Краткое руководство по git

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 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. 40MB download):
$ git clone git://git.kernel.org/pub/scm/git/git.git
	# the Linux kernel (approx. 640MB download):
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.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 in the examples above). After you cd into this
directory, you will see that it contains a copy of the project files,
called the 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 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 branches, which may
merge and diverge.

A single Git repository can track development on multiple branches. It
does this by keeping a list of heads which reference the
latest commit on each branch; the git-branch[1] command shows
you the list of branch heads:

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 tags. Tags, like heads, are
references into the project’s history, and can be listed using the
git-tag[1] command:

$ git tag -l
v2.6.11
v2.6.11-tree
v2.6.12
v2.6.12-rc2
v2.6.12-rc3
v2.6.12-rc4
v2.6.12-rc5
v2.6.12-rc6
v2.6.13
...

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 git-switch[1]:

$ git switch -c new v2.6.13

The working directory then reflects the contents that the project had
when it was tagged v2.6.13, and git-branch[1] shows two
branches, with an asterisk marking the currently checked-out branch:

$ git branch
  master
* 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
carefully.

Understanding History: Commits

Every change in the history of a project is represented by a commit.
The git-show[1] command shows the most recent commit on the
current branch:

$ git show
commit 17cf781661e6d38f737f15f53ab552f1e95960d7
Author: Linus Torvalds <torvalds@ppc970.osdl.org.(none)>
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.

         o--o--o <-- Branch A
        /
 o--o--o <-- master
        
         o--o--o <-- Branch B

Manipulating branches

Creating, deleting, and modifying branches is quick and easy; here’s
a summary of the commands:

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 switch command normally expects a branch head, but will also
accept an arbitrary commit when invoked with —detach; for example,
you can check out the commit referenced by a tag:

$ git switch --detach v2.6.17
Note: checking out 'v2.6.17'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another switch.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command again. Example:

  git switch -c 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
427abfa28afedffadfca9dd8b067eb6d36bac53f
$ git branch
* (detached from v2.6.17)
  master

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 git-branch[1]:

$ git branch -r
  origin/HEAD
  origin/html
  origin/maint
  origin/man
  origin/master
  origin/next
  origin/seen
  origin/todo

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 switch -c 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.

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
shorthand:

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
git-pack-refs[1]).

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 gitrevisions[7].

Updating a repository with git fetch

After you clone a repository and commit a few changes of your own, you
may wish to check the original repository for updates.

The git-fetch command, with no arguments, will update all of the
remote-tracking branches to the latest version found in the original
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 git-remote[1]:

$ git remote add staging git://git.kernel.org/.../gregkh/staging.git
$ git fetch staging
...
From git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging
 * [new branch]      master     -> staging/master
 * [new branch]      staging-linus -> staging/staging-linus
 * [new branch]      staging-next -> staging/staging-next

New remote-tracking branches will be stored under the shorthand name
that you gave git remote add, in this case staging:

$ git branch -r
  origin/HEAD -> origin/master
  origin/master
  staging/master
  staging/staging-linus
  staging/staging-next

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 "staging"]
	url = git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git
	fetch = +refs/heads/*:refs/remotes/staging/*
...

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
git-config[1] for details.)

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
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
git-show[1], find out who wrote it, and mail them your bug
report with the commit id. Finally, run

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;
run

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
continue.

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:

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
git-bisect[1] for more information about this and other git
bisect features.

Naming commits

We have seen several ways of naming commits already:

There are many more; see the «SPECIFYING REVISIONS» section of the
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
commits:

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://example.com/proj.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
branch.

The 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
e05db0fd4f31dde7005f075a84f96b360d05984b

Creating tags

We can also create a tag to refer to a particular commit; after
running

$ 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 git-tag[1] man page
for details.

Browsing revisions

The 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:

See the --pretty option in the 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[1]:

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:

Sometimes what you want instead is a set of patches; for this you can
use 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.

Examples

$ git log --pretty=oneline origin..mybranch | wc -l

$ git rev-list origin..mybranch | wc -l

$ git diff origin..master

$ git rev-list origin
e05db0fd4f31dde7005f075a84f96b360d05984b
$ git rev-list master
e05db0fd4f31dde7005f075a84f96b360d05984b

$ git log origin...master

$ git name-rev --tags e05db0fd
e05db0fd tags/v1.5.0-rc1^0~23

$ git describe e05db0fd
v1.5.0-rc0-260-ge05db0f

$ git merge-base e05db0fd v1.5.0-rc1
e05db0fd4f31dde7005f075a84f96b360d05984b

$ git log v1.5.0-rc1..e05db0fd

$ 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
available
 ! [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
...

