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Git User's Manual (for version 1.5.3 or newer)
Git is a fast distributed revision control system.
This manual is designed to be readable by someone with basic UNIX
command-line skills, but no previous knowledge of git.
<<repositories-and-branches>> and <<exploring-git-history>> explain how
to fetch and study a project using git--read these chapters to learn how
to build and test a particular version of a software project, search for
regressions, and so on.
People needing to do actual development will also want to read
<<Developing-With-git>> and <<sharing-development>>.
Further chapters cover more specialized topics.
Comprehensive reference documentation is available through the man
pages, or linkgit:git-help[1] command. For example, for the command
"git clone <repo>", you can either use:
$ man git-clone
$ git help clone
With the latter, you can use the manual viewer of your choice; see
linkgit:git-help[1] for more information.
See also <<git-quick-start>> for a brief overview of git commands,
without any explanation.
Finally, see <<todo>> for ways that you can help make this manual more
Repositories and Branches
How to get a git repository
It will be useful to have a git repository to experiment with as you
read this manual.
The best way to get one is by using the linkgit:git-clone[1] command to
download a copy of an existing repository. If you don't already have a
project in mind, here are some interesting examples:
# git itself (approx. 10MB download):
$ git clone git://
# the Linux kernel (approx. 150MB download):
$ git clone git://
The initial clone may be time-consuming for a large project, but you
will only need to clone once.
The clone command creates a new directory named after the project ("git"
or "linux-2.6" in the examples above). After you cd into this
directory, you will see that it contains a copy of the project files,
called the <<def_working_tree,working tree>>, together with a special
top-level directory named ".git", which contains all the information
about the history of the project.
How to check out a different version of a project
Git is best thought of as a tool for storing the history of a collection
of files. It stores the history as a compressed collection of
interrelated snapshots of the project's contents. In git each such
version is called a <<def_commit,commit>>.
Those snapshots aren't necessarily all arranged in a single line from
oldest to newest; instead, work may simultaneously proceed along
parallel lines of development, called <<def_branch,branches>>, which may
merge and diverge.
A single git repository can track development on multiple branches. It
does this by keeping a list of <<def_head,heads>> which reference the
latest commit on each branch; the linkgit:git-branch[1] command shows
you the list of branch heads:
$ git branch
* master
A freshly cloned repository contains a single branch head, by default
named "master", with the working directory initialized to the state of
the project referred to by that branch head.
Most projects also use <<def_tag,tags>>. Tags, like heads, are
references into the project's history, and can be listed using the
linkgit:git-tag[1] command:
$ git tag -l
Tags are expected to always point at the same version of a project,
while heads are expected to advance as development progresses.
Create a new branch head pointing to one of these versions and check it
out using linkgit:git-checkout[1]:
$ git checkout -b new v2.6.13
The working directory then reflects the contents that the project had
when it was tagged v2.6.13, and linkgit:git-branch[1] shows two
branches, with an asterisk marking the currently checked-out branch:
$ git branch
* new
If you decide that you'd rather see version 2.6.17, you can modify
the current branch to point at v2.6.17 instead, with
$ git reset --hard v2.6.17
Note that if the current branch head was your only reference to a
particular point in history, then resetting that branch may leave you
with no way to find the history it used to point to; so use this command
Understanding History: Commits
Every change in the history of a project is represented by a commit.
The linkgit:git-show[1] command shows the most recent commit on the
current branch:
$ git show
commit 17cf781661e6d38f737f15f53ab552f1e95960d7
Author: Linus Torvalds <>
Date: Tue Apr 19 14:11:06 2005 -0700
Remove duplicate getenv(DB_ENVIRONMENT) call
Noted by Tony Luck.
diff --git a/init-db.c b/init-db.c
index 65898fa..b002dc6 100644
--- a/init-db.c
+++ b/init-db.c
@@ -7,7 +7,7 @@
int main(int argc, char **argv)
- char *sha1_dir = getenv(DB_ENVIRONMENT), *path;
+ char *sha1_dir, *path;
int len, i;
if (mkdir(".git", 0755) < 0) {
As you can see, a commit shows who made the latest change, what they
did, and why.
Every commit has a 40-hexdigit id, sometimes called the "object name" or the
"SHA-1 id", shown on the first line of the "git show" output. You can usually
refer to a commit by a shorter name, such as a tag or a branch name, but this
longer name can also be useful. Most importantly, it is a globally unique
name for this commit: so if you tell somebody else the object name (for
example in email), then you are guaranteed that name will refer to the same
commit in their repository that it does in yours (assuming their repository
has that commit at all). Since the object name is computed as a hash over the
contents of the commit, you are guaranteed that the commit can never change
without its name also changing.
In fact, in <<git-concepts>> we shall see that everything stored in git
history, including file data and directory contents, is stored in an object
with a name that is a hash of its contents.
Understanding history: commits, parents, and reachability
Every commit (except the very first commit in a project) also has a
parent commit which shows what happened before this commit.
Following the chain of parents will eventually take you back to the
beginning of the project.
However, the commits do not form a simple list; git allows lines of
development to diverge and then reconverge, and the point where two
lines of development reconverge is called a "merge". The commit
representing a merge can therefore have more than one parent, with
each parent representing the most recent commit on one of the lines
of development leading to that point.
The best way to see how this works is using the linkgit:gitk[1]
command; running gitk now on a git repository and looking for merge
commits will help understand how the git organizes history.
In the following, we say that commit X is "reachable" from commit Y
if commit X is an ancestor of commit Y. Equivalently, you could say
that Y is a descendant of X, or that there is a chain of parents
leading from commit Y to commit X.
Understanding history: History diagrams
We will sometimes represent git history using diagrams like the one
below. Commits are shown as "o", and the links between them with
lines drawn with - / and \. Time goes left to right:
o--o--o <-- Branch A
o--o--o <-- master
o--o--o <-- Branch B
If we need to talk about a particular commit, the character "o" may
be replaced with another letter or number.
Understanding history: What is a branch?
When we need to be precise, we will use the word "branch" to mean a line
of development, and "branch head" (or just "head") to mean a reference
to the most recent commit on a branch. In the example above, the branch
head named "A" is a pointer to one particular commit, but we refer to
the line of three commits leading up to that point as all being part of
"branch A".
However, when no confusion will result, we often just use the term
"branch" both for branches and for branch heads.
Manipulating branches
Creating, deleting, and modifying branches is quick and easy; here's
a summary of the commands:
git branch::
list all branches
git branch <branch>::
create a new branch named <branch>, referencing the same
point in history as the current branch
git branch <branch> <start-point>::
create a new branch named <branch>, referencing
<start-point>, which may be specified any way you like,
including using a branch name or a tag name
git branch -d <branch>::
delete the branch <branch>; if the branch you are deleting
points to a commit which is not reachable from the current
branch, this command will fail with a warning.
git branch -D <branch>::
even if the branch points to a commit not reachable
from the current branch, you may know that that commit
is still reachable from some other branch or tag. In that
case it is safe to use this command to force git to delete
the branch.
git checkout <branch>::
make the current branch <branch>, updating the working
directory to reflect the version referenced by <branch>
git checkout -b <new> <start-point>::
create a new branch <new> referencing <start-point>, and
check it out.
The special symbol "HEAD" can always be used to refer to the current
branch. In fact, git uses a file named "HEAD" in the .git directory to
remember which branch is current:
$ cat .git/HEAD
ref: refs/heads/master
Examining an old version without creating a new branch
The `git checkout` command normally expects a branch head, but will also
accept an arbitrary commit; for example, you can check out the commit
referenced by a tag:
$ git checkout v2.6.17
Note: moving to "v2.6.17" which isn't a local branch
If you want to create a new branch from this checkout, you may do so
(now or later) by using -b with the checkout command again. Example:
git checkout -b <new_branch_name>
HEAD is now at 427abfa... Linux v2.6.17
The HEAD then refers to the SHA-1 of the commit instead of to a branch,
and git branch shows that you are no longer on a branch:
$ cat .git/HEAD
$ git branch
* (no branch)
In this case we say that the HEAD is "detached".
