runs a number of self-consistency checks
1191ee18 1587on the repository, and reports on any problems. This may take some
c6a13b2c 1588time.
21dcb3b7
BF
1589
1590-------------------------------------------------
04e50e94 1591$ git fsck
21dcb3b7
BF
1592dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
1593dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
1594dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
1595dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb
1596dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f
1597dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e
1598dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085
1599dangling tree b24c2473f1fd3d91352a624795be026d64c8841f
1600...
1601-------------------------------------------------
1602
c6a13b2c
JH
1603You will see informational messages on dangling objects. They are objects
1604that still exist in the repository but are no longer referenced by any of
1605your branches, and can (and will) be removed after a while with "gc".
b4ab1980 1606You can run `git fsck --no-dangling` to suppress these messages, and still
c6a13b2c 1607view real errors.
1cdade2c 1608
e34caace 1609[[recovering-lost-changes]]
11e016a3
BF
1610Recovering lost changes
1611~~~~~~~~~~~~~~~~~~~~~~~
1612
e34caace 1613[[reflogs]]
559e4d7a
BF
1614Reflogs
1615^^^^^^^
1616
6cf378f0 1617Say you modify a branch with +linkgit:git-reset[1] \--hard+, and then
559e4d7a
BF
1618realize that the branch was the only reference you had to that point in
1619history.
1620
1621Fortunately, git also keeps a log, called a "reflog", of all the
1622previous values of each branch. So in this case you can still find the
a6080a0a 1623old history using, for example,
559e4d7a
BF
1624
1625-------------------------------------------------
1626$ git log master@{1}
1627-------------------------------------------------
1628
e502c2c3
SO
1629This lists the commits reachable from the previous version of the
1630"master" branch head. This syntax can be used with any git command
1631that accepts a commit, not just with git log. Some other examples:
559e4d7a
BF
1632
1633-------------------------------------------------
1634$ git show master@{2} # See where the branch pointed 2,
1635$ git show master@{3} # 3, ... changes ago.
1636$ gitk master@{yesterday} # See where it pointed yesterday,
1637$ gitk master@{"1 week ago"} # ... or last week
953f3d6f
BF
1638$ git log --walk-reflogs master # show reflog entries for master
1639-------------------------------------------------
1640
1641A separate reflog is kept for the HEAD, so
1642
1643-------------------------------------------------
1644$ git show HEAD@{"1 week ago"}
559e4d7a
BF
1645-------------------------------------------------
1646
953f3d6f
BF
1647will show what HEAD pointed to one week ago, not what the current branch
1648pointed to one week ago. This allows you to see the history of what
1649you've checked out.
1650
559e4d7a 1651The reflogs are kept by default for 30 days, after which they may be
5162e697 1652pruned. See linkgit:git-reflog[1] and linkgit:git-gc[1] to learn
559e4d7a 1653how to control this pruning, and see the "SPECIFYING REVISIONS"
9d83e382 1654section of linkgit:gitrevisions[7] for details.
559e4d7a
BF
1655
1656Note that the reflog history is very different from normal git history.
1657While normal history is shared by every repository that works on the
1658same project, the reflog history is not shared: it tells you only about
1659how the branches in your local repository have changed over time.
1660
59723040 1661[[dangling-object-recovery]]
559e4d7a
BF
1662Examining dangling objects
1663^^^^^^^^^^^^^^^^^^^^^^^^^^
1664
59723040
BF
1665In some situations the reflog may not be able to save you. For example,
1666suppose you delete a branch, then realize you need the history it
1667contained. The reflog is also deleted; however, if you have not yet
1668pruned the repository, then you may still be able to find the lost
6127c086 1669commits in the dangling objects that `git fsck` reports. See
59723040 1670<<dangling-objects>> for the details.
559e4d7a
BF
1671
1672-------------------------------------------------
1673$ git fsck
1674dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
1675dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
1676dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
1677...
1678-------------------------------------------------
1679
aacd404e 1680You can examine
559e4d7a
BF
1681one of those dangling commits with, for example,
1682
1683------------------------------------------------
1684$ gitk 7281251ddd --not --all
1685------------------------------------------------
1686
1687which does what it sounds like: it says that you want to see the commit
1688history that is described by the dangling commit(s), but not the
1689history that is described by all your existing branches and tags. Thus
1690you get exactly the history reachable from that commit that is lost.
1691(And notice that it might not be just one commit: we only report the
1692"tip of the line" as being dangling, but there might be a whole deep
79c96c57 1693and complex commit history that was dropped.)
559e4d7a
BF
1694
1695If you decide you want the history back, you can always create a new
1696reference pointing to it, for example, a new branch:
1697
1698------------------------------------------------
a6080a0a 1699$ git branch recovered-branch 7281251ddd
559e4d7a
BF
1700------------------------------------------------
1701
59723040
BF
1702Other types of dangling objects (blobs and trees) are also possible, and
1703dangling objects can arise in other situations.
1704
11e016a3 1705
e34caace 1706[[sharing-development]]
d19fbc3c 1707Sharing development with others
b684f830 1708===============================
d19fbc3c 1709
aa971cb9 1710[[getting-updates-With-git-pull]]
6127c086 1711Getting updates with git pull
b684f830 1712-----------------------------
d19fbc3c 1713
e63ec003 1714After you clone a repository and commit a few changes of your own, you
d19fbc3c
BF
1715may wish to check the original repository for updates and merge them
1716into your own work.
1717
aa971cb9 1718We have already seen <<Updating-a-repository-With-git-fetch,how to
0e615b25 1719keep remote-tracking branches up to date>> with linkgit:git-fetch[1],
d19fbc3c
BF
1720and how to merge two branches. So you can merge in changes from the
1721original repository's master branch with:
1722
1723-------------------------------------------------
1724$ git fetch
1725$ git merge origin/master
1726-------------------------------------------------
1727
5162e697 1728However, the linkgit:git-pull[1] command provides a way to do this in
d19fbc3c
BF
1729one step:
1730
1731-------------------------------------------------
1732$ git pull origin master
1733-------------------------------------------------
1734
66a062a1
MM
1735In fact, if you have "master" checked out, then this branch has been
1736configured by "git clone" to get changes from the HEAD branch of the
1737origin repository. So often you can
0eb4f7cd 1738accomplish the above with just a simple
d19fbc3c
BF
1739
1740-------------------------------------------------
1741$ git pull
1742-------------------------------------------------
1743
66a062a1
MM
1744This command will fetch changes from the remote branches to your
1745remote-tracking branches `origin/*`, and merge the default branch into
1746the current branch.
1747
29b9a66f
MM
1748More generally, a branch that is created from a remote-tracking branch
1749will pull
0eb4f7cd
BF
1750by default from that branch. See the descriptions of the
1751branch.<name>.remote and branch.<name>.merge options in
5162e697
DM
1752linkgit:git-config[1], and the discussion of the `--track` option in
1753linkgit:git-checkout[1], to learn how to control these defaults.
d19fbc3c
BF
1754
1755In addition to saving you keystrokes, "git pull" also helps you by
1756producing a default commit message documenting the branch and
1757repository that you pulled from.
1758
1759(But note that no such commit will be created in the case of a
a75d7b54 1760<<fast-forwards,fast-forward>>; instead, your branch will just be
79c96c57 1761updated to point to the latest commit from the upstream branch.)
d19fbc3c 1762
6127c086 1763The `git pull` command can also be given "." as the "remote" repository,
1191ee18 1764in which case it just merges in a branch from the current repository; so
4c63ff45
BF
1765the commands
1766
1767-------------------------------------------------
1768$ git pull . branch
1769$ git merge branch
1770-------------------------------------------------
1771
1772are roughly equivalent. The former is actually very commonly used.
1773
e34caace 1774[[submitting-patches]]
d19fbc3c 1775Submitting patches to a project
b684f830 1776-------------------------------
d19fbc3c
BF
1777
1778If you just have a few changes, the simplest way to submit them may
1779just be to send them as patches in email:
1780
5162e697 1781First, use linkgit:git-format-patch[1]; for example:
d19fbc3c
BF
1782
1783-------------------------------------------------
eb6ae7f4 1784$ git format-patch origin
d19fbc3c
BF
1785-------------------------------------------------
1786
1787will produce a numbered series of files in the current directory, one
1788for each patch in the current branch but not in origin/HEAD.
1789
d84cef18
PO
1790`git format-patch` can include an initial "cover letter". You can insert
1791commentary on individual patches after the three dash line which
1792`format-patch` places after the commit message but before the patch
1793itself. If you use `git notes` to track your cover letter material,
1794`git format-patch --notes` will include the commit's notes in a similar
1795manner.