+ ++ [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
available

$ 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

$ git show-ref --heads | cut -d' ' -f2 | grep -v '^refs/heads/master'
refs/heads/core-tutorial
refs/heads/maint
refs/heads/tutorial-2
refs/heads/tutorial-fixes

$ gitk master --not $( git show-ref --heads | cut -d' ' -f2 |
				grep -v '^refs/heads/master' )

$ gitk $( git show-ref --heads ) --not  $( git show-ref --tags )

$ git archive -o latest.tar.gz --prefix=project/ HEAD

$ git archive --format=tar --prefix=project/ HEAD | gzip >latest.tar.gz

$ release-script 2.6.12 2.6.13-rc6 2.6.13-rc7

#!/bin/sh
stable="$1"
last="$2"
new="$3"
echo "# git tag v$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"

$  git log --raw --abbrev=40 --pretty=oneline |
	grep -B 1 `git hash-object filename`

Telling Git your name

Before creating any commits, you should introduce yourself to Git.
The easiest way to do so is to use git-config[1]:

$ git config --global user.name 'Your Name Comes Here'
$ git config --global user.email 'you@yourdomain.example.com'

Which will add the following to a file named .gitconfig in your
home directory:

[user]
	name = Your Name Comes Here
	email = you@yourdomain.example.com

See the «CONFIGURATION FILE» section of git-config[1] for
details on the configuration file. The file is plain text, so you can
also edit it with your favorite editor.

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:

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 contents of a new or modified file, use

To remove a file from the index and from the working tree, use

After each step you can verify that

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

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

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

As a special shortcut,

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 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, 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.
foo.txt
# Ignore (generated) html files,
*.html
# except foo.html which is maintained by hand.
!foo.html
# Ignore objects and archives.
*.[oa]

See 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 gitignore[5] for the details.

How to merge

You can rejoin two diverging branches of development using
git-merge[1]:

merges the development in the branch branchname into the current
branch.

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 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-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, git-commit[1] will
fail:

$ git commit
file.txt: needs merge

Also, 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
=======
Goodbye
>>>>>>> 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:

$ 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

$ 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.

$ 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
++Goodbye world

$ 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.

$ git log --merge
$ gitk --merge

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

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 an ancestor of the other—​so every commit
present in the current branch is already contained in the other branch—​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 restore --staged --worktree :/

If you make a commit that you later wish you hadn’t, there are two
fundamentally different ways to fix the problem:

$ git restore --source=HEAD^ path/to/file

$ git show HEAD^:path/to/file

$ git stash push -m "work in progress for foo feature"

... edit and test ...
$ git commit -a -m "blorpl: typofix"

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. Some
Git commands may automatically run git-gc[1], so you don’t
have to worry about running it manually. However, compressing a large
repository may take a while, so you may want to call gc explicitly
to avoid automatic compression kicking in when it is not convenient.

Ensuring reliability

$ 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
...

$ 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

$ git show HEAD@{"1 week ago"}

$ git fsck
dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
...

$ gitk 7281251ddd --not --all

$ git branch recovered-branch 7281251ddd

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 how to
keep remote-tracking branches up to date with 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 git-pull[1] command provides a way to do this in
one step:

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

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
git-config[1], and the discussion of the --track option in
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-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.

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:

$ 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.

git format-patch can include an initial «cover letter». You can insert
commentary on individual patches after the three dash line which
format-patch places after the commit message but before the patch
itself. If you use git notes to track your cover letter material,
git format-patch --notes will include the commit’s notes in a similar
manner.

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 git-send-email[1] script to automate the process.
Consult the mailing list for your project first to determine
their requirements for submitting patches.

Importing patches to a project

Git also provides a tool called 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 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». (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

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
git-pull[1]. In the section «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.