This is an easy way to check out a particular version without having to
make up a name for the new branch. You can still create a new branch
(or tag) for this version later if you decide to.
Examining branches from a remote repository
The "master" branch that was created at the time you cloned is a copy
of the HEAD in the repository that you cloned from. That repository
may also have had other branches, though, and your local repository
keeps branches which track each of those remote branches, called
remote-tracking branches, which you
can view using the "-r" option to linkgit:git-branch[1]:
$ git branch -r
In this example, "origin" is called a remote repository, or "remote"
for short. The branches of this repository are called "remote
branches" from our point of view. The remote-tracking branches listed
above were created based on the remote branches at clone time and will
be updated by "git fetch" (hence "git pull") and "git push". See
<<Updating-a-repository-With-git-fetch>> for details.
You might want to build on one of these remote-tracking branches
on a branch of your own, just as you would for a tag:
$ git checkout -b my-todo-copy origin/todo
You can also check out "origin/todo" directly to examine it or
write a one-off patch. See <<detached-head,detached head>>.
Note that the name "origin" is just the name that git uses by default
to refer to the repository that you cloned from.
Naming branches, tags, and other references
Branches, remote-tracking branches, and tags are all references to
commits. All references are named with a slash-separated path name
starting with "refs"; the names we've been using so far are actually
- The branch "test" is short for "refs/heads/test".
- The tag "v2.6.18" is short for "refs/tags/v2.6.18".
- "origin/master" is short for "refs/remotes/origin/master".
The full name is occasionally useful if, for example, there ever
exists a tag and a branch with the same name.
(Newly created refs are actually stored in the .git/refs directory,
under the path given by their name. However, for efficiency reasons
they may also be packed together in a single file; see
As another useful shortcut, the "HEAD" of a repository can be referred
to just using the name of that repository. So, for example, "origin"
is usually a shortcut for the HEAD branch in the repository "origin".
For the complete list of paths which git checks for references, and
the order it uses to decide which to choose when there are multiple
references with the same shorthand name, see the "SPECIFYING
REVISIONS" section of linkgit:gitrevisions[7].
Updating a repository with git fetch
Eventually the developer cloned from will do additional work in her
repository, creating new commits and advancing the branches to point
at the new commits.
The command "git fetch", with no arguments, will update all of the
remote-tracking branches to the latest version found in her
repository. It will not touch any of your own branches--not even the
"master" branch that was created for you on clone.
Fetching branches from other repositories
You can also track branches from repositories other than the one you
cloned from, using linkgit:git-remote[1]:
$ git remote add linux-nfs git://
$ git fetch linux-nfs
* refs/remotes/linux-nfs/master: storing branch 'master' ...
commit: bf81b46
New remote-tracking branches will be stored under the shorthand name
that you gave "git remote add", in this case linux-nfs:
$ git branch -r
If you run "git fetch <remote>" later, the remote-tracking branches for the
named <remote> will be updated.
If you examine the file .git/config, you will see that git has added
a new stanza:
$ cat .git/config
[remote "linux-nfs"]
url = git://
fetch = +refs/heads/*:refs/remotes/linux-nfs/*
This is what causes git to track the remote's branches; you may modify
or delete these configuration options by editing .git/config with a
text editor. (See the "CONFIGURATION FILE" section of
linkgit:git-config[1] for details.)
Exploring git history
Git is best thought of as a tool for storing the history of a
collection of files. It does this by storing compressed snapshots of
the contents of a file hierarchy, together with "commits" which show
the relationships between these snapshots.
Git provides extremely flexible and fast tools for exploring the
history of a project.
We start with one specialized tool that is useful for finding the
commit that introduced a bug into a project.
How to use bisect to find a regression
Suppose version 2.6.18 of your project worked, but the version at
"master" crashes. Sometimes the best way to find the cause of such a
regression is to perform a brute-force search through the project's
history to find the particular commit that caused the problem. The
linkgit:git-bisect[1] command can help you do this:
$ git bisect start
$ git bisect good v2.6.18
$ git bisect bad master
Bisecting: 3537 revisions left to test after this
[65934a9a028b88e83e2b0f8b36618fe503349f8e] BLOCK: Make USB storage depend on SCSI rather than selecting it [try #6]
If you run "git branch" at this point, you'll see that git has
temporarily moved you in "(no branch)". HEAD is now detached from any
branch and points directly to a commit (with commit id 65934...) that
is reachable from "master" but not from v2.6.18. Compile and test it,
and see whether it crashes. Assume it does crash. Then:
$ git bisect bad
Bisecting: 1769 revisions left to test after this
[7eff82c8b1511017ae605f0c99ac275a7e21b867] i2c-core: Drop useless bitmaskings
checks out an older version. Continue like this, telling git at each
stage whether the version it gives you is good or bad, and notice
that the number of revisions left to test is cut approximately in
half each time.
After about 13 tests (in this case), it will output the commit id of
the guilty commit. You can then examine the commit with
linkgit:git-show[1], find out who wrote it, and mail them your bug
report with the commit id. Finally, run
$ git bisect reset
to return you to the branch you were on before.
Note that the version which `git bisect` checks out for you at each
point is just a suggestion, and you're free to try a different
version if you think it would be a good idea. For example,
occasionally you may land on a commit that broke something unrelated;
$ git bisect visualize
which will run gitk and label the commit it chose with a marker that
says "bisect". Choose a safe-looking commit nearby, note its commit
id, and check it out with:
$ git reset --hard fb47ddb2db...
then test, run "bisect good" or "bisect bad" as appropriate, and
Instead of "git bisect visualize" and then "git reset --hard
fb47ddb2db...", you might just want to tell git that you want to skip
the current commit:
$ git bisect skip
In this case, though, git may not eventually be able to tell the first
bad one between some first skipped commits and a later bad commit.
There are also ways to automate the bisecting process if you have a
test script that can tell a good from a bad commit. See
linkgit:git-bisect[1] for more information about this and other "git
bisect" features.
Naming commits
We have seen several ways of naming commits already:
- 40-hexdigit object name
- branch name: refers to the commit at the head of the given
- tag name: refers to the commit pointed to by the given tag
(we've seen branches and tags are special cases of
- HEAD: refers to the head of the current branch
There are many more; see the "SPECIFYING REVISIONS" section of the
linkgit:gitrevisions[7] man page for the complete list of ways to
name revisions. Some examples:
$ git show fb47ddb2 # the first few characters of the object name
# are usually enough to specify it uniquely
$ git show HEAD^ # the parent of the HEAD commit
$ git show HEAD^^ # the grandparent
$ git show HEAD~4 # the great-great-grandparent
Recall that merge commits may have more than one parent; by default,
^ and ~ follow the first parent listed in the commit, but you can
also choose:
$ git show HEAD^1 # show the first parent of HEAD
$ git show HEAD^2 # show the second parent of HEAD
In addition to HEAD, there are several other special names for
Merges (to be discussed later), as well as operations such as
`git reset`, which change the currently checked-out commit, generally
set ORIG_HEAD to the value HEAD had before the current operation.
The `git fetch` operation always stores the head of the last fetched
branch in FETCH_HEAD. For example, if you run `git fetch` without
specifying a local branch as the target of the operation
$ git fetch git:// theirbranch
the fetched commits will still be available from FETCH_HEAD.
When we discuss merges we'll also see the special name MERGE_HEAD,
which refers to the other branch that we're merging in to the current
The linkgit:git-rev-parse[1] command is a low-level command that is
occasionally useful for translating some name for a commit to the object
name for that commit:
$ git rev-parse origin
Creating tags
We can also create a tag to refer to a particular commit; after
$ git tag stable-1 1b2e1d63ff
You can use stable-1 to refer to the commit 1b2e1d63ff.