1796
d19fbc3c
BF
1797You can then import these into your mail client and send them by
1798hand. However, if you have a lot to send at once, you may prefer to
5162e697 1799use the linkgit:git-send-email[1] script to automate the process.
d19fbc3c
BF
1800Consult the mailing list for your project first to determine how they
1801prefer such patches be handled.
1802
e34caace 1803[[importing-patches]]
d19fbc3c 1804Importing patches to a project
b684f830 1805------------------------------
d19fbc3c 1806
5162e697 1807Git also provides a tool called linkgit:git-am[1] (am stands for
d19fbc3c
BF
1808"apply mailbox"), for importing such an emailed series of patches.
1809Just save all of the patch-containing messages, in order, into a
1810single mailbox file, say "patches.mbox", then run
1811
1812-------------------------------------------------
eb6ae7f4 1813$ git am -3 patches.mbox
d19fbc3c
BF
1814-------------------------------------------------
1815
1816Git will apply each patch in order; if any conflicts are found, it
1817will stop, and you can fix the conflicts as described in
01997b4a
BF
1818"<<resolving-a-merge,Resolving a merge>>". (The "-3" option tells
1819git to perform a merge; if you would prefer it just to abort and
1820leave your tree and index untouched, you may omit that option.)
1821
1822Once the index is updated with the results of the conflict
1823resolution, instead of creating a new commit, just run
d19fbc3c
BF
1824
1825-------------------------------------------------
1826$ git am --resolved
1827-------------------------------------------------
1828
1829and git will create the commit for you and continue applying the
1830remaining patches from the mailbox.
1831
1832The final result will be a series of commits, one for each patch in
1833the original mailbox, with authorship and commit log message each
1834taken from the message containing each patch.
1835
eda69449
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1836[[public-repositories]]
1837Public git repositories
1838-----------------------
d19fbc3c 1839
6e30fb0c
DK
1840Another way to submit changes to a project is to tell the maintainer
1841of that project to pull the changes from your repository using
aa971cb9 1842linkgit:git-pull[1]. In the section "<<getting-updates-With-git-pull,
6127c086 1843Getting updates with `git pull`>>" we described this as a way to get
6e30fb0c
DK
1844updates from the "main" repository, but it works just as well in the
1845other direction.
d19fbc3c 1846
eda69449
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1847If you and the maintainer both have accounts on the same machine, then
1848you can just pull changes from each other's repositories directly;
11d51533 1849commands that accept repository URLs as arguments will also accept a
eda69449 1850local directory name:
d19fbc3c
BF
1851
1852-------------------------------------------------
1853$ git clone /path/to/repository
1854$ git pull /path/to/other/repository
1855-------------------------------------------------
1856
c9016158 1857or an ssh URL:
11d51533
BF
1858
1859-------------------------------------------------
1860$ git clone ssh://yourhost/~you/repository
1861-------------------------------------------------
1862
1863For projects with few developers, or for synchronizing a few private
1864repositories, this may be all you need.
1865
eda69449
BF
1866However, the more common way to do this is to maintain a separate public
1867repository (usually on a different host) for others to pull changes
1868from. This is usually more convenient, and allows you to cleanly
1869separate private work in progress from publicly visible work.
d19fbc3c
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1870
1871You will continue to do your day-to-day work in your personal
1872repository, but periodically "push" changes from your personal
1873repository into your public repository, allowing other developers to
1874pull from that repository. So the flow of changes, in a situation
1875where there is one other developer with a public repository, looks
1876like this:
1877
1878 you push
1879 your personal repo ------------------> your public repo
a6080a0a 1880 ^ |
d19fbc3c
BF
1881 | |
1882 | you pull | they pull
1883 | |
1884 | |
1885 | they push V
1886 their public repo <------------------- their repo
1887
11d51533
BF
1888We explain how to do this in the following sections.
1889
eda69449
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1890[[setting-up-a-public-repository]]
1891Setting up a public repository
1892~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1893
1894Assume your personal repository is in the directory ~/proj. We
6127c086 1895first create a new clone of the repository and tell `git daemon` that it
eda69449 1896is meant to be public:
d19fbc3c
BF
1897
1898-------------------------------------------------
52c80037 1899$ git clone --bare ~/proj proj.git
eda69449 1900$ touch proj.git/git-daemon-export-ok
d19fbc3c
BF
1901-------------------------------------------------
1902
52c80037 1903The resulting directory proj.git contains a "bare" git repository--it is
eda69449
BF
1904just the contents of the ".git" directory, without any files checked out
1905around it.
d19fbc3c 1906
c64415e2 1907Next, copy proj.git to the server where you plan to host the
d19fbc3c
BF
1908public repository. You can use scp, rsync, or whatever is most
1909convenient.
1910
eda69449
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1911[[exporting-via-git]]
1912Exporting a git repository via the git protocol
1913~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1914
1915This is the preferred method.
1916
1917If someone else administers the server, they should tell you what
c9016158 1918directory to put the repository in, and what git:// URL it will appear
eda69449 1919at. You can then skip to the section
d19fbc3c
BF
1920"<<pushing-changes-to-a-public-repository,Pushing changes to a public
1921repository>>", below.
1922
5162e697 1923Otherwise, all you need to do is start linkgit:git-daemon[1]; it will
eda69449
BF
1924listen on port 9418. By default, it will allow access to any directory
1925that looks like a git directory and contains the magic file
6127c086 1926git-daemon-export-ok. Passing some directory paths as `git daemon`
eda69449
BF
1927arguments will further restrict the exports to those paths.
1928
6127c086 1929You can also run `git daemon` as an inetd service; see the
5162e697 1930linkgit:git-daemon[1] man page for details. (See especially the
eda69449 1931examples section.)
d19fbc3c
BF
1932
1933[[exporting-via-http]]
1934Exporting a git repository via http
eda69449 1935~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d19fbc3c
BF
1936
1937The git protocol gives better performance and reliability, but on a
1938host with a web server set up, http exports may be simpler to set up.
1939
1940All you need to do is place the newly created bare git repository in
1941a directory that is exported by the web server, and make some
1942adjustments to give web clients some extra information they need:
1943
1944-------------------------------------------------
1945$ mv proj.git /home/you/public_html/proj.git
1946$ cd proj.git
c64415e2 1947$ git --bare update-server-info
7dce9918 1948$ mv hooks/post-update.sample hooks/post-update
d19fbc3c
BF
1949-------------------------------------------------
1950
1951(For an explanation of the last two lines, see
6998e4db 1952linkgit:git-update-server-info[1] and linkgit:githooks[5].)
d19fbc3c 1953
c9016158
RW
1954Advertise the URL of proj.git. Anybody else should then be able to
1955clone or pull from that URL, for example with a command line like:
d19fbc3c
BF
1956
1957-------------------------------------------------
1958$ git clone http://yourserver.com/~you/proj.git
1959-------------------------------------------------
1960
1961(See also
1962link:howto/setup-git-server-over-http.txt[setup-git-server-over-http]
1963for a slightly more sophisticated setup using WebDAV which also
1964allows pushing over http.)
1965
d19fbc3c
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1966[[pushing-changes-to-a-public-repository]]
1967Pushing changes to a public repository
eda69449 1968~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d19fbc3c 1969
eda69449 1970Note that the two techniques outlined above (exporting via
d19fbc3c
BF
1971<<exporting-via-http,http>> or <<exporting-via-git,git>>) allow other
1972maintainers to fetch your latest changes, but they do not allow write
1973access, which you will need to update the public repository with the
1974latest changes created in your private repository.
1975
5162e697 1976The simplest way to do this is using linkgit:git-push[1] and ssh; to
d19fbc3c
BF
1977update the remote branch named "master" with the latest state of your
1978branch named "master", run
1979
1980-------------------------------------------------
1981$ git push ssh://yourserver.com/~you/proj.git master:master
1982-------------------------------------------------
1983
1984or just
1985
1986-------------------------------------------------
1987$ git push ssh://yourserver.com/~you/proj.git master
1988-------------------------------------------------
1989
6127c086 1990As with `git fetch`, `git push` will complain if this does not result in a
a75d7b54 1991<<fast-forwards,fast-forward>>; see the following section for details on
81eb417a 1992handling this case.
d19fbc3c 1993
11d51533
BF
1994Note that the target of a "push" is normally a
1995<<def_bare_repository,bare>> repository. You can also push to a
1996repository that has a checked-out working tree, but the working tree
1997will not be updated by the push. This may lead to unexpected results if
1998the branch you push to is the currently checked-out branch!