$ git clone --bare ~/proj proj.git
$ touch proj.git/git-daemon-export-ok

$ 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

$ git clone http://yourserver.com/~you/proj.git

$ git push ssh://yourserver.com/~you/proj.git master:master

$ git push ssh://yourserver.com/~you/proj.git master

$ git remote add public-repo ssh://yourserver.com/~you/proj.git

[remote "public-repo"]
	url = yourserver.com:proj.git
	fetch = +refs/heads/*:refs/remotes/example/*

$ git push public-repo master

 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to '...'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

$ git push ssh://yourserver.com/~you/proj.git +master

$ git push -f ssh://yourserver.com/~you/proj.git master

How to get a Git repository with minimal history

A shallow clone, with its truncated
history, is useful when one is interested only in recent history
of a project and getting full history from the upstream is
expensive.

A shallow clone is created by specifying
the git-clone[1] --depth switch. The depth can later be
changed with the git-fetch[1] --depth switch, or full
history restored with --unshallow.

Merging inside a shallow clone will work as long
as a merge base is in the recent history.
Otherwise, it will be like merging unrelated histories and may
have to result in huge conflicts. This limitation may make such
a repository unsuitable to be used in merge based workflows.

Examples

$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git work
$ cd work

$ git branch --track test origin/master
$ git branch --track release origin/master

$ git switch test && git pull
$ git switch release && git pull

$ cat >> .git/config <<EOF
[remote "mytree"]
	url =  master.kernel.org:/pub/scm/linux/kernel/git/aegl/linux.git
	push = release
	push = test
EOF

$ git push mytree release

$ git switch -c speed-up-spinlocks v2.6.35

$ ... patch ... test  ... commit [ ... patch ... test ... commit ]*

$ git switch test && git merge speed-up-spinlocks

$ git switch release && git merge speed-up-spinlocks

$ git log linux..branchname | git shortlog

$ git log test..branchname

$ git log release..branchname

$ git log origin..branchname

$ git branch -d branchname

$ git push mytree
$ git request-pull origin mytree release

==== update script ====
# Update a branch in my Git tree.  If the branch to be updated
# is origin, then pull from kernel.org.  Otherwise merge
# origin/master branch into test|release branch

case "$1" in
test|release)
	git checkout $1 && git pull . origin
	;;
origin)
	before=$(git rev-parse refs/remotes/origin/master)
	git fetch origin
	after=$(git rev-parse refs/remotes/origin/master)
	if [ $before != $after ]
	then
		git log $before..$after | git shortlog
	fi
	;;
*)
	echo "usage: $0 origin|test|release" 1>&2
	exit 1
	;;
esac

==== merge script ====
# Merge a branch into either the test or release branch

pname=$0

usage()
{
	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
	usage
}

case "$2" in
test|release)
	if [ $(git log $2..$1 | wc -c) -eq 0 ]
	then
		echo $1 already merged into $2 1>&2
		exit 1
	fi
	git checkout $2 && git pull . $1
	;;
*)
	usage
	;;
esac

==== 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 ]
then
	echo $rb Warning: commits in release that are not in test $restore
	git log test..release
fi

for branch in `git show-ref --heads | sed 's|^.*/||'`
do
	if [ $branch = test -o $branch = release ]
	then
		continue
	fi

	echo -n $gb ======= $branch ====== $restore " "
	status=
	for ref in test release origin/master
	do
		if [ `git rev-list $ref..$branch | wc -c` -gt 0 ]
		then
			status=$status${ref:0:1}
		fi
	done
	case $status in
	trl)
		echo $rb Need to pull into test $restore
		;;
	rl)
		echo "In test"
		;;
	l)
		echo "Waiting for linus"
		;;
	"")
		echo $rb All done $restore
		;;
	*)
		echo $rb "<$status>" $restore
		;;
	esac
	git log origin/master..$branch | git shortlog
done

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:

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 switch -c 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-rebase[1]:

$ git switch 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

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:

If you need to reorder or edit a number of commits in a branch, it may
be easier to use git rebase -i, which allows you to reorder and
squash commits, as well as marking them for individual editing during
the rebase. See Using interactive rebases for details, and
Reordering or selecting from a patch series for alternatives.

Rewriting a single commit

We saw in Fixing a mistake by rewriting history that you can replace the
most recent commit using

which will replace the old commit by a new commit incorporating your
changes, giving you a chance to edit the old commit message first.
This is useful for fixing typos in your last commit, or for adjusting
the patch contents of a poorly staged commit.