This creates a "lightweight" tag. If you would also like to include a
comment with the tag, and possibly sign it cryptographically, then you
should create a tag object instead; see the linkgit:git-tag[1] man page
for details.
Browsing revisions
The linkgit:git-log[1] command can show lists of commits. On its
own, it shows all commits reachable from the parent commit; but you
can also make more specific requests:
$ git log v2.5.. # commits since (not reachable from) v2.5
$ git log test..master # commits reachable from master but not test
$ git log master..test # ...reachable from test but not master
$ git log master...test # ...reachable from either test or master,
# but not both
$ git log --since="2 weeks ago" # commits from the last 2 weeks
$ git log Makefile # commits which modify Makefile
$ git log fs/ # ... which modify any file under fs/
$ git log -S'foo()' # commits which add or remove any file data
# matching the string 'foo()'
And of course you can combine all of these; the following finds
commits since v2.5 which touch the Makefile or any file under fs:
$ git log v2.5.. Makefile fs/
You can also ask git log to show patches:
$ git log -p
See the "--pretty" option in the linkgit:git-log[1] man page for more
display options.
Note that git log starts with the most recent commit and works
backwards through the parents; however, since git history can contain
multiple independent lines of development, the particular order that
commits are listed in may be somewhat arbitrary.
Generating diffs
You can generate diffs between any two versions using
$ git diff master..test
That will produce the diff between the tips of the two branches. If
you'd prefer to find the diff from their common ancestor to test, you
can use three dots instead of two:
$ git diff master...test
Sometimes what you want instead is a set of patches; for this you can
use linkgit:git-format-patch[1]:
$ git format-patch master..test
will generate a file with a patch for each commit reachable from test
but not from master.
Viewing old file versions
You can always view an old version of a file by just checking out the
correct revision first. But sometimes it is more convenient to be
able to view an old version of a single file without checking
anything out; this command does that:
$ git show v2.5:fs/locks.c
Before the colon may be anything that names a commit, and after it
may be any path to a file tracked by git.
Counting the number of commits on a branch
Suppose you want to know how many commits you've made on "mybranch"
since it diverged from "origin":
$ git log --pretty=oneline origin..mybranch | wc -l
Alternatively, you may often see this sort of thing done with the
lower-level command linkgit:git-rev-list[1], which just lists the SHA-1's
of all the given commits:
$ git rev-list origin..mybranch | wc -l
Check whether two branches point at the same history
Suppose you want to check whether two branches point at the same point
in history.
$ git diff origin..master
will tell you whether the contents of the project are the same at the
two branches; in theory, however, it's possible that the same project
contents could have been arrived at by two different historical
routes. You could compare the object names:
$ git rev-list origin
$ git rev-list master
Or you could recall that the ... operator selects all commits
contained reachable from either one reference or the other but not
both: so
$ git log origin...master
will return no commits when the two branches are equal.
Find first tagged version including a given fix
Suppose you know that the commit e05db0fd fixed a certain problem.
You'd like to find the earliest tagged release that contains that
Of course, there may be more than one answer--if the history branched
after commit e05db0fd, then there could be multiple "earliest" tagged
You could just visually inspect the commits since e05db0fd:
$ gitk e05db0fd..
Or you can use linkgit:git-name-rev[1], which will give the commit a
name based on any tag it finds pointing to one of the commit's
$ git name-rev --tags e05db0fd
e05db0fd tags/v1.5.0-rc1^0~23
The linkgit:git-describe[1] command does the opposite, naming the
revision using a tag on which the given commit is based:
$ git describe e05db0fd
but that may sometimes help you guess which tags might come after the
given commit.
If you just want to verify whether a given tagged version contains a
given commit, you could use linkgit:git-merge-base[1]:
$ git merge-base e05db0fd v1.5.0-rc1
The merge-base command finds a common ancestor of the given commits,
and always returns one or the other in the case where one is a
descendant of the other; so the above output shows that e05db0fd
actually is an ancestor of v1.5.0-rc1.
Alternatively, note that
$ git log v1.5.0-rc1..e05db0fd
will produce empty output if and only if v1.5.0-rc1 includes e05db0fd,
because it outputs only commits that are not reachable from v1.5.0-rc1.
As yet another alternative, the linkgit:git-show-branch[1] command lists
the commits reachable from its arguments with a display on the left-hand
side that indicates which arguments that commit is reachable from. So,
you can run something like
$ git show-branch e05db0fd v1.5.0-rc0 v1.5.0-rc1 v1.5.0-rc2
! [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
! [v1.5.0-rc0] GIT v1.5.0 preview
! [v1.5.0-rc1] GIT v1.5.0-rc1
! [v1.5.0-rc2] GIT v1.5.0-rc2
then search for a line that looks like
+ ++ [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
Which shows that e05db0fd is reachable from itself, from v1.5.0-rc1, and
from v1.5.0-rc2, but not from v1.5.0-rc0.
Showing commits unique to a given branch
Suppose you would like to see all the commits reachable from the branch
head named "master" but not from any other head in your repository.
We can list all the heads in this repository with
$ git show-ref --heads
bf62196b5e363d73353a9dcf094c59595f3153b7 refs/heads/core-tutorial
db768d5504c1bb46f63ee9d6e1772bd047e05bf9 refs/heads/maint
a07157ac624b2524a059a3414e99f6f44bebc1e7 refs/heads/master
24dbc180ea14dc1aebe09f14c8ecf32010690627 refs/heads/tutorial-2
1e87486ae06626c2f31eaa63d26fc0fd646c8af2 refs/heads/tutorial-fixes
We can get just the branch-head names, and remove "master", with
the help of the standard utilities cut and grep:
$ git show-ref --heads | cut -d' ' -f2 | grep -v '^refs/heads/master'
And then we can ask to see all the commits reachable from master
but not from these other heads:
$ gitk master --not $( git show-ref --heads | cut -d' ' -f2 |
grep -v '^refs/heads/master' )
Obviously, endless variations are possible; for example, to see all
commits reachable from some head but not from any tag in the repository:
$ gitk $( git show-ref --heads ) --not $( git show-ref --tags )
(See linkgit:gitrevisions[7] for explanations of commit-selecting
syntax such as `--not`.)
Creating a changelog and tarball for a software release
The linkgit:git-archive[1] command can create a tar or zip archive from
any version of a project; for example:
$ git archive --format=tar --prefix=project/ HEAD | gzip >latest.tar.gz
will use HEAD to produce a tar archive in which each filename is
preceded by "project/".
If you're releasing a new version of a software project, you may want
to simultaneously make a changelog to include in the release
Linus Torvalds, for example, makes new kernel releases by tagging them,
then running:
$ release-script 2.6.12 2.6.13-rc6 2.6.13-rc7
where release-script is a shell script that looks like:
echo "# git tag v$new"
echo "git archive --prefix=linux-$new/ v$new | gzip -9 > ../linux-$new.tar.gz"
echo "git diff v$stable v$new | gzip -9 > ../patch-$new.gz"
echo "git log --no-merges v$new ^v$last > ../ChangeLog-$new"
echo "git shortlog --no-merges v$new ^v$last > ../ShortLog"
echo "git diff --stat --summary -M v$last v$new > ../diffstat-$new"
and then he just cut-and-pastes the output commands after verifying that
they look OK.
Finding commits referencing a file with given content
Somebody hands you a copy of a file, and asks which commits modified a
file such that it contained the given content either before or after the
commit. You can find out with this:
$ git log --raw --abbrev=40 --pretty=oneline |
grep -B 1 `git hash-object filename`
Figuring out why this works is left as an exercise to the (advanced)
student. The linkgit:git-log[1], linkgit:git-diff-tree[1], and
linkgit:git-hash-object[1] man pages may prove helpful.
Developing with git
Telling git your name
Before creating any commits, you should introduce yourself to git. The
easiest way to do so is to make sure the following lines appear in a
file named .gitconfig in your home directory:
name = Your Name Comes Here
email =
(See the "CONFIGURATION FILE" section of linkgit:git-config[1] for
details on the configuration file.)