1999
6127c086 2000As with `git fetch`, you may also set up configuration options to
d19fbc3c
BF
2001save typing; so, for example, after
2002
2003-------------------------------------------------
c64415e2 2004$ cat >>.git/config <<EOF
d19fbc3c
BF
2005[remote "public-repo"]
2006 url = ssh://yourserver.com/~you/proj.git
2007EOF
2008-------------------------------------------------
2009
2010you should be able to perform the above push with just
2011
2012-------------------------------------------------
2013$ git push public-repo master
2014-------------------------------------------------
2015
2016See the explanations of the remote.<name>.url, branch.<name>.remote,
5162e697 2017and remote.<name>.push options in linkgit:git-config[1] for
d19fbc3c
BF
2018details.
2019
81eb417a
BF
2020[[forcing-push]]
2021What to do when a push fails
2022~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2023
a75d7b54 2024If a push would not result in a <<fast-forwards,fast-forward>> of the
81eb417a
BF
2025remote branch, then it will fail with an error like:
2026
2027-------------------------------------------------
2028error: remote 'refs/heads/master' is not an ancestor of
2029 local 'refs/heads/master'.
2030 Maybe you are not up-to-date and need to pull first?
2031error: failed to push to 'ssh://yourserver.com/~you/proj.git'
2032-------------------------------------------------
2033
2034This can happen, for example, if you:
2035
6127c086
FC
2036 - use `git reset --hard` to remove already-published commits, or
2037 - use `git commit --amend` to replace already-published commits
7cb192ea 2038 (as in <<fixing-a-mistake-by-rewriting-history>>), or
6127c086 2039 - use `git rebase` to rebase any already-published commits (as
81eb417a
BF
2040 in <<using-git-rebase>>).
2041
6127c086 2042You may force `git push` to perform the update anyway by preceding the
81eb417a
BF
2043branch name with a plus sign:
2044
2045-------------------------------------------------
2046$ git push ssh://yourserver.com/~you/proj.git +master
2047-------------------------------------------------
2048
2049Normally whenever a branch head in a public repository is modified, it
9e5d87d4 2050is modified to point to a descendant of the commit that it pointed to
81eb417a 2051before. By forcing a push in this situation, you break that convention.
aa971cb9 2052(See <<problems-With-rewriting-history>>.)
81eb417a
BF
2053
2054Nevertheless, this is a common practice for people that need a simple
2055way to publish a work-in-progress patch series, and it is an acceptable
2056compromise as long as you warn other developers that this is how you
2057intend to manage the branch.
2058
2059It's also possible for a push to fail in this way when other people have
2060the right to push to the same repository. In that case, the correct
843c81dc
EH
2061solution is to retry the push after first updating your work: either by a
2062pull, or by a fetch followed by a rebase; see the
81eb417a 2063<<setting-up-a-shared-repository,next section>> and
6998e4db 2064linkgit:gitcvs-migration[7] for more.
81eb417a 2065
e34caace 2066[[setting-up-a-shared-repository]]
d19fbc3c 2067Setting up a shared repository
eda69449 2068~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d19fbc3c
BF
2069
2070Another way to collaborate is by using a model similar to that
2071commonly used in CVS, where several developers with special rights
2072all push to and pull from a single shared repository. See
6998e4db 2073linkgit:gitcvs-migration[7] for instructions on how to
d19fbc3c
BF
2074set this up.
2075
8fae2225
BF
2076However, while there is nothing wrong with git's support for shared
2077repositories, this mode of operation is not generally recommended,
2078simply because the mode of collaboration that git supports--by
2079exchanging patches and pulling from public repositories--has so many
2080advantages over the central shared repository:
2081
2082 - Git's ability to quickly import and merge patches allows a
2083 single maintainer to process incoming changes even at very
6127c086 2084 high rates. And when that becomes too much, `git pull` provides
8fae2225
BF
2085 an easy way for that maintainer to delegate this job to other
2086 maintainers while still allowing optional review of incoming
2087 changes.
2088 - Since every developer's repository has the same complete copy
2089 of the project history, no repository is special, and it is
2090 trivial for another developer to take over maintenance of a
2091 project, either by mutual agreement, or because a maintainer
2092 becomes unresponsive or difficult to work with.
2093 - The lack of a central group of "committers" means there is
2094 less need for formal decisions about who is "in" and who is
2095 "out".
2096
e34caace 2097[[setting-up-gitweb]]
eda69449
BF
2098Allowing web browsing of a repository
2099~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d19fbc3c 2100
a8cd1402
BF
2101The gitweb cgi script provides users an easy way to browse your
2102project's files and history without having to install git; see the file
04483524 2103gitweb/INSTALL in the git source tree for instructions on setting it up.
d19fbc3c 2104
e34caace 2105[[sharing-development-examples]]
b684f830
BF
2106Examples
2107--------
d19fbc3c 2108
9e2163ea
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2109[[maintaining-topic-branches]]
2110Maintaining topic branches for a Linux subsystem maintainer
2111~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2112
2113This describes how Tony Luck uses git in his role as maintainer of the
2114IA64 architecture for the Linux kernel.
2115
2116He uses two public branches:
2117
2118 - A "test" tree into which patches are initially placed so that they
2119 can get some exposure when integrated with other ongoing development.
2120 This tree is available to Andrew for pulling into -mm whenever he
2121 wants.
2122
2123 - A "release" tree into which tested patches are moved for final sanity
2124 checking, and as a vehicle to send them upstream to Linus (by sending
2125 him a "please pull" request.)
2126
2127He also uses a set of temporary branches ("topic branches"), each
2128containing a logical grouping of patches.
2129
2130To set this up, first create your work tree by cloning Linus's public
2131tree:
2132
2133-------------------------------------------------
2134$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git work
2135$ cd work
2136-------------------------------------------------
2137
29b9a66f 2138Linus's tree will be stored in the remote-tracking branch named origin/master,
5162e697
DM
2139and can be updated using linkgit:git-fetch[1]; you can track other
2140public trees using linkgit:git-remote[1] to set up a "remote" and
2141linkgit:git-fetch[1] to keep them up-to-date; see
6e30fb0c 2142<<repositories-and-branches>>.
9e2163ea
BF
2143
2144Now create the branches in which you are going to work; these start out
2145at the current tip of origin/master branch, and should be set up (using
5162e697 2146the --track option to linkgit:git-branch[1]) to merge changes in from
9e2163ea
BF
2147Linus by default.
2148
2149-------------------------------------------------
2150$ git branch --track test origin/master
2151$ git branch --track release origin/master
2152-------------------------------------------------
2153
5162e697 2154These can be easily kept up to date using linkgit:git-pull[1].
9e2163ea
BF
2155
2156-------------------------------------------------
2157$ git checkout test && git pull
2158$ git checkout release && git pull
2159-------------------------------------------------
2160
2161Important note! If you have any local changes in these branches, then
2162this merge will create a commit object in the history (with no local
a75d7b54 2163changes git will simply do a "fast-forward" merge). Many people dislike
9e2163ea
BF
2164the "noise" that this creates in the Linux history, so you should avoid
2165doing this capriciously in the "release" branch, as these noisy commits
2166will become part of the permanent history when you ask Linus to pull
2167from the release branch.
2168
5162e697 2169A few configuration variables (see linkgit:git-config[1]) can
9e2163ea
BF
2170make it easy to push both branches to your public tree. (See
2171<<setting-up-a-public-repository>>.)
2172
2173-------------------------------------------------
2174$ cat >> .git/config <<EOF
2175[remote "mytree"]
2176 url = master.kernel.org:/pub/scm/linux/kernel/git/aegl/linux-2.6.git
2177 push = release
2178 push = test
2179EOF
2180-------------------------------------------------
2181
2182Then you can push both the test and release trees using
5162e697 2183linkgit:git-push[1]:
9e2163ea
BF
2184
2185-------------------------------------------------
2186$ git push mytree
2187-------------------------------------------------
2188
2189or push just one of the test and release branches using:
2190
2191-------------------------------------------------
2192$ git push mytree test
2193-------------------------------------------------
2194
2195or
2196
2197-------------------------------------------------
2198$ git push mytree release
2199-------------------------------------------------
2200
2201Now to apply some patches from the community. Think of a short
2202snappy name for a branch to hold this patch (or related group of
352953a5
TL
2203patches), and create a new branch from a recent stable tag of
2204Linus's branch. Picking a stable base for your branch will:
22051) help you: by avoiding inclusion of unrelated and perhaps lightly
2206tested changes
22072) help future bug hunters that use "git bisect" to find problems
9e2163ea
BF
2208
2209-------------------------------------------------
352953a5 2210$ git checkout -b speed-up-spinlocks v2.6.35
9e2163ea
BF
2211-------------------------------------------------
2212
2213Now you apply the patch(es), run some tests, and commit the change(s). If
2214the patch is a multi-part series, then you should apply each as a separate
2215commit to this branch.