If you need to amend commits from deeper in your history, you can
use interactive rebase’s edit instruction.

Reordering or selecting from a patch series

Sometimes you want to edit a commit deeper in your history. One
approach is to use git format-patch to create a series of patches
and then reset the state to before the patches:

$ git format-patch origin
$ git reset --hard origin

Then modify, reorder, or eliminate patches as needed before applying
them again with git-am[1]:

Using interactive rebases

You can also edit a patch series with an interactive rebase. This is
the same as reordering a patch series using
format-patch, so use whichever interface you like best.

Rebase your current HEAD on the last commit you want to retain as-is.
For example, if you want to reorder the last 5 commits, use:

This will open your editor with a list of steps to be taken to perform
your rebase.

pick deadbee The oneline of this commit
pick fa1afe1 The oneline of the next commit
...

# Rebase c0ffeee..deadbee onto c0ffeee
#
# Commands:
#  p, pick = use commit
#  r, reword = use commit, but edit the commit message
#  e, edit = use commit, but stop for amending
#  s, squash = use commit, but meld into previous commit
#  f, fixup = like "squash", but discard this commit's log message
#  x, exec = run command (the rest of the line) using shell
#
# These lines can be re-ordered; they are executed from top to bottom.
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
# However, if you remove everything, the rebase will be aborted.
#
# Note that empty commits are commented out

As explained in the comments, you can reorder commits, squash them
together, edit commit messages, etc. by editing the list. Once you
are satisfied, save the list and close your editor, and the rebase
will begin.

The rebase will stop where pick has been replaced with edit or
when a step in the list fails to mechanically resolve conflicts and
needs your help. When you are done editing and/or resolving conflicts
you can continue with git rebase --continue. If you decide that
things are getting too hairy, you can always bail out with git rebase
--abort. Even after the rebase is complete, you can still recover
the original branch by using the reflog.

For a more detailed discussion of the procedure and additional tips,
see the «INTERACTIVE MODE» section of git-rebase[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 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:

      ---Z---o---X---...---o---A---C---D
                                 /
           o---o---Y---...---o---B

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
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:

    ---Z---o---X--...---o---A---o---o---Y*--...---o---B*--D*

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
publishing.

Fetching individual branches

Instead of using 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

The first argument, origin, just tells Git to fetch from the
repository you originally cloned from. The second argument tells Git
to fetch the branch named todo from the remote repository, and to
store it locally under the name refs/heads/my-todo-work.

You can also fetch branches from other repositories; so

$ git fetch git://example.com/proj.git master:example-master

will create a new branch named example-master and store in it the
branch named master from the repository at the given URL. If you
already have a branch named example-master, it will attempt to
fast-forward to the commit given by example.com’s
master branch. In more detail:

git fetch and fast-forwards

In the previous example, when updating an existing branch, git fetch
checks to make sure that the most recent commit on the remote
branch is a descendant of the most recent commit on your copy of the
branch before updating your copy of the branch to point at the new
commit. Git calls this process a fast-forward.

A fast-forward looks something like this:

 o--o--o--o <-- old head of the branch
           
            o--o--o <-- new head of the branch

In some cases it is possible that the new head will not actually be
a descendant of the old head. For example, the developer may have
realized a serious mistake was made and decided to backtrack,
resulting in a situation like:

 o--o--o--o--a--b <-- old head of the branch
           
            o--o--o <-- new head of the branch

In this case, git fetch will fail, and print out a warning.

In that case, you can still force Git to update to the new head, as
described in the following section. However, note that in the
situation above this may mean losing the commits labeled a and b,
unless you’ve already created a reference of your own pointing to
them.

Forcing git fetch to do non-fast-forward updates

If git fetch fails because the new head of a branch is not a
descendant of the old head, you may force the update with:

$ git fetch git://example.com/proj.git +master:refs/remotes/example/master

Note the addition of the + sign. Alternatively, you can use the -f
flag to force updates of all the fetched branches, as in:

Be aware that commits that the old version of example/master pointed at
may be lost, as we saw in the previous section.