Creating a new repository
Creating a new repository from scratch is very easy:
$ mkdir project
$ cd project
$ git init
If you have some initial content (say, a tarball):
$ tar xzvf project.tar.gz
$ cd project
$ git init
$ git add . # include everything below ./ in the first commit:
$ git commit
How to make a commit
Creating a new commit takes three steps:
1. Making some changes to the working directory using your
favorite editor.
2. Telling git about your changes.
3. Creating the commit using the content you told git about
in step 2.
In practice, you can interleave and repeat steps 1 and 2 as many
times as you want: in order to keep track of what you want committed
at step 3, git maintains a snapshot of the tree's contents in a
special staging area called "the index."
At the beginning, the content of the index will be identical to
that of the HEAD. The command "git diff --cached", which shows
the difference between the HEAD and the index, should therefore
produce no output at that point.
Modifying the index is easy:
To update the index with the new contents of a modified file, use
$ git add path/to/file
To add the contents of a new file to the index, use
$ git add path/to/file
To remove a file from the index and from the working tree,
$ git rm path/to/file
After each step you can verify that
$ git diff --cached
always shows the difference between the HEAD and the index file--this
is what you'd commit if you created the commit now--and that
$ git diff
shows the difference between the working tree and the index file.
Note that "git add" always adds just the current contents of a file
to the index; further changes to the same file will be ignored unless
you run `git add` on the file again.
When you're ready, just run
$ git commit
and git will prompt you for a commit message and then create the new
commit. Check to make sure it looks like what you expected with
$ git show
As a special shortcut,
$ git commit -a
will update the index with any files that you've modified or removed
and create a commit, all in one step.
A number of commands are useful for keeping track of what you're
about to commit:
$ git diff --cached # difference between HEAD and the index; what
# would be committed if you ran "commit" now.
$ git diff # difference between the index file and your
# working directory; changes that would not
# be included if you ran "commit" now.
$ git diff HEAD # difference between HEAD and working tree; what
# would be committed if you ran "commit -a" now.
$ git status # a brief per-file summary of the above.
You can also use linkgit:git-gui[1] to create commits, view changes in
the index and the working tree files, and individually select diff hunks
for inclusion in the index (by right-clicking on the diff hunk and
choosing "Stage Hunk For Commit").
Creating good commit messages
Though not required, it's a good idea to begin the commit message
with a single short (less than 50 character) line summarizing the
change, followed by a blank line and then a more thorough
description. The text up to the first blank line in a commit
message is treated as the commit title, and that title is used
throughout git. For example, linkgit:git-format-patch[1] turns a
commit into email, and it uses the title on the Subject line and the
rest of the commit in the body.
Ignoring files
A project will often generate files that you do 'not' want to track with git.
This typically includes files generated by a build process or temporary
backup files made by your editor. Of course, 'not' tracking files with git
is just a matter of 'not' calling `git add` on them. But it quickly becomes
annoying to have these untracked files lying around; e.g. they make
`git add .` practically useless, and they keep showing up in the output of
`git status`.
You can tell git to ignore certain files by creating a file called .gitignore
in the top level of your working directory, with contents such as:
# Lines starting with '#' are considered comments.
# Ignore any file named foo.txt.
# Ignore (generated) html files,
# except foo.html which is maintained by hand.
# Ignore objects and archives.
See linkgit:gitignore[5] for a detailed explanation of the syntax. You can
also place .gitignore files in other directories in your working tree, and they
will apply to those directories and their subdirectories. The `.gitignore`
files can be added to your repository like any other files (just run `git add
.gitignore` and `git commit`, as usual), which is convenient when the exclude
patterns (such as patterns matching build output files) would also make sense
for other users who clone your repository.
If you wish the exclude patterns to affect only certain repositories
(instead of every repository for a given project), you may instead put
them in a file in your repository named .git/info/exclude, or in any file
specified by the `core.excludesfile` configuration variable. Some git
commands can also take exclude patterns directly on the command line.
See linkgit:gitignore[5] for the details.
How to merge
You can rejoin two diverging branches of development using
$ git merge branchname
merges the development in the branch "branchname" into the current
A merge is made by combining the changes made in "branchname" and the
changes made up to the latest commit in your current branch since
their histories forked. The work tree is overwritten by the result of
the merge when this combining is done cleanly, or overwritten by a
half-merged results when this combining results in conflicts.
Therefore, if you have uncommitted changes touching the same files as
the ones impacted by the merge, Git will refuse to proceed. Most of
the time, you will want to commit your changes before you can merge,
and if you don't, then linkgit:git-stash[1] can take these changes
away while you're doing the merge, and reapply them afterwards.
If the changes are independent enough, Git will automatically complete
the merge and commit the result (or reuse an existing commit in case
of <<fast-forwards,fast-forward>>, see below). On the other hand,
if there are conflicts--for example, if the same file is
modified in two different ways in the remote branch and the local
branch--then you are warned; the output may look something like this:
$ git merge next
100% (4/4) done
Auto-merged file.txt
CONFLICT (content): Merge conflict in file.txt
Automatic merge failed; fix conflicts and then commit the result.
Conflict markers are left in the problematic files, and after
you resolve the conflicts manually, you can update the index
with the contents and run git commit, as you normally would when
creating a new file.
If you examine the resulting commit using gitk, you will see that it
has two parents, one pointing to the top of the current branch, and
one to the top of the other branch.
Resolving a merge
When a merge isn't resolved automatically, git leaves the index and
the working tree in a special state that gives you all the
information you need to help resolve the merge.
Files with conflicts are marked specially in the index, so until you
resolve the problem and update the index, linkgit:git-commit[1] will
$ git commit
file.txt: needs merge
Also, linkgit:git-status[1] will list those files as "unmerged", and the
files with conflicts will have conflict markers added, like this:
<<<<<<< HEAD:file.txt
Hello world
>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
All you need to do is edit the files to resolve the conflicts, and then
$ git add file.txt
$ git commit
Note that the commit message will already be filled in for you with
some information about the merge. Normally you can just use this
default message unchanged, but you may add additional commentary of
your own if desired.
The above is all you need to know to resolve a simple merge. But git
also provides more information to help resolve conflicts:
Getting conflict-resolution help during a merge
All of the changes that git was able to merge automatically are
already added to the index file, so linkgit:git-diff[1] shows only
the conflicts. It uses an unusual syntax:
$ git diff
diff --cc file.txt
index 802992c,2b60207..0000000
--- a/file.txt
+++ b/file.txt
@@@ -1,1 -1,1 +1,5 @@@
++<<<<<<< HEAD:file.txt
+Hello world
+ Goodbye
++>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
Recall that the commit which will be committed after we resolve this
conflict will have two parents instead of the usual one: one parent
will be HEAD, the tip of the current branch; the other will be the
tip of the other branch, which is stored temporarily in MERGE_HEAD.
During the merge, the index holds three versions of each file. Each of
these three "file stages" represents a different version of the file:
$ git show :1:file.txt # the file in a common ancestor of both branches
$ git show :2:file.txt # the version from HEAD.
$ git show :3:file.txt # the version from MERGE_HEAD.
When you ask linkgit:git-diff[1] to show the conflicts, it runs a
three-way diff between the conflicted merge results in the work tree with
stages 2 and 3 to show only hunks whose contents come from both sides,
mixed (in other words, when a hunk's merge results come only from stage 2,
that part is not conflicting and is not shown. Same for stage 3).
The diff above shows the differences between the working-tree version of
file.txt and the stage 2 and stage 3 versions. So instead of preceding
each line by a single "+" or "-", it now uses two columns: the first
column is used for differences between the first parent and the working
directory copy, and the second for differences between the second parent
and the working directory copy. (See the "COMBINED DIFF FORMAT" section
of linkgit:git-diff-files[1] for a details of the format.)