2216
2217-------------------------------------------------
2218$ ... patch ... test ... commit [ ... patch ... test ... commit ]*
2219-------------------------------------------------
2220
2221When you are happy with the state of this change, you can pull it into the
2222"test" branch in preparation to make it public:
2223
2224-------------------------------------------------
2225$ git checkout test && git pull . speed-up-spinlocks
2226-------------------------------------------------
2227
2228It is unlikely that you would have any conflicts here ... but you might if you
2229spent a while on this step and had also pulled new versions from upstream.
2230
2231Some time later when enough time has passed and testing done, you can pull the
2232same branch into the "release" tree ready to go upstream. This is where you
2233see the value of keeping each patch (or patch series) in its own branch. It
2234means that the patches can be moved into the "release" tree in any order.
2235
2236-------------------------------------------------
2237$ git checkout release && git pull . speed-up-spinlocks
2238-------------------------------------------------
2239
2240After a while, you will have a number of branches, and despite the
2241well chosen names you picked for each of them, you may forget what
2242they are for, or what status they are in. To get a reminder of what
2243changes are in a specific branch, use:
2244
2245-------------------------------------------------
467c0197 2246$ git log linux..branchname | git shortlog
9e2163ea
BF
2247-------------------------------------------------
2248
06ada152 2249To see whether it has already been merged into the test or release branches,
9e2163ea
BF
2250use:
2251
2252-------------------------------------------------
2253$ git log test..branchname
2254-------------------------------------------------
2255
2256or
2257
2258-------------------------------------------------
2259$ git log release..branchname
2260-------------------------------------------------
2261
06ada152 2262(If this branch has not yet been merged, you will see some log entries.
9e2163ea
BF
2263If it has been merged, then there will be no output.)
2264
2265Once a patch completes the great cycle (moving from test to release,
2266then pulled by Linus, and finally coming back into your local
06ada152 2267"origin/master" branch), the branch for this change is no longer needed.
9e2163ea
BF
2268You detect this when the output from:
2269
2270-------------------------------------------------
2271$ git log origin..branchname
2272-------------------------------------------------
2273
2274is empty. At this point the branch can be deleted:
2275
2276-------------------------------------------------
2277$ git branch -d branchname
2278-------------------------------------------------
2279
2280Some changes are so trivial that it is not necessary to create a separate
2281branch and then merge into each of the test and release branches. For
2282these changes, just apply directly to the "release" branch, and then
2283merge that into the "test" branch.
2284
2285To create diffstat and shortlog summaries of changes to include in a "please
2286pull" request to Linus you can use:
2287
2288-------------------------------------------------
2289$ git diff --stat origin..release
2290-------------------------------------------------
2291
2292and
2293
2294-------------------------------------------------
2295$ git log -p origin..release | git shortlog
2296-------------------------------------------------
2297
2298Here are some of the scripts that simplify all this even further.
2299
2300-------------------------------------------------
2301==== update script ====
48a8c26c 2302# Update a branch in my Git tree. If the branch to be updated
9e2163ea
BF
2303# is origin, then pull from kernel.org. Otherwise merge
2304# origin/master branch into test|release branch
2305
2306case "$1" in
2307test|release)
2308 git checkout $1 && git pull . origin
2309 ;;
2310origin)
fc74ecc1 2311 before=$(git rev-parse refs/remotes/origin/master)
9e2163ea 2312 git fetch origin
fc74ecc1 2313 after=$(git rev-parse refs/remotes/origin/master)
9e2163ea
BF
2314 if [ $before != $after ]
2315 then
2316 git log $before..$after | git shortlog
2317 fi
2318 ;;
2319*)
2320 echo "Usage: $0 origin|test|release" 1>&2
2321 exit 1
2322 ;;
2323esac
2324-------------------------------------------------
2325
2326-------------------------------------------------
2327==== merge script ====
2328# Merge a branch into either the test or release branch
2329
2330pname=$0
2331
2332usage()
2333{
2334 echo "Usage: $pname branch test|release" 1>&2
2335 exit 1
2336}
2337
fc74ecc1 2338git show-ref -q --verify -- refs/heads/"$1" || {
9e2163ea
BF
2339 echo "Can't see branch <$1>" 1>&2
2340 usage
fc74ecc1 2341}
9e2163ea
BF
2342
2343case "$2" in
2344test|release)
2345 if [ $(git log $2..$1 | wc -c) -eq 0 ]
2346 then
2347 echo $1 already merged into $2 1>&2
2348 exit 1
2349 fi
2350 git checkout $2 && git pull . $1
2351 ;;
2352*)
2353 usage
2354 ;;
2355esac
2356-------------------------------------------------
2357
2358-------------------------------------------------
2359==== status script ====
48a8c26c 2360# report on status of my ia64 Git tree
9e2163ea
BF
2361
2362gb=$(tput setab 2)
2363rb=$(tput setab 1)
2364restore=$(tput setab 9)
2365
2366if [ `git rev-list test..release | wc -c` -gt 0 ]
2367then
2368 echo $rb Warning: commits in release that are not in test $restore
2369 git log test..release
2370fi
2371
fc74ecc1 2372for branch in `git show-ref --heads | sed 's|^.*/||'`
9e2163ea
BF
2373do
2374 if [ $branch = test -o $branch = release ]
2375 then
2376 continue
2377 fi
2378
2379 echo -n $gb ======= $branch ====== $restore " "
2380 status=
2381 for ref in test release origin/master
2382 do
2383 if [ `git rev-list $ref..$branch | wc -c` -gt 0 ]
2384 then
2385 status=$status${ref:0:1}
2386 fi
2387 done
2388 case $status in
2389 trl)
2390 echo $rb Need to pull into test $restore
2391 ;;
2392 rl)
2393 echo "In test"
2394 ;;
2395 l)
2396 echo "Waiting for linus"
2397 ;;
2398 "")
2399 echo $rb All done $restore
2400 ;;
2401 *)
2402 echo $rb "<$status>" $restore
2403 ;;
2404 esac
2405 git log origin/master..$branch | git shortlog
2406done
2407-------------------------------------------------
d19fbc3c 2408
d19fbc3c 2409
d19fbc3c 2410[[cleaning-up-history]]
4c63ff45
BF
2411Rewriting history and maintaining patch series
2412==============================================
2413
2414Normally commits are only added to a project, never taken away or
2415replaced. Git is designed with this assumption, and violating it will
2416cause git's merge machinery (for example) to do the wrong thing.
2417
2418However, there is a situation in which it can be useful to violate this
2419assumption.
2420
e34caace 2421[[patch-series]]
4c63ff45
BF
2422Creating the perfect patch series
2423---------------------------------
2424
2425Suppose you are a contributor to a large project, and you want to add a
2426complicated feature, and to present it to the other developers in a way
2427that makes it easy for them to read your changes, verify that they are
2428correct, and understand why you made each change.
2429
b181d57f 2430If you present all of your changes as a single patch (or commit), they
79c96c57 2431may find that it is too much to digest all at once.
4c63ff45
BF
2432
2433If you present them with the entire history of your work, complete with
2434mistakes, corrections, and dead ends, they may be overwhelmed.
2435
2436So the ideal is usually to produce a series of patches such that:
2437
2438 1. Each patch can be applied in order.
2439
2440 2. Each patch includes a single logical change, together with a
2441 message explaining the change.
2442
2443 3. No patch introduces a regression: after applying any initial
2444 part of the series, the resulting project still compiles and
2445 works, and has no bugs that it didn't have before.
2446
2447 4. The complete series produces the same end result as your own
2448 (probably much messier!) development process did.
2449
b181d57f
BF
2450We will introduce some tools that can help you do this, explain how to
2451use them, and then explain some of the problems that can arise because
2452you are rewriting history.
4c63ff45 2453
e34caace 2454[[using-git-rebase]]
6127c086 2455Keeping a patch series up to date using git rebase
4c63ff45
BF
2456--------------------------------------------------
2457
79c96c57
MC
2458Suppose that you create a branch "mywork" on a remote-tracking branch
2459"origin", and create some commits on top of it:
4c63ff45
BF
2460
2461-------------------------------------------------
2462$ git checkout -b mywork origin
2463$ vi file.txt
2464$ git commit
2465$ vi otherfile.txt
2466$ git commit
2467...
2468-------------------------------------------------
2469
2470You have performed no merges into mywork, so it is just a simple linear
2471sequence of patches on top of "origin":
2472
1dc71a91 2473................................................
fa8347b8 2474 o--o--O <-- origin
4c63ff45 2475 \
fa8347b8 2476 a--b--c <-- mywork
1dc71a91 2477................................................