Configuring remote-tracking branches

We saw above that origin is just a shortcut to refer to the
repository that you originally cloned from. This information is
stored in Git configuration variables, which you can see using
git-config[1]:

$ git config -l
core.repositoryformatversion=0
core.filemode=true
core.logallrefupdates=true
remote.origin.url=git://git.kernel.org/pub/scm/git/git.git
remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
branch.master.remote=origin
branch.master.merge=refs/heads/master

If there are other repositories that you also use frequently, you can
create similar configuration options to save typing; for example,

$ git remote add example git://example.com/proj.git

adds the following to .git/config:

[remote "example"]
	url = git://example.com/proj.git
	fetch = +refs/heads/*:refs/remotes/example/*

Also note that the above configuration can be performed by directly
editing the file .git/config instead of using git-remote[1].

After configuring the remote, the following three commands will do the
same thing:

$ git fetch git://example.com/proj.git +refs/heads/*:refs/remotes/example/*
$ git fetch example +refs/heads/*:refs/remotes/example/*
$ git fetch example

See git-config[1] for more details on the configuration
options mentioned above and git-fetch[1] for more details on
the refspec syntax.

The Object Database

We already saw in Understanding History: Commits that all commits are stored
under a 40-digit «object name». In fact, all the information needed to
represent the history of a project is stored in objects with such names.
In each case the name is calculated by taking the SHA-1 hash of the
contents of the object. The SHA-1 hash is a cryptographic hash function.
What that means to us is that it is impossible to find two different
objects with the same name. This has a number of advantages; among
others:

(See Object storage format for the details of the object formatting and
SHA-1 calculation.)

There are four different types of objects: «blob», «tree», «commit», and
«tag».

The object types in some more detail:

$ git show -s --pretty=raw 2be7fcb476
commit 2be7fcb4764f2dbcee52635b91fedb1b3dcf7ab4
tree fb3a8bdd0ceddd019615af4d57a53f43d8cee2bf
parent 257a84d9d02e90447b149af58b271c19405edb6a
author Dave Watson <dwatson@mimvista.com> 1187576872 -0400
committer Junio C Hamano <gitster@pobox.com> 1187591163 -0700

    Fix misspelling of 'suppress' in docs

    Signed-off-by: Junio C Hamano <gitster@pobox.com>

$ git ls-tree fb3a8bdd0ce
100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c    .gitignore
100644 blob 5529b198e8d14decbe4ad99db3f7fb632de0439d    .mailmap
100644 blob 6ff87c4664981e4397625791c8ea3bbb5f2279a3    COPYING
040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745    Documentation
100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200    GIT-VERSION-GEN
100644 blob 289b046a443c0647624607d471289b2c7dcd470b    INSTALL
100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1    Makefile
100644 blob 548142c327a6790ff8821d67c2ee1eff7a656b52    README
...

$ git show 6ff87c4664

 Note that the only valid version of the GPL as far as this project
 is concerned is _this_ particular version of the license (ie v2, not
 v2.2 or v3.x or whatever), unless explicitly otherwise stated.
...

$ git cat-file tag v1.5.0
object 437b1b20df4b356c9342dac8d38849f24ef44f27
type commit
tag v1.5.0
tagger Junio C Hamano <junkio@cox.net> 1171411200 +0000

GIT 1.5.0
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.6 (GNU/Linux)

iD8DBQBF0lGqwMbZpPMRm5oRAuRiAJ9ohBLd7s2kqjkKlq1qqC57SbnmzQCdG4ui
nLE/L9aUXdWeTFPron96DLA=
=2E+0
-----END PGP SIGNATURE-----

$ git count-objects
6930 objects, 47620 kilobytes

$ git repack
Counting objects: 6020, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (6020/6020), done.
Writing objects: 100% (6020/6020), done.
Total 6020 (delta 4070), reused 0 (delta 0)

$ git count-objects
0 objects, 0 kilobytes

$ gitk <dangling-commit-sha-goes-here> --not --all

$ git branch recovered-branch <dangling-commit-sha-goes-here>

$ git show <dangling-blob/tree-sha-goes-here>

$ git fsck --full --no-dangling
broken link from    tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
              to    blob 4b9458b3786228369c63936db65827de3cc06200
missing blob 4b9458b3786228369c63936db65827de3cc06200

$ git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8	.gitignore
100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883	.mailmap
100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c	COPYING
...
100644 blob 4b9458b3786228369c63936db65827de3cc06200	myfile
...