After resolving the conflict in the obvious way (but before updating the
index), the diff will look like:
$ git diff
diff --cc file.txt
index 802992c,2b60207..0000000
--- a/file.txt
+++ b/file.txt
@@@ -1,1 -1,1 +1,1 @@@
- Hello world
++Goodbye world
This shows that our resolved version deleted "Hello world" from the
first parent, deleted "Goodbye" from the second parent, and added
"Goodbye world", which was previously absent from both.
Some special diff options allow diffing the working directory against
any of these stages:
$ git diff -1 file.txt # diff against stage 1
$ git diff --base file.txt # same as the above
$ git diff -2 file.txt # diff against stage 2
$ git diff --ours file.txt # same as the above
$ git diff -3 file.txt # diff against stage 3
$ git diff --theirs file.txt # same as the above.
The linkgit:git-log[1] and linkgit:gitk[1] commands also provide special help
for merges:
$ git log --merge
$ gitk --merge
These will display all commits which exist only on HEAD or on
MERGE_HEAD, and which touch an unmerged file.
You may also use linkgit:git-mergetool[1], which lets you merge the
unmerged files using external tools such as Emacs or kdiff3.
Each time you resolve the conflicts in a file and update the index:
$ git add file.txt
the different stages of that file will be "collapsed", after which
`git diff` will (by default) no longer show diffs for that file.
Undoing a merge
If you get stuck and decide to just give up and throw the whole mess
away, you can always return to the pre-merge state with
$ git reset --hard HEAD
Or, if you've already committed the merge that you want to throw away,
$ git reset --hard ORIG_HEAD
However, this last command can be dangerous in some cases--never
throw away a commit you have already committed if that commit may
itself have been merged into another branch, as doing so may confuse
further merges.
Fast-forward merges
There is one special case not mentioned above, which is treated
differently. Normally, a merge results in a merge commit, with two
parents, one pointing at each of the two lines of development that
were merged.
However, if the current branch is a descendant of the other--so every
commit present in the one is already contained in the other--then git
just performs a "fast-forward"; the head of the current branch is moved
forward to point at the head of the merged-in branch, without any new
commits being created.
Fixing mistakes
If you've messed up the working tree, but haven't yet committed your
mistake, you can return the entire working tree to the last committed
state with
$ git reset --hard HEAD
If you make a commit that you later wish you hadn't, there are two
fundamentally different ways to fix the problem:
1. You can create a new commit that undoes whatever was done
by the old commit. This is the correct thing if your
mistake has already been made public.
2. You can go back and modify the old commit. You should
never do this if you have already made the history public;
git does not normally expect the "history" of a project to
change, and cannot correctly perform repeated merges from
a branch that has had its history changed.
Fixing a mistake with a new commit
Creating a new commit that reverts an earlier change is very easy;
just pass the linkgit:git-revert[1] command a reference to the bad
commit; for example, to revert the most recent commit:
$ git revert HEAD
This will create a new commit which undoes the change in HEAD. You
will be given a chance to edit the commit message for the new commit.
You can also revert an earlier change, for example, the next-to-last:
$ git revert HEAD^
In this case git will attempt to undo the old change while leaving
intact any changes made since then. If more recent changes overlap
with the changes to be reverted, then you will be asked to fix
conflicts manually, just as in the case of <<resolving-a-merge,
resolving a merge>>.
Fixing a mistake by rewriting history
If the problematic commit is the most recent commit, and you have not
yet made that commit public, then you may just
<<undoing-a-merge,destroy it using `git reset`>>.
Alternatively, you
can edit the working directory and update the index to fix your
mistake, just as if you were going to <<how-to-make-a-commit,create a
new commit>>, then run
$ git commit --amend
which will replace the old commit by a new commit incorporating your
changes, giving you a chance to edit the old commit message first.
Again, you should never do this to a commit that may already have
been merged into another branch; use linkgit:git-revert[1] instead in
that case.
It is also possible to replace commits further back in the history, but
this is an advanced topic to be left for
<<cleaning-up-history,another chapter>>.
Checking out an old version of a file
In the process of undoing a previous bad change, you may find it
useful to check out an older version of a particular file using
linkgit:git-checkout[1]. We've used `git checkout` before to switch
branches, but it has quite different behavior if it is given a path
name: the command
$ git checkout HEAD^ path/to/file
replaces path/to/file by the contents it had in the commit HEAD^, and
also updates the index to match. It does not change branches.
If you just want to look at an old version of the file, without
modifying the working directory, you can do that with
$ git show HEAD^:path/to/file
which will display the given version of the file.
Temporarily setting aside work in progress
While you are in the middle of working on something complicated, you
find an unrelated but obvious and trivial bug. You would like to fix it
before continuing. You can use linkgit:git-stash[1] to save the current
state of your work, and after fixing the bug (or, optionally after doing
so on a different branch and then coming back), unstash the
work-in-progress changes.
$ git stash save "work in progress for foo feature"
This command will save your changes away to the `stash`, and
reset your working tree and the index to match the tip of your
current branch. Then you can make your fix as usual.
... edit and test ...
$ git commit -a -m "blorpl: typofix"
After that, you can go back to what you were working on with
`git stash pop`:
$ git stash pop
Ensuring good performance
On large repositories, git depends on compression to keep the history
information from taking up too much space on disk or in memory.
This compression is not performed automatically. Therefore you
should occasionally run linkgit:git-gc[1]:
$ git gc
to recompress the archive. This can be very time-consuming, so
you may prefer to run `git gc` when you are not doing other work.
Ensuring reliability
Checking the repository for corruption
The linkgit:git-fsck[1] command runs a number of self-consistency checks
on the repository, and reports on any problems. This may take some
$ git fsck
dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb
dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f
dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e
dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085
dangling tree b24c2473f1fd3d91352a624795be026d64c8841f
You will see informational messages on dangling objects. They are objects
that still exist in the repository but are no longer referenced by any of
your branches, and can (and will) be removed after a while with "gc".
You can run `git fsck --no-dangling` to suppress these messages, and still
view real errors.
Recovering lost changes
Say you modify a branch with +linkgit:git-reset[1] \--hard+, and then
realize that the branch was the only reference you had to that point in
Fortunately, git also keeps a log, called a "reflog", of all the
previous values of each branch. So in this case you can still find the
old history using, for example,
$ git log master@{1}
This lists the commits reachable from the previous version of the
"master" branch head. This syntax can be used with any git command
that accepts a commit, not just with git log. Some other examples:
$ git show master@{2} # See where the branch pointed 2,
$ git show master@{3} # 3, ... changes ago.
$ gitk master@{yesterday} # See where it pointed yesterday,
$ gitk master@{"1 week ago"} # ... or last week
$ git log --walk-reflogs master # show reflog entries for master
A separate reflog is kept for the HEAD, so
$ git show HEAD@{"1 week ago"}
will show what HEAD pointed to one week ago, not what the current branch
pointed to one week ago. This allows you to see the history of what
you've checked out.
The reflogs are kept by default for 30 days, after which they may be
pruned. See linkgit:git-reflog[1] and linkgit:git-gc[1] to learn
how to control this pruning, and see the "SPECIFYING REVISIONS"
section of linkgit:gitrevisions[7] for details.
Note that the reflog history is very different from normal git history.
While normal history is shared by every repository that works on the
same project, the reflog history is not shared: it tells you only about
how the branches in your local repository have changed over time.
Examining dangling objects
In some situations the reflog may not be able to save you. For example,
suppose you delete a branch, then realize you need the history it
contained. The reflog is also deleted; however, if you have not yet
pruned the repository, then you may still be able to find the lost
commits in the dangling objects that `git fsck` reports. See
<<dangling-objects>> for the details.
$ git fsck
dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
You can examine
one of those dangling commits with, for example,
$ gitk 7281251ddd --not --all
which does what it sounds like: it says that you want to see the commit
history that is described by the dangling commit(s), but not the
history that is described by all your existing branches and tags. Thus
you get exactly the history reachable from that commit that is lost.
(And notice that it might not be just one commit: we only report the
"tip of the line" as being dangling, but there might be a whole deep
and complex commit history that was dropped.)