4c63ff45
BF
2478
2479Some more interesting work has been done in the upstream project, and
2480"origin" has advanced:
2481
1dc71a91 2482................................................
4c63ff45
BF
2483 o--o--O--o--o--o <-- origin
2484 \
2485 a--b--c <-- mywork
1dc71a91 2486................................................
4c63ff45
BF
2487
2488At this point, you could use "pull" to merge your changes back in;
2489the result would create a new merge commit, like this:
2490
1dc71a91 2491................................................
4c63ff45
BF
2492 o--o--O--o--o--o <-- origin
2493 \ \
2494 a--b--c--m <-- mywork
1dc71a91 2495................................................
a6080a0a 2496
4c63ff45
BF
2497However, if you prefer to keep the history in mywork a simple series of
2498commits without any merges, you may instead choose to use
5162e697 2499linkgit:git-rebase[1]:
4c63ff45
BF
2500
2501-------------------------------------------------
2502$ git checkout mywork
2503$ git rebase origin
2504-------------------------------------------------
2505
b181d57f 2506This will remove each of your commits from mywork, temporarily saving
51ef1daa 2507them as patches (in a directory named ".git/rebase-apply"), update mywork to
b181d57f
BF
2508point at the latest version of origin, then apply each of the saved
2509patches to the new mywork. The result will look like:
4c63ff45
BF
2510
2511
1dc71a91 2512................................................
4c63ff45
BF
2513 o--o--O--o--o--o <-- origin
2514 \
2515 a'--b'--c' <-- mywork
1dc71a91 2516................................................
4c63ff45 2517
b181d57f 2518In the process, it may discover conflicts. In that case it will stop
6127c086 2519and allow you to fix the conflicts; after fixing conflicts, use `git add`
7a7d4ef6 2520to update the index with those contents, and then, instead of
6127c086 2521running `git commit`, just run
4c63ff45
BF
2522
2523-------------------------------------------------
2524$ git rebase --continue
2525-------------------------------------------------
2526
2527and git will continue applying the rest of the patches.
2528
b6cbca38 2529At any point you may use the `--abort` option to abort this process and
4c63ff45
BF
2530return mywork to the state it had before you started the rebase:
2531
2532-------------------------------------------------
2533$ git rebase --abort
2534-------------------------------------------------
2535
7cb192ea
BF
2536[[rewriting-one-commit]]
2537Rewriting a single commit
365aa199
BF
2538-------------------------
2539
7cb192ea 2540We saw in <<fixing-a-mistake-by-rewriting-history>> that you can replace the
365aa199
BF
2541most recent commit using
2542
2543-------------------------------------------------
2544$ git commit --amend
2545-------------------------------------------------
2546
2547which will replace the old commit by a new commit incorporating your
2548changes, giving you a chance to edit the old commit message first.
2549
5162e697 2550You can also use a combination of this and linkgit:git-rebase[1] to
7cb192ea
BF
2551replace a commit further back in your history and recreate the
2552intervening changes on top of it. First, tag the problematic commit
2553with
365aa199
BF
2554
2555-------------------------------------------------
2556$ git tag bad mywork~5
2557-------------------------------------------------
2558
6127c086 2559(Either gitk or `git log` may be useful for finding the commit.)
365aa199 2560
25d9f3fa
BF
2561Then check out that commit, edit it, and rebase the rest of the series
2562on top of it (note that we could check out the commit on a temporary
2563branch, but instead we're using a <<detached-head,detached head>>):
365aa199
BF
2564
2565-------------------------------------------------
25d9f3fa 2566$ git checkout bad
365aa199
BF
2567$ # make changes here and update the index
2568$ git commit --amend
25d9f3fa 2569$ git rebase --onto HEAD bad mywork
365aa199
BF
2570-------------------------------------------------
2571
25d9f3fa
BF
2572When you're done, you'll be left with mywork checked out, with the top
2573patches on mywork reapplied on top of your modified commit. You can
365aa199
BF
2574then clean up with
2575
2576-------------------------------------------------
365aa199
BF
2577$ git tag -d bad
2578-------------------------------------------------
2579
2580Note that the immutable nature of git history means that you haven't really
2581"modified" existing commits; instead, you have replaced the old commits with
2582new commits having new object names.
2583
e34caace 2584[[reordering-patch-series]]
4c63ff45
BF
2585Reordering or selecting from a patch series
2586-------------------------------------------
2587
5162e697 2588Given one existing commit, the linkgit:git-cherry-pick[1] command
b181d57f
BF
2589allows you to apply the change introduced by that commit and create a
2590new commit that records it. So, for example, if "mywork" points to a
2591series of patches on top of "origin", you might do something like:
2592
2593-------------------------------------------------
2594$ git checkout -b mywork-new origin
2595$ gitk origin..mywork &
2596-------------------------------------------------
2597
06ada152 2598and browse through the list of patches in the mywork branch using gitk,
b181d57f 2599applying them (possibly in a different order) to mywork-new using
6127c086 2600cherry-pick, and possibly modifying them as you go using `git commit --amend`.
5162e697 2601The linkgit:git-gui[1] command may also help as it allows you to
6e30fb0c
DK
2602individually select diff hunks for inclusion in the index (by
2603right-clicking on the diff hunk and choosing "Stage Hunk for Commit").
b181d57f 2604
6127c086 2605Another technique is to use `git format-patch` to create a series of
b181d57f 2606patches, then reset the state to before the patches:
4c63ff45 2607
b181d57f
BF
2608-------------------------------------------------
2609$ git format-patch origin
2610$ git reset --hard origin
2611-------------------------------------------------
4c63ff45 2612
b181d57f 2613Then modify, reorder, or eliminate patches as preferred before applying
5162e697 2614them again with linkgit:git-am[1].
4c63ff45 2615
e34caace 2616[[patch-series-tools]]
4c63ff45
BF
2617Other tools
2618-----------
2619
73a1d050 2620There are numerous other tools, such as StGit, which exist for the
79c96c57 2621purpose of maintaining a patch series. These are outside of the scope of
b181d57f 2622this manual.
4c63ff45 2623
aa971cb9 2624[[problems-With-rewriting-history]]
4c63ff45
BF
2625Problems with rewriting history
2626-------------------------------
2627
b181d57f
BF
2628The primary problem with rewriting the history of a branch has to do
2629with merging. Suppose somebody fetches your branch and merges it into
2630their branch, with a result something like this:
2631
1dc71a91 2632................................................
b181d57f
BF
2633 o--o--O--o--o--o <-- origin
2634 \ \
2635 t--t--t--m <-- their branch:
1dc71a91 2636................................................
b181d57f
BF
2637
2638Then suppose you modify the last three commits:
2639
1dc71a91 2640................................................
b181d57f
BF
2641 o--o--o <-- new head of origin
2642 /
2643 o--o--O--o--o--o <-- old head of origin
1dc71a91 2644................................................
b181d57f
BF
2645
2646If we examined all this history together in one repository, it will
2647look like:
2648
1dc71a91 2649................................................
b181d57f
BF
2650 o--o--o <-- new head of origin
2651 /
2652 o--o--O--o--o--o <-- old head of origin
2653 \ \
2654 t--t--t--m <-- their branch:
1dc71a91 2655................................................
b181d57f
BF
2656
2657Git has no way of knowing that the new head is an updated version of
2658the old head; it treats this situation exactly the same as it would if
2659two developers had independently done the work on the old and new heads
2660in parallel. At this point, if someone attempts to merge the new head
2661in to their branch, git will attempt to merge together the two (old and
2662new) lines of development, instead of trying to replace the old by the
2663new. The results are likely to be unexpected.
2664
2665You may still choose to publish branches whose history is rewritten,
2666and it may be useful for others to be able to fetch those branches in
2667order to examine or test them, but they should not attempt to pull such
2668branches into their own work.
2669
2670For true distributed development that supports proper merging,
2671published branches should never be rewritten.
2672
3fb00282
SP
2673[[bisect-merges]]
2674Why bisecting merge commits can be harder than bisecting linear history
2675-----------------------------------------------------------------------
2676
5162e697 2677The linkgit:git-bisect[1] command correctly handles history that
3fb00282
SP
2678includes merge commits. However, when the commit that it finds is a
2679merge commit, the user may need to work harder than usual to figure out
2680why that commit introduced a problem.
2681
2682Imagine this history:
2683
2684................................................
2685 ---Z---o---X---...---o---A---C---D
2686 \ /
2687 o---o---Y---...---o---B
2688................................................
2689
2690Suppose that on the upper line of development, the meaning of one
2691of the functions that exists at Z is changed at commit X. The
2692commits from Z leading to A change both the function's
2693implementation and all calling sites that exist at Z, as well
2694as new calling sites they add, to be consistent. There is no
2695bug at A.