$ git hash-object -w somedirectory/myfile

$ git log --raw --all --full-history -- somedirectory/myfile

commit abc
Author:
Date:
...
:100644 100644 4b9458b newsha M somedirectory/myfile


commit xyz
Author:
Date:

...
:100644 100644 oldsha 4b9458b M somedirectory/myfile

$ git hash-object -w <recreated-file>

The index

The index is a binary file (generally kept in .git/index) containing a
sorted list of path names, each with permissions and the SHA-1 of a blob
object; git-ls-files[1] can show you the contents of the index:

$ git ls-files --stage
100644 63c918c667fa005ff12ad89437f2fdc80926e21c 0	.gitignore
100644 5529b198e8d14decbe4ad99db3f7fb632de0439d 0	.mailmap
100644 6ff87c4664981e4397625791c8ea3bbb5f2279a3 0	COPYING
100644 a37b2152bd26be2c2289e1f57a292534a51a93c7 0	Documentation/.gitignore
100644 fbefe9a45b00a54b58d94d06eca48b03d40a50e0 0	Documentation/Makefile
...
100644 2511aef8d89ab52be5ec6a5e46236b4b6bcd07ea 0	xdiff/xtypes.h
100644 2ade97b2574a9f77e7ae4002a4e07a6a38e46d07 0	xdiff/xutils.c
100644 d5de8292e05e7c36c4b68857c1cf9855e3d2f70a 0	xdiff/xutils.h

Note that in older documentation you may see the index called the
«current directory cache» or just the «cache». It has three important
properties:

The index is thus a sort of temporary staging area, which is filled with
a tree which you are in the process of working on.

If you blow the index away entirely, you generally haven’t lost any
information as long as you have the name of the tree that it described.

$ mkdir ~/git
$ cd ~/git
$ for i in a b c d
do
	mkdir $i
	cd $i
	git init
	echo "module $i" > $i.txt
	git add $i.txt
	git commit -m "Initial commit, submodule $i"
	cd ..
done

$ mkdir super
$ cd super
$ git init
$ for i in a b c d
do
	git submodule add ~/git/$i $i
done

$ ls -a
.  ..  .git  .gitmodules  a  b  c  d

• It clones the submodule from <repo> to the given <path> under the
  current directory and by default checks out the master branch.

• It adds the submodule’s clone path to the gitmodules[5] file and
  adds this file to the index, ready to be committed.

• It adds the submodule’s current commit ID to the index, ready to be
  committed.

$ git commit -m "Add submodules a, b, c and d."

$ cd ..
$ git clone super cloned
$ cd cloned

$ ls -a a
.  ..
$ git submodule status
-d266b9873ad50488163457f025db7cdd9683d88b a
-e81d457da15309b4fef4249aba9b50187999670d b
-c1536a972b9affea0f16e0680ba87332dc059146 c
-d96249ff5d57de5de093e6baff9e0aafa5276a74 d

$ git submodule update
$ cd a
$ ls -a
.  ..  .git  a.txt

$ git branch
* (detached from d266b98)
  master

$ echo "adding a line again" >> a.txt
$ git commit -a -m "Updated the submodule from within the superproject."
$ git push
$ cd ..
$ git diff
diff --git a/a b/a
index d266b98..261dfac 160000
--- a/a
+++ b/a
@@ -1 +1 @@
-Subproject commit d266b9873ad50488163457f025db7cdd9683d88b
+Subproject commit 261dfac35cb99d380eb966e102c1197139f7fa24
$ git add a
$ git commit -m "Updated submodule a."
$ git push

Pitfalls with submodules

Always publish the submodule change before publishing the change to the
superproject that references it. If you forget to publish the submodule change,
others won’t be able to clone the repository:

$ cd ~/git/super/a
$ echo i added another line to this file >> a.txt
$ git commit -a -m "doing it wrong this time"
$ cd ..
$ git add a
$ git commit -m "Updated submodule a again."
$ git push
$ cd ~/git/cloned
$ git pull
$ git submodule update
error: pathspec '261dfac35cb99d380eb966e102c1197139f7fa24' did not match any file(s) known to git.
Did you forget to 'git add'?
Unable to checkout '261dfac35cb99d380eb966e102c1197139f7fa24' in submodule path 'a'