If you decide you want the history back, you can always create a new
reference pointing to it, for example, a new branch:
$ git branch recovered-branch 7281251ddd
Other types of dangling objects (blobs and trees) are also possible, and
dangling objects can arise in other situations.
Sharing development with others
Getting updates with git pull
After you clone a repository and commit a few changes of your own, you
may wish to check the original repository for updates and merge them
into your own work.
We have already seen <<Updating-a-repository-With-git-fetch,how to
keep remote-tracking branches up to date>> with linkgit:git-fetch[1],
and how to merge two branches. So you can merge in changes from the
original repository's master branch with:
$ git fetch
$ git merge origin/master
However, the linkgit:git-pull[1] command provides a way to do this in
one step:
$ git pull origin master
In fact, if you have "master" checked out, then this branch has been
configured by "git clone" to get changes from the HEAD branch of the
origin repository. So often you can
accomplish the above with just a simple
$ git pull
This command will fetch changes from the remote branches to your
remote-tracking branches `origin/*`, and merge the default branch into
the current branch.
More generally, a branch that is created from a remote-tracking branch
will pull
by default from that branch. See the descriptions of the
branch.<name>.remote and branch.<name>.merge options in
linkgit:git-config[1], and the discussion of the `--track` option in
linkgit:git-checkout[1], to learn how to control these defaults.
In addition to saving you keystrokes, "git pull" also helps you by
producing a default commit message documenting the branch and
repository that you pulled from.
(But note that no such commit will be created in the case of a
<<fast-forwards,fast-forward>>; instead, your branch will just be
updated to point to the latest commit from the upstream branch.)
The `git pull` command can also be given "." as the "remote" repository,
in which case it just merges in a branch from the current repository; so
the commands
$ git pull . branch
$ git merge branch
are roughly equivalent. The former is actually very commonly used.
Submitting patches to a project
If you just have a few changes, the simplest way to submit them may
just be to send them as patches in email:
First, use linkgit:git-format-patch[1]; for example:
$ git format-patch origin
will produce a numbered series of files in the current directory, one
for each patch in the current branch but not in origin/HEAD.
You can then import these into your mail client and send them by
hand. However, if you have a lot to send at once, you may prefer to
use the linkgit:git-send-email[1] script to automate the process.
Consult the mailing list for your project first to determine how they
prefer such patches be handled.
Importing patches to a project
Git also provides a tool called linkgit:git-am[1] (am stands for
"apply mailbox"), for importing such an emailed series of patches.
Just save all of the patch-containing messages, in order, into a
single mailbox file, say "patches.mbox", then run
$ git am -3 patches.mbox
Git will apply each patch in order; if any conflicts are found, it
will stop, and you can fix the conflicts as described in
"<<resolving-a-merge,Resolving a merge>>". (The "-3" option tells
git to perform a merge; if you would prefer it just to abort and
leave your tree and index untouched, you may omit that option.)
Once the index is updated with the results of the conflict
resolution, instead of creating a new commit, just run
$ git am --resolved
and git will create the commit for you and continue applying the
remaining patches from the mailbox.
The final result will be a series of commits, one for each patch in
the original mailbox, with authorship and commit log message each
taken from the message containing each patch.
Public git repositories
Another way to submit changes to a project is to tell the maintainer
of that project to pull the changes from your repository using
linkgit:git-pull[1]. In the section "<<getting-updates-With-git-pull,
Getting updates with `git pull`>>" we described this as a way to get
updates from the "main" repository, but it works just as well in the
other direction.
If you and the maintainer both have accounts on the same machine, then
you can just pull changes from each other's repositories directly;
commands that accept repository URLs as arguments will also accept a
local directory name:
$ git clone /path/to/repository
$ git pull /path/to/other/repository
or an ssh URL:
$ git clone ssh://yourhost/~you/repository
For projects with few developers, or for synchronizing a few private
repositories, this may be all you need.
However, the more common way to do this is to maintain a separate public
repository (usually on a different host) for others to pull changes
from. This is usually more convenient, and allows you to cleanly
separate private work in progress from publicly visible work.
You will continue to do your day-to-day work in your personal
repository, but periodically "push" changes from your personal
repository into your public repository, allowing other developers to
pull from that repository. So the flow of changes, in a situation
where there is one other developer with a public repository, looks
like this:
you push
your personal repo ------------------> your public repo
^ |
| |
| you pull | they pull
| |
| |
| they push V
their public repo <------------------- their repo
We explain how to do this in the following sections.
Setting up a public repository
Assume your personal repository is in the directory ~/proj. We
first create a new clone of the repository and tell `git daemon` that it
is meant to be public:
$ git clone --bare ~/proj proj.git
$ touch proj.git/git-daemon-export-ok
The resulting directory proj.git contains a "bare" git repository--it is
just the contents of the ".git" directory, without any files checked out
around it.
Next, copy proj.git to the server where you plan to host the
public repository. You can use scp, rsync, or whatever is most
Exporting a git repository via the git protocol
This is the preferred method.
If someone else administers the server, they should tell you what
directory to put the repository in, and what git:// URL it will appear
at. You can then skip to the section
"<<pushing-changes-to-a-public-repository,Pushing changes to a public
repository>>", below.
Otherwise, all you need to do is start linkgit:git-daemon[1]; it will
listen on port 9418. By default, it will allow access to any directory
that looks like a git directory and contains the magic file
git-daemon-export-ok. Passing some directory paths as `git daemon`
arguments will further restrict the exports to those paths.
You can also run `git daemon` as an inetd service; see the
linkgit:git-daemon[1] man page for details. (See especially the
examples section.)
Exporting a git repository via http
The git protocol gives better performance and reliability, but on a
host with a web server set up, http exports may be simpler to set up.
All you need to do is place the newly created bare git repository in
a directory that is exported by the web server, and make some
adjustments to give web clients some extra information they need:
$ mv proj.git /home/you/public_html/proj.git
$ cd proj.git
$ git --bare update-server-info
$ mv hooks/post-update.sample hooks/post-update
(For an explanation of the last two lines, see
linkgit:git-update-server-info[1] and linkgit:githooks[5].)
Advertise the URL of proj.git. Anybody else should then be able to
clone or pull from that URL, for example with a command line like:
$ git clone
(See also
for a slightly more sophisticated setup using WebDAV which also
allows pushing over http.)
Pushing changes to a public repository
Note that the two techniques outlined above (exporting via
<<exporting-via-http,http>> or <<exporting-via-git,git>>) allow other
maintainers to fetch your latest changes, but they do not allow write
access, which you will need to update the public repository with the
latest changes created in your private repository.
The simplest way to do this is using linkgit:git-push[1] and ssh; to
update the remote branch named "master" with the latest state of your
branch named "master", run
$ git push ssh:// master:master
or just
$ git push ssh:// master
As with `git fetch`, `git push` will complain if this does not result in a
<<fast-forwards,fast-forward>>; see the following section for details on
handling this case.
Note that the target of a "push" is normally a
<<def_bare_repository,bare>> repository. You can also push to a
repository that has a checked-out working tree, but the working tree
will not be updated by the push. This may lead to unexpected results if
the branch you push to is the currently checked-out branch!
As with `git fetch`, you may also set up configuration options to
save typing; so, for example, after
$ cat >>.git/config <<EOF
[remote "public-repo"]
url = ssh://
you should be able to perform the above push with just
$ git push public-repo master
See the explanations of the remote.<name>.url, branch.<name>.remote,
and remote.<name>.push options in linkgit:git-config[1] for
What to do when a push fails
If a push would not result in a <<fast-forwards,fast-forward>> of the
remote branch, then it will fail with an error like:
error: remote 'refs/heads/master' is not an ancestor of
local 'refs/heads/master'.