2696
2697Suppose that in the meantime on the lower line of development somebody
2698adds a new calling site for that function at commit Y. The
2699commits from Z leading to B all assume the old semantics of that
2700function and the callers and the callee are consistent with each
2701other. There is no bug at B, either.
2702
2703Suppose further that the two development lines merge cleanly at C,
2704so no conflict resolution is required.
2705
2706Nevertheless, the code at C is broken, because the callers added
2707on the lower line of development have not been converted to the new
2708semantics introduced on the upper line of development. So if all
2709you know is that D is bad, that Z is good, and that
5162e697 2710linkgit:git-bisect[1] identifies C as the culprit, how will you
3fb00282
SP
2711figure out that the problem is due to this change in semantics?
2712
6127c086 2713When the result of a `git bisect` is a non-merge commit, you should
3fb00282
SP
2714normally be able to discover the problem by examining just that commit.
2715Developers can make this easy by breaking their changes into small
2716self-contained commits. That won't help in the case above, however,
2717because the problem isn't obvious from examination of any single
2718commit; instead, a global view of the development is required. To
2719make matters worse, the change in semantics in the problematic
2720function may be just one small part of the changes in the upper
2721line of development.
2722
2723On the other hand, if instead of merging at C you had rebased the
2724history between Z to B on top of A, you would have gotten this
2725linear history:
2726
2727................................................................
2728 ---Z---o---X--...---o---A---o---o---Y*--...---o---B*--D*
2729................................................................
2730
2731Bisecting between Z and D* would hit a single culprit commit Y*,
2732and understanding why Y* was broken would probably be easier.
2733
2734Partly for this reason, many experienced git users, even when
2735working on an otherwise merge-heavy project, keep the history
2736linear by rebasing against the latest upstream version before
2737publishing.
2738
e34caace 2739[[advanced-branch-management]]
b181d57f
BF
2740Advanced branch management
2741==========================
4c63ff45 2742
e34caace 2743[[fetching-individual-branches]]
b181d57f
BF
2744Fetching individual branches
2745----------------------------
2746
5162e697 2747Instead of using linkgit:git-remote[1], you can also choose just
b181d57f
BF
2748to update one branch at a time, and to store it locally under an
2749arbitrary name:
2750
2751-------------------------------------------------
2752$ git fetch origin todo:my-todo-work
2753-------------------------------------------------
2754
2755The first argument, "origin", just tells git to fetch from the
2756repository you originally cloned from. The second argument tells git
2757to fetch the branch named "todo" from the remote repository, and to
2758store it locally under the name refs/heads/my-todo-work.
2759
2760You can also fetch branches from other repositories; so
2761
2762-------------------------------------------------
2763$ git fetch git://example.com/proj.git master:example-master
2764-------------------------------------------------
2765
2766will create a new branch named "example-master" and store in it the
2767branch named "master" from the repository at the given URL. If you
2768already have a branch named example-master, it will attempt to
59723040
BF
2769<<fast-forwards,fast-forward>> to the commit given by example.com's
2770master branch. In more detail:
b181d57f 2771
59723040
BF
2772[[fetch-fast-forwards]]
2773git fetch and fast-forwards
2774---------------------------
b181d57f 2775
6127c086 2776In the previous example, when updating an existing branch, "git fetch"
7a7d4ef6 2777checks to make sure that the most recent commit on the remote
b181d57f
BF
2778branch is a descendant of the most recent commit on your copy of the
2779branch before updating your copy of the branch to point at the new
a75d7b54 2780commit. Git calls this process a <<fast-forwards,fast-forward>>.
b181d57f 2781
a75d7b54 2782A fast-forward looks something like this:
b181d57f 2783
1dc71a91 2784................................................
b181d57f
BF
2785 o--o--o--o <-- old head of the branch
2786 \
2787 o--o--o <-- new head of the branch
1dc71a91 2788................................................
b181d57f
BF
2789
2790
2791In some cases it is possible that the new head will *not* actually be
2792a descendant of the old head. For example, the developer may have
2793realized she made a serious mistake, and decided to backtrack,
2794resulting in a situation like:
2795
1dc71a91 2796................................................
b181d57f
BF
2797 o--o--o--o--a--b <-- old head of the branch
2798 \
2799 o--o--o <-- new head of the branch
1dc71a91 2800................................................
b181d57f 2801
6127c086 2802In this case, "git fetch" will fail, and print out a warning.
b181d57f
BF
2803
2804In that case, you can still force git to update to the new head, as
2805described in the following section. However, note that in the
2806situation above this may mean losing the commits labeled "a" and "b",
2807unless you've already created a reference of your own pointing to
2808them.
2809
e34caace 2810[[forcing-fetch]]
6127c086 2811Forcing git fetch to do non-fast-forward updates
b181d57f
BF
2812------------------------------------------------
2813
2814If git fetch fails because the new head of a branch is not a
2815descendant of the old head, you may force the update with:
2816
2817-------------------------------------------------
2818$ git fetch git://example.com/proj.git +master:refs/remotes/example/master
2819-------------------------------------------------
2820
c64415e2
BF
2821Note the addition of the "+" sign. Alternatively, you can use the "-f"
2822flag to force updates of all the fetched branches, as in:
2823
2824-------------------------------------------------
2825$ git fetch -f origin
2826-------------------------------------------------
2827
2828Be aware that commits that the old version of example/master pointed at
2829may be lost, as we saw in the previous section.
b181d57f 2830
e34caace 2831[[remote-branch-configuration]]
29b9a66f
MM
2832Configuring remote-tracking branches
2833------------------------------------
b181d57f
BF
2834
2835We saw above that "origin" is just a shortcut to refer to the
79c96c57 2836repository that you originally cloned from. This information is
b181d57f 2837stored in git configuration variables, which you can see using
5162e697 2838linkgit:git-config[1]:
b181d57f
BF
2839
2840-------------------------------------------------
9d13bda3 2841$ git config -l
b181d57f
BF
2842core.repositoryformatversion=0
2843core.filemode=true
2844core.logallrefupdates=true
2845remote.origin.url=git://git.kernel.org/pub/scm/git/git.git
2846remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
2847branch.master.remote=origin
2848branch.master.merge=refs/heads/master
2849-------------------------------------------------
2850
2851If there are other repositories that you also use frequently, you can
2852create similar configuration options to save typing; for example,
2853after
2854
2855-------------------------------------------------
9d13bda3 2856$ git config remote.example.url git://example.com/proj.git
b181d57f
BF
2857-------------------------------------------------
2858
2859then the following two commands will do the same thing:
2860
2861-------------------------------------------------
2862$ git fetch git://example.com/proj.git master:refs/remotes/example/master
2863$ git fetch example master:refs/remotes/example/master
2864-------------------------------------------------
2865
2866Even better, if you add one more option:
2867
2868-------------------------------------------------
9d13bda3 2869$ git config remote.example.fetch master:refs/remotes/example/master
b181d57f
BF
2870-------------------------------------------------
2871
2872then the following commands will all do the same thing:
2873
2874-------------------------------------------------
52c80037
BF
2875$ git fetch git://example.com/proj.git master:refs/remotes/example/master
2876$ git fetch example master:refs/remotes/example/master
b181d57f
BF
2877$ git fetch example
2878-------------------------------------------------
2879
2880You can also add a "+" to force the update each time:
2881
2882-------------------------------------------------
edfbbf7e 2883$ git config remote.example.fetch +master:refs/remotes/example/master
b181d57f
BF
2884-------------------------------------------------
2885
2886Don't do this unless you're sure you won't mind "git fetch" possibly
4f80b27d 2887throwing away commits on 'example/master'.
b181d57f
BF
2888
2889Also note that all of the above configuration can be performed by
2890directly editing the file .git/config instead of using
5162e697 2891linkgit:git-config[1].
b181d57f 2892
5162e697 2893See linkgit:git-config[1] for more details on the configuration
b181d57f 2894options mentioned above.
d19fbc3c 2895
d19fbc3c 2896
036f8199
BF
2897[[git-concepts]]
2898Git concepts
2899============
d19fbc3c 2900
036f8199
BF
2901Git is built on a small number of simple but powerful ideas. While it
2902is possible to get things done without understanding them, you will find
2903git much more intuitive if you do.
2904
2905We start with the most important, the <<def_object_database,object
2906database>> and the <<def_index,index>>.
b181d57f 2907
e34caace 2908[[the-object-database]]
b181d57f
BF
2909The Object Database
2910-------------------
2911
1bbf1c79
BF
2912
2913We already saw in <<understanding-commits>> that all commits are stored
2914under a 40-digit "object name". In fact, all the information needed to
2915represent the history of a project is stored in objects with such names.
a6e5ef7d
FC
2916In each case the name is calculated by taking the SHA-1 hash of the
2917contents of the object. The SHA-1 hash is a cryptographic hash function.