Maybe you are not up-to-date and need to pull first?
error: failed to push to 'ssh://'
This can happen, for example, if you:
- use `git reset --hard` to remove already-published commits, or
- use `git commit --amend` to replace already-published commits
(as in <<fixing-a-mistake-by-rewriting-history>>), or
- use `git rebase` to rebase any already-published commits (as
in <<using-git-rebase>>).
You may force `git push` to perform the update anyway by preceding the
branch name with a plus sign:
$ git push ssh:// +master
Normally whenever a branch head in a public repository is modified, it
is modified to point to a descendant of the commit that it pointed to
before. By forcing a push in this situation, you break that convention.
(See <<problems-With-rewriting-history>>.)
Nevertheless, this is a common practice for people that need a simple
way to publish a work-in-progress patch series, and it is an acceptable
compromise as long as you warn other developers that this is how you
intend to manage the branch.
It's also possible for a push to fail in this way when other people have
the right to push to the same repository. In that case, the correct
solution is to retry the push after first updating your work: either by a
pull, or by a fetch followed by a rebase; see the
<<setting-up-a-shared-repository,next section>> and
linkgit:gitcvs-migration[7] for more.
Setting up a shared repository
Another way to collaborate is by using a model similar to that
commonly used in CVS, where several developers with special rights
all push to and pull from a single shared repository. See
linkgit:gitcvs-migration[7] for instructions on how to
set this up.
However, while there is nothing wrong with git's support for shared
repositories, this mode of operation is not generally recommended,
simply because the mode of collaboration that git supports--by
exchanging patches and pulling from public repositories--has so many
advantages over the central shared repository:
- Git's ability to quickly import and merge patches allows a
single maintainer to process incoming changes even at very
high rates. And when that becomes too much, `git pull` provides
an easy way for that maintainer to delegate this job to other
maintainers while still allowing optional review of incoming
- Since every developer's repository has the same complete copy
of the project history, no repository is special, and it is
trivial for another developer to take over maintenance of a
project, either by mutual agreement, or because a maintainer
becomes unresponsive or difficult to work with.
- The lack of a central group of "committers" means there is
less need for formal decisions about who is "in" and who is
Allowing web browsing of a repository
The gitweb cgi script provides users an easy way to browse your
project's files and history without having to install git; see the file
gitweb/INSTALL in the git source tree for instructions on setting it up.
Maintaining topic branches for a Linux subsystem maintainer
This describes how Tony Luck uses git in his role as maintainer of the
IA64 architecture for the Linux kernel.
He uses two public branches:
- A "test" tree into which patches are initially placed so that they
can get some exposure when integrated with other ongoing development.
This tree is available to Andrew for pulling into -mm whenever he
- A "release" tree into which tested patches are moved for final sanity
checking, and as a vehicle to send them upstream to Linus (by sending
him a "please pull" request.)
He also uses a set of temporary branches ("topic branches"), each
containing a logical grouping of patches.
To set this up, first create your work tree by cloning Linus's public
$ git clone git:// work
$ cd work
Linus's tree will be stored in the remote-tracking branch named origin/master,
and can be updated using linkgit:git-fetch[1]; you can track other
public trees using linkgit:git-remote[1] to set up a "remote" and
linkgit:git-fetch[1] to keep them up-to-date; see
Now create the branches in which you are going to work; these start out
at the current tip of origin/master branch, and should be set up (using
the --track option to linkgit:git-branch[1]) to merge changes in from
Linus by default.
$ git branch --track test origin/master
$ git branch --track release origin/master
These can be easily kept up to date using linkgit:git-pull[1].
$ git checkout test && git pull
$ git checkout release && git pull
Important note! If you have any local changes in these branches, then
this merge will create a commit object in the history (with no local
changes git will simply do a "fast-forward" merge). Many people dislike
the "noise" that this creates in the Linux history, so you should avoid
doing this capriciously in the "release" branch, as these noisy commits
will become part of the permanent history when you ask Linus to pull
from the release branch.
A few configuration variables (see linkgit:git-config[1]) can
make it easy to push both branches to your public tree. (See
$ cat >> .git/config <<EOF
[remote "mytree"]
url =
push = release
push = test
Then you can push both the test and release trees using
$ git push mytree
or push just one of the test and release branches using:
$ git push mytree test
$ git push mytree release
Now to apply some patches from the community. Think of a short
snappy name for a branch to hold this patch (or related group of
patches), and create a new branch from a recent stable tag of
Linus's branch. Picking a stable base for your branch will:
1) help you: by avoiding inclusion of unrelated and perhaps lightly
tested changes
2) help future bug hunters that use "git bisect" to find problems
$ git checkout -b speed-up-spinlocks v2.6.35
Now you apply the patch(es), run some tests, and commit the change(s). If
the patch is a multi-part series, then you should apply each as a separate
commit to this branch.
$ ... patch ... test ... commit [ ... patch ... test ... commit ]*
When you are happy with the state of this change, you can pull it into the
"test" branch in preparation to make it public:
$ git checkout test && git pull . speed-up-spinlocks
It is unlikely that you would have any conflicts here ... but you might if you
spent a while on this step and had also pulled new versions from upstream.
Some time later when enough time has passed and testing done, you can pull the
same branch into the "release" tree ready to go upstream. This is where you
see the value of keeping each patch (or patch series) in its own branch. It
means that the patches can be moved into the "release" tree in any order.
$ git checkout release && git pull . speed-up-spinlocks
After a while, you will have a number of branches, and despite the
well chosen names you picked for each of them, you may forget what
they are for, or what status they are in. To get a reminder of what
changes are in a specific branch, use:
$ git log linux..branchname | git shortlog
To see whether it has already been merged into the test or release branches,
$ git log test..branchname
$ git log release..branchname
(If this branch has not yet been merged, you will see some log entries.
If it has been merged, then there will be no output.)
Once a patch completes the great cycle (moving from test to release,
then pulled by Linus, and finally coming back into your local
"origin/master" branch), the branch for this change is no longer needed.
You detect this when the output from:
$ git log origin..branchname
is empty. At this point the branch can be deleted:
$ git branch -d branchname
Some changes are so trivial that it is not necessary to create a separate
branch and then merge into each of the test and release branches. For
these changes, just apply directly to the "release" branch, and then
merge that into the "test" branch.
To create diffstat and shortlog summaries of changes to include in a "please
pull" request to Linus you can use:
$ git diff --stat origin..release
$ git log -p origin..release | git shortlog
Here are some of the scripts that simplify all this even further.
==== update script ====
# Update a branch in my GIT tree. If the branch to be updated
# is origin, then pull from Otherwise merge
# origin/master branch into test|release branch
case "$1" in
git checkout $1 && git pull . origin
before=$(git rev-parse refs/remotes/origin/master)
git fetch origin
after=$(git rev-parse refs/remotes/origin/master)
if [ $before != $after ]
git log $before..$after | git shortlog
echo "Usage: $0 origin|test|release" 1>&2
exit 1
==== merge script ====
# Merge a branch into either the test or release branch
echo "Usage: $pname branch test|release" 1>&2
exit 1
git show-ref -q --verify -- refs/heads/"$1" || {
echo "Can't see branch <$1>" 1>&2
case "$2" in
if [ $(git log $2..$1 | wc -c) -eq 0 ]
echo $1 already merged into $2 1>&2
exit 1
git checkout $2 && git pull . $1
==== status script ====
# report on status of my ia64 GIT tree
gb=$(tput setab 2)
rb=$(tput setab 1)
restore=$(tput setab 9)
if [ `git rev-list test..release | wc -c` -gt 0 ]
echo $rb Warning: commits in release that are not in test $restore
git log test..release
for branch in `git show-ref --heads | sed 's|^.*/||'`
if [ $branch = test -o $branch = release ]
echo -n $gb ======= $branch ====== $restore " "
for ref in test release origin/master
if [ `git rev-list $ref..$branch | wc -c` -gt 0 ]
case $status in
echo $rb Need to pull into test $restore
echo "In test"
echo "Waiting for linus"
echo $rb All done $restore
echo $rb "<$status>" $restore
git log origin/master..$branch | git shortlog
Rewriting history and maintaining patch series
Normally commits are only added to a project, never taken away or
replaced. Git is designed with this assumption, and violating it will
cause git's merge machinery (for example) to do the wrong thing.