1bbf1c79
BF
2918What that means to us is that it is impossible to find two different
2919objects with the same name. This has a number of advantages; among
2920others:
2921
2922- Git can quickly determine whether two objects are identical or not,
2923 just by comparing names.
06ada152 2924- Since object names are computed the same way in every repository, the
1bbf1c79
BF
2925 same content stored in two repositories will always be stored under
2926 the same name.
2927- Git can detect errors when it reads an object, by checking that the
a6e5ef7d 2928 object's name is still the SHA-1 hash of its contents.
1bbf1c79
BF
2929
2930(See <<object-details>> for the details of the object formatting and
a6e5ef7d 2931SHA-1 calculation.)
1bbf1c79
BF
2932
2933There are four different types of objects: "blob", "tree", "commit", and
2934"tag".
2935
2936- A <<def_blob_object,"blob" object>> is used to store file data.
843c81dc 2937- A <<def_tree_object,"tree" object>> ties one or more
1bbf1c79
BF
2938 "blob" objects into a directory structure. In addition, a tree object
2939 can refer to other tree objects, thus creating a directory hierarchy.
2940- A <<def_commit_object,"commit" object>> ties such directory hierarchies
2ef8ac1b 2941 together into a <<def_DAG,directed acyclic graph>> of revisions--each
1bbf1c79
BF
2942 commit contains the object name of exactly one tree designating the
2943 directory hierarchy at the time of the commit. In addition, a commit
2944 refers to "parent" commit objects that describe the history of how we
2945 arrived at that directory hierarchy.
2946- A <<def_tag_object,"tag" object>> symbolically identifies and can be
2947 used to sign other objects. It contains the object name and type of
2948 another object, a symbolic name (of course!) and, optionally, a
2949 signature.
b181d57f 2950
b181d57f
BF
2951The object types in some more detail:
2952
513d419c
BF
2953[[commit-object]]
2954Commit Object
2955~~~~~~~~~~~~~
b181d57f 2956
1bbf1c79
BF
2957The "commit" object links a physical state of a tree with a description
2958of how we got there and why. Use the --pretty=raw option to
5162e697 2959linkgit:git-show[1] or linkgit:git-log[1] to examine your favorite
1bbf1c79
BF
2960commit:
2961
2962------------------------------------------------
2963$ git show -s --pretty=raw 2be7fcb476
2964commit 2be7fcb4764f2dbcee52635b91fedb1b3dcf7ab4
2965tree fb3a8bdd0ceddd019615af4d57a53f43d8cee2bf
2966parent 257a84d9d02e90447b149af58b271c19405edb6a
2967author Dave Watson <dwatson@mimvista.com> 1187576872 -0400
2968committer Junio C Hamano <gitster@pobox.com> 1187591163 -0700
2969
2970 Fix misspelling of 'suppress' in docs
2971
2972 Signed-off-by: Junio C Hamano <gitster@pobox.com>
2973------------------------------------------------
2974
2975As you can see, a commit is defined by:
2976
a6e5ef7d 2977- a tree: The SHA-1 name of a tree object (as defined below), representing
1bbf1c79 2978 the contents of a directory at a certain point in time.
edfbbf7e 2979- parent(s): The SHA-1 name(s) of some number of commits which represent the
9e5d87d4 2980 immediately previous step(s) in the history of the project. The
1bbf1c79
BF
2981 example above has one parent; merge commits may have more than
2982 one. A commit with no parents is called a "root" commit, and
2983 represents the initial revision of a project. Each project must have
2984 at least one root. A project can also have multiple roots, though
2985 that isn't common (or necessarily a good idea).
2986- an author: The name of the person responsible for this change, together
2987 with its date.
2988- a committer: The name of the person who actually created the commit,
2989 with the date it was done. This may be different from the author, for
2990 example, if the author was someone who wrote a patch and emailed it
2991 to the person who used it to create the commit.
2992- a comment describing this commit.
2993
2994Note that a commit does not itself contain any information about what
2995actually changed; all changes are calculated by comparing the contents
2996of the tree referred to by this commit with the trees associated with
2997its parents. In particular, git does not attempt to record file renames
2998explicitly, though it can identify cases where the existence of the same
2999file data at changing paths suggests a rename. (See, for example, the
5162e697 3000-M option to linkgit:git-diff[1]).
1bbf1c79 3001
5162e697 3002A commit is usually created by linkgit:git-commit[1], which creates a
1bbf1c79
BF
3003commit whose parent is normally the current HEAD, and whose tree is
3004taken from the content currently stored in the index.
b181d57f 3005
e34caace 3006[[tree-object]]
b181d57f 3007Tree Object
971aa71f 3008~~~~~~~~~~~
b181d57f 3009
5162e697
DM
3010The ever-versatile linkgit:git-show[1] command can also be used to
3011examine tree objects, but linkgit:git-ls-tree[1] will give you more
1bbf1c79
BF
3012details:
3013
3014------------------------------------------------
3015$ git ls-tree fb3a8bdd0ce
3016100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c .gitignore
3017100644 blob 5529b198e8d14decbe4ad99db3f7fb632de0439d .mailmap
3018100644 blob 6ff87c4664981e4397625791c8ea3bbb5f2279a3 COPYING
3019040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745 Documentation
3020100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200 GIT-VERSION-GEN
3021100644 blob 289b046a443c0647624607d471289b2c7dcd470b INSTALL
3022100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1 Makefile
3023100644 blob 548142c327a6790ff8821d67c2ee1eff7a656b52 README
3024...
3025------------------------------------------------
3026
3027As you can see, a tree object contains a list of entries, each with a
a6e5ef7d 3028mode, object type, SHA-1 name, and name, sorted by name. It represents
1bbf1c79
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3029the contents of a single directory tree.
3030
3031The object type may be a blob, representing the contents of a file, or
3032another tree, representing the contents of a subdirectory. Since trees
a6e5ef7d
FC
3033and blobs, like all other objects, are named by the SHA-1 hash of their
3034contents, two trees have the same SHA-1 name if and only if their
1bbf1c79
BF
3035contents (including, recursively, the contents of all subdirectories)
3036are identical. This allows git to quickly determine the differences
3037between two related tree objects, since it can ignore any entries with
3038identical object names.
3039
3040(Note: in the presence of submodules, trees may also have commits as
6dd14366 3041entries. See <<submodules>> for documentation.)
1bbf1c79
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3042
3043Note that the files all have mode 644 or 755: git actually only pays
3044attention to the executable bit.
b181d57f 3045
513d419c
BF
3046[[blob-object]]
3047Blob Object
3048~~~~~~~~~~~
b181d57f 3049
5162e697 3050You can use linkgit:git-show[1] to examine the contents of a blob; take,
1bbf1c79 3051for example, the blob in the entry for "COPYING" from the tree above:
b181d57f 3052
1bbf1c79
BF
3053------------------------------------------------
3054$ git show 6ff87c4664
3055
3056 Note that the only valid version of the GPL as far as this project
3057 is concerned is _this_ particular version of the license (ie v2, not
3058 v2.2 or v3.x or whatever), unless explicitly otherwise stated.
3059...
3060------------------------------------------------
b181d57f 3061
1bbf1c79
BF
3062A "blob" object is nothing but a binary blob of data. It doesn't refer
3063to anything else or have attributes of any kind.
3064
3065Since the blob is entirely defined by its data, if two files in a
3066directory tree (or in multiple different versions of the repository)
3067have the same contents, they will share the same blob object. The object
3068is totally independent of its location in the directory tree, and
3069renaming a file does not change the object that file is associated with.
3070
3071Note that any tree or blob object can be examined using
5162e697 3072linkgit:git-show[1] with the <revision>:<path> syntax. This can
1bbf1c79
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3073sometimes be useful for browsing the contents of a tree that is not
3074currently checked out.
b181d57f 3075
e34caace 3076[[trust]]
b181d57f 3077Trust
971aa71f 3078~~~~~
b181d57f 3079
a6e5ef7d 3080If you receive the SHA-1 name of a blob from one source, and its contents
1bbf1c79 3081from another (possibly untrusted) source, you can still trust that those
a6e5ef7d
FC
3082contents are correct as long as the SHA-1 name agrees. This is because
3083the SHA-1 is designed so that it is infeasible to find different contents
1bbf1c79 3084that produce the same hash.
b181d57f 3085
a6e5ef7d 3086Similarly, you need only trust the SHA-1 name of a top-level tree object
1bbf1c79 3087to trust the contents of the entire directory that it refers to, and if
a6e5ef7d 3088you receive the SHA-1 name of a commit from a trusted source, then you
1bbf1c79
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3089can easily verify the entire history of commits reachable through
3090parents of that commit, and all of those contents of the trees referred
3091to by those commits.
b181d57f
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3092
3093So to introduce some real trust in the system, the only thing you need
3094to do is to digitally sign just 'one' special note, which includes the
3095name of a top-level commit. Your digital signature shows others
3096that you trust that commit, and the immutability of the history of
3097commits tells others that they can trust the whole history.
3098
3099In other words, you can easily validate a whole archive by just
a6e5ef7d 3100sending out a single email that tells the people the name (SHA-1 hash)
b181d57f
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3101of the top commit, and digitally sign that email using something
3102like GPG/PGP.
3103
3104To assist in this, git also provides the tag object...
3105
e34caace 3106[[tag-object]]
b181d57f 3107Tag Object
971aa71f 3108~~~~~~~~~~
b181d57f 3109
1bbf1c79
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3110A tag object contains an object, object type, tag name, the name of the
3111person ("tagger") who created the tag, and a message, which may contain
843c81dc 3112a signature, as can be seen using linkgit:git-cat-file[1]:
b181d57f 3113
1bbf1c79
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3114------------------------------------------------
3115$ git cat-file tag v1.5.0
3116object 437b1b20df4b356c9342dac8d38849f24ef44f27
3117type commit
3118tag v1.5.0
3119tagger Junio C Hamano <junkio@cox.net> 1171411200 +0000
3120
3121GIT 1.5.0
3122-----BEGIN PGP SIGNATURE-----
3123Version: GnuPG v1.4.6 (GNU/Linux)
3124
3125iD8DBQBF0lGqwMbZpPMRm5oRAuRiAJ9ohBLd7s2kqjkKlq1qqC57SbnmzQCdG4ui
3126nLE/L9aUXdWeTFPron96DLA=
3127=2E+0
3128-----END PGP SIGNATURE-----
3129------------------------------------------------
b181d57f 3130
5162e697
DM
3131See the linkgit:git-tag[1] command to learn how to create and verify tag
3132objects. (Note that linkgit:git-tag[1] can also be used to create
1bbf1c79 3133"lightweight tags", which are not tag objects at all, but just simple
fc74ecc1 3134references whose names begin with "refs/tags/").
b181d57f 3135
09eff7b0
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3136[[pack-files]]
3137How git stores objects efficiently: pack files
3138~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3139
9644ffdd 3140Newly created objects are initially created in a file named after the
a6e5ef7d 3141object's SHA-1 hash (stored in .git/objects).
09eff7b0
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3142
3143Unfortunately this system becomes inefficient once a project has a
3144lot of objects. Try this on an old project:
3145
3146------------------------------------------------
3147$ git count-objects
31486930 objects, 47620 kilobytes
3149------------------------------------------------
3150
3151The first number is the number of objects which are kept in
3152individual files. The second is the amount of space taken up by
3153those "loose" objects.
3154
3155You can save space and make git faster by moving these loose objects in
3156to a "pack file", which stores a group of objects in an efficient
3157compressed format; the details of how pack files are formatted can be
3158found in link:technical/pack-format.txt[technical/pack-format.txt].
3159
3160To put the loose objects into a pack, just run git repack:
3161
3162------------------------------------------------
3163$ git repack
3164Generating pack...
3165Done counting 6020 objects.
3166Deltifying 6020 objects.
3167 100% (6020/6020) done
3168Writing 6020 objects.
3169 100% (6020/6020) done
3170Total 6020, written 6020 (delta 4070), reused 0 (delta 0)
3171Pack pack-3e54ad29d5b2e05838c75df582c65257b8d08e1c created.
3172------------------------------------------------
3173
3174You can then run
3175
3176------------------------------------------------
3177$ git prune
3178------------------------------------------------
3179
3180to remove any of the "loose" objects that are now contained in the
3181pack. This will also remove any unreferenced objects (which may be
6127c086 3182created when, for example, you use "git reset" to remove a commit).
09eff7b0
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3183You can verify that the loose objects are gone by looking at the
3184.git/objects directory or by running
3185
3186------------------------------------------------
3187$ git count-objects
31880 objects, 0 kilobytes
3189------------------------------------------------
3190
3191Although the object files are gone, any commands that refer to those
3192objects will work exactly as they did before.
3193
5162e697 3194The linkgit:git-gc[1] command performs packing, pruning, and more for
09eff7b0
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3195you, so is normally the only high-level command you need.
3196
3197[[dangling-objects]]
3198Dangling objects
3199~~~~~~~~~~~~~~~~
3200
5162e697 3201The linkgit:git-fsck[1] command will sometimes complain about dangling
09eff7b0
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3202objects. They are not a problem.
3203
3204The most common cause of dangling objects is that you've rebased a
3205branch, or you have pulled from somebody else who rebased a branch--see
3206<<cleaning-up-history>>. In that case, the old head of the original
3207branch still exists, as does everything it pointed to. The branch
3208pointer itself just doesn't, since you replaced it with another one.
3209
3210There are also other situations that cause dangling objects. For
6127c086 3211example, a "dangling blob" may arise because you did a "git add" of a
09eff7b0
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3212file, but then, before you actually committed it and made it part of the
3213bigger picture, you changed something else in that file and committed
2ef8ac1b 3214that *updated* thing--the old state that you added originally ends up
09eff7b0
BF
3215not being pointed to by any commit or tree, so it's now a dangling blob
3216object.
3217
3218Similarly, when the "recursive" merge strategy runs, and finds that
3219there are criss-cross merges and thus more than one merge base (which is
3220fairly unusual, but it does happen), it will generate one temporary
3221midway tree (or possibly even more, if you had lots of criss-crossing
3222merges and more than two merge bases) as a temporary internal merge
3223base, and again, those are real objects, but the end result will not end
3224up pointing to them, so they end up "dangling" in your repository.
3225
3226Generally, dangling objects aren't anything to worry about. They can
3227even be very useful: if you screw something up, the dangling objects can
3228be how you recover your old tree (say, you did a rebase, and realized
2ef8ac1b 3229that you really didn't want to--you can look at what dangling objects
09eff7b0
BF
3230you have, and decide to reset your head to some old dangling state).
3231
3232For commits, you can just use:
3233
3234------------------------------------------------
3235$ gitk <dangling-commit-sha-goes-here> --not --all
3236------------------------------------------------
3237
3238This asks for all the history reachable from the given commit but not
3239from any branch, tag, or other reference. If you decide it's something
3240you want, you can always create a new reference to it, e.g.,
3241
3242------------------------------------------------
3243$ git branch recovered-branch <dangling-commit-sha-goes-here>
3244------------------------------------------------
3245
3246For blobs and trees, you can't do the same, but you can still examine
3247them. You can just do
3248
3249------------------------------------------------
3250$ git show <dangling-blob/tree-sha-goes-here>
3251------------------------------------------------
3252
3253to show what the contents of the blob were (or, for a tree, basically
3254what the "ls" for that directory was), and that may give you some idea
3255of what the operation was that left that dangling object.
3256
3257Usually, dangling blobs and trees aren't very interesting. They're
3258almost always the result of either being a half-way mergebase (the blob
3259will often even have the conflict markers from a merge in it, if you
3260have had conflicting merges that you fixed up by hand), or simply
6127c086 3261because you interrupted a "git fetch" with ^C or something like that,
09eff7b0
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3262leaving _some_ of the new objects in the object database, but just
3263dangling and useless.
3264
3265Anyway, once you are sure that you're not interested in any dangling
3266state, you can just prune all unreachable objects:
3267
3268------------------------------------------------
3269$ git prune
3270------------------------------------------------
3271
3272and they'll be gone. But you should only run "git prune" on a quiescent
2ef8ac1b 3273repository--it's kind of like doing a filesystem fsck recovery: you
09eff7b0
BF
3274don't want to do that while the filesystem is mounted.
3275
6127c086
FC
3276(The same is true of "git fsck" itself, btw, but since
3277`git fsck` never actually *changes* the repository, it just reports
3278on what it found, `git fsck` itself is never 'dangerous' to run.
09eff7b0
BF
3279Running it while somebody is actually changing the repository can cause
3280confusing and scary messages, but it won't actually do anything bad. In
3281contrast, running "git prune" while somebody is actively changing the
3282repository is a *BAD* idea).
b181d57f 3283
1cdade2c
BF
3284[[recovering-from-repository-corruption]]
3285Recovering from repository corruption
3286~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3287
3288By design, git treats data trusted to it with caution. However, even in
3289the absence of bugs in git itself, it is still possible that hardware or
3290operating system errors could corrupt data.
3291