However, there is a situation in which it can be useful to violate this
Creating the perfect patch series
Suppose you are a contributor to a large project, and you want to add a
complicated feature, and to present it to the other developers in a way
that makes it easy for them to read your changes, verify that they are
correct, and understand why you made each change.
If you present all of your changes as a single patch (or commit), they
may find that it is too much to digest all at once.
If you present them with the entire history of your work, complete with
mistakes, corrections, and dead ends, they may be overwhelmed.
So the ideal is usually to produce a series of patches such that:
1. Each patch can be applied in order.
2. Each patch includes a single logical change, together with a
message explaining the change.
3. No patch introduces a regression: after applying any initial
part of the series, the resulting project still compiles and
works, and has no bugs that it didn't have before.
4. The complete series produces the same end result as your own
(probably much messier!) development process did.
We will introduce some tools that can help you do this, explain how to
use them, and then explain some of the problems that can arise because
you are rewriting history.
Keeping a patch series up to date using git rebase
Suppose that you create a branch "mywork" on a remote-tracking branch
"origin", and create some commits on top of it:
$ git checkout -b mywork origin
$ vi file.txt
$ git commit
$ vi otherfile.txt
$ git commit
You have performed no merges into mywork, so it is just a simple linear
sequence of patches on top of "origin":
o--o--O <-- origin
a--b--c <-- mywork
Some more interesting work has been done in the upstream project, and
"origin" has advanced:
o--o--O--o--o--o <-- origin
a--b--c <-- mywork
At this point, you could use "pull" to merge your changes back in;
the result would create a new merge commit, like this:
o--o--O--o--o--o <-- origin
\ \
a--b--c--m <-- mywork
However, if you prefer to keep the history in mywork a simple series of
commits without any merges, you may instead choose to use
$ git checkout mywork
$ git rebase origin
This will remove each of your commits from mywork, temporarily saving
them as patches (in a directory named ".git/rebase-apply"), update mywork to
point at the latest version of origin, then apply each of the saved
patches to the new mywork. The result will look like:
o--o--O--o--o--o <-- origin
a'--b'--c' <-- mywork
In the process, it may discover conflicts. In that case it will stop
and allow you to fix the conflicts; after fixing conflicts, use `git add`
to update the index with those contents, and then, instead of
running `git commit`, just run
$ git rebase --continue
and git will continue applying the rest of the patches.
At any point you may use the `--abort` option to abort this process and
return mywork to the state it had before you started the rebase:
$ git rebase --abort
Rewriting a single commit
We saw in <<fixing-a-mistake-by-rewriting-history>> that you can replace the
most recent commit using
$ git commit --amend
which will replace the old commit by a new commit incorporating your
changes, giving you a chance to edit the old commit message first.
You can also use a combination of this and linkgit:git-rebase[1] to
replace a commit further back in your history and recreate the
intervening changes on top of it. First, tag the problematic commit
$ git tag bad mywork~5
(Either gitk or `git log` may be useful for finding the commit.)
Then check out that commit, edit it, and rebase the rest of the series
on top of it (note that we could check out the commit on a temporary
branch, but instead we're using a <<detached-head,detached head>>):
$ git checkout bad
$ # make changes here and update the index
$ git commit --amend
$ git rebase --onto HEAD bad mywork
When you're done, you'll be left with mywork checked out, with the top
patches on mywork reapplied on top of your modified commit. You can
then clean up with
$ git tag -d bad
Note that the immutable nature of git history means that you haven't really
"modified" existing commits; instead, you have replaced the old commits with
new commits having new object names.
Reordering or selecting from a patch series
Given one existing commit, the linkgit:git-cherry-pick[1] command
allows you to apply the change introduced by that commit and create a
new commit that records it. So, for example, if "mywork" points to a
series of patches on top of "origin", you might do something like:
$ git checkout -b mywork-new origin
$ gitk origin..mywork &
and browse through the list of patches in the mywork branch using gitk,
applying them (possibly in a different order) to mywork-new using
cherry-pick, and possibly modifying them as you go using `git commit --amend`.
The linkgit:git-gui[1] command may also help as it allows you to
individually select diff hunks for inclusion in the index (by
right-clicking on the diff hunk and choosing "Stage Hunk for Commit").
Another technique is to use `git format-patch` to create a series of
patches, then reset the state to before the patches:
$ git format-patch origin
$ git reset --hard origin
Then modify, reorder, or eliminate patches as preferred before applying
them again with linkgit:git-am[1].
Other tools
There are numerous other tools, such as StGit, which exist for the
purpose of maintaining a patch series. These are outside of the scope of
this manual.
Problems with rewriting history
The primary problem with rewriting the history of a branch has to do
with merging. Suppose somebody fetches your branch and merges it into
their branch, with a result something like this:
o--o--O--o--o--o <-- origin
\ \
t--t--t--m <-- their branch:
Then suppose you modify the last three commits:
o--o--o <-- new head of origin
o--o--O--o--o--o <-- old head of origin
If we examined all this history together in one repository, it will
look like:
o--o--o <-- new head of origin
o--o--O--o--o--o <-- old head of origin
\ \
t--t--t--m <-- their branch:
Git has no way of knowing that the new head is an updated version of
the old head; it treats this situation exactly the same as it would if
two developers had independently done the work on the old and new heads
in parallel. At this point, if someone attempts to merge the new head
in to their branch, git will attempt to merge together the two (old and
new) lines of development, instead of trying to replace the old by the
new. The results are likely to be unexpected.
You may still choose to publish branches whose history is rewritten,
and it may be useful for others to be able to fetch those branches in
order to examine or test them, but they should not attempt to pull such
branches into their own work.
For true distributed development that supports proper merging,
published branches should never be rewritten.
Why bisecting merge commits can be harder than bisecting linear history
The linkgit:git-bisect[1] command correctly handles history that
includes merge commits. However, when the commit that it finds is a
merge commit, the user may need to work harder than usual to figure out
why that commit introduced a problem.
Imagine this history:
\ /
Suppose that on the upper line of development, the meaning of one
of the functions that exists at Z is changed at commit X. The
commits from Z leading to A change both the function's
implementation and all calling sites that exist at Z, as well
as new calling sites they add, to be consistent. There is no
bug at A.
Suppose that in the meantime on the lower line of development somebody
adds a new calling site for that function at commit Y. The
commits from Z leading to B all assume the old semantics of that
function and the callers and the callee are consistent with each
other. There is no bug at B, either.
Suppose further that the two development lines merge cleanly at C,
so no conflict resolution is required.
Nevertheless, the code at C is broken, because the callers added
on the lower line of development have not been converted to the new
semantics introduced on the upper line of development. So if all
you know is that D is bad, that Z is good, and that
linkgit:git-bisect[1] identifies C as the culprit, how will you
figure out that the problem is due to this change in semantics?
When the result of a `git bisect` is a non-merge commit, you should
normally be able to discover the problem by examining just that commit.
Developers can make this easy by breaking their changes into small
self-contained commits. That won't help in the case above, however,
because the problem isn't obvious from examination of any single
commit; instead, a global view of the development is required. To
make matters worse, the change in semantics in the problematic
function may be just one small part of the changes in the upper
line of development.
On the other hand, if instead of merging at C you had rebased the
history between Z to B on top of A, you would have gotten this
linear history:
Bisecting between Z and D* would hit a single culprit commit Y*,
and understanding why Y* was broken would probably be easier.
Partly for this reason, many experienced git users, even when
working on an otherwise merge-heavy project, keep the history
linear by rebasing against the latest upstream version before
Advanced branch management
Fetching individual branches
Instead of using linkgit:git-remote[1], you can also choose just
to update one branch at a time, and to store it locally under an
arbitrary name:
$ git fetch origin todo:my-todo-work