In older Git versions it could be easily forgotten to commit new or modified
files in a submodule, which silently leads to similar problems as not pushing
the submodule changes. Starting with Git 1.7.0 both git status and git diff
in the superproject show submodules as modified when they contain new or
modified files to protect against accidentally committing such a state. git
diff will also add a -dirty to the work tree side when generating patch
output or used with the --submodule option:

$ git diff
diff --git a/sub b/sub
--- a/sub
+++ b/sub
@@ -1 +1 @@
-Subproject commit 3f356705649b5d566d97ff843cf193359229a453
+Subproject commit 3f356705649b5d566d97ff843cf193359229a453-dirty
$ git diff --submodule
Submodule sub 3f35670..3f35670-dirty:

You also should not rewind branches in a submodule beyond commits that were
ever recorded in any superproject.

It’s not safe to run git submodule update if you’ve made and committed
changes within a submodule without checking out a branch first. They will be
silently overwritten:

$ cat a.txt
module a
$ echo line added from private2 >> a.txt
$ git commit -a -m "line added inside private2"
$ cd ..
$ git submodule update
Submodule path 'a': checked out 'd266b9873ad50488163457f025db7cdd9683d88b'
$ cd a
$ cat a.txt
module a

If you have uncommitted changes in your submodule working tree, git
submodule update will not overwrite them. Instead, you get the usual
warning about not being able switch from a dirty branch.

Object access and manipulation

The git-cat-file[1] command can show the contents of any object,
though the higher-level git-show[1] is usually more useful.

The git-commit-tree[1] command allows constructing commits with
arbitrary parents and trees.

The Workflow

High-level operations such as git-commit[1] and
git-restore[1] work by moving data
between the working tree, the index, and the object database. Git
provides low-level operations which perform each of these steps
individually.

Generally, all Git operations work on the index file. Some operations
work purely on the index file (showing the current state of the
index), but most operations move data between the index file and either
the database or the working directory. Thus there are four main
combinations:

$ git update-index filename

$ git read-tree <SHA-1 of tree>

$ git checkout-index filename

$ git commit-tree <tree> -p <parent> [(-p <parent2>)...]

                     commit-tree
                      commit obj
                       +----+
                       |    |
                       |    |
                       V    V
                    +-----------+
                    | Object DB |
                    |  Backing  |
                    |   Store   |
                    +-----------+
                       ^
           write-tree  |     |
             tree obj  |     |
                       |     |  read-tree
                       |     |  tree obj
                             V
                    +-----------+
                    |   Index   |
                    |  "cache"  |
                    +-----------+
         update-index  ^
             blob obj  |     |
                       |     |
    checkout-index -u  |     |  checkout-index
             stat      |     |  blob obj
                             V
                    +-----------+
                    |  Working  |
                    | Directory |
                    +-----------+

Examining the data

You can examine the data represented in the object database and the
index with various helper tools. For every object, you can use
git-cat-file[1] to examine details about the
object:

$ git cat-file -t <objectname>

shows the type of the object, and once you have the type (which is
usually implicit in where you find the object), you can use

$ git cat-file blob|tree|commit|tag <objectname>

to show its contents. NOTE! Trees have binary content, and as a result
there is a special helper for showing that content, called
git ls-tree, which turns the binary content into a more easily
readable form.

It’s especially instructive to look at «commit» objects, since those
tend to be small and fairly self-explanatory. In particular, if you
follow the convention of having the top commit name in .git/HEAD,
you can do

$ git cat-file commit HEAD

to see what the top commit was.

Merging multiple trees

Git can help you perform a three-way merge, which can in turn be
used for a many-way merge by repeating the merge procedure several
times. The usual situation is that you only do one three-way merge
(reconciling two lines of history) and commit the result, but if
you like to, you can merge several branches in one go.

To perform a three-way merge, you start with the two commits you
want to merge, find their closest common parent (a third commit),
and compare the trees corresponding to these three commits.

To get the «base» for the merge, look up the common parent of two
commits:

$ git merge-base <commit1> <commit2>

This prints the name of a commit they are both based on. You should
now look up the tree objects of those commits, which you can easily
do with

$ git cat-file commit <commitname> | head -1

since the tree object information is always the first line in a commit
object.

Once you know the three trees you are going to merge (the one «original»
tree, aka the common tree, and the two «result» trees, aka the branches
you want to merge), you do a «merge» read into the index. This will
complain if it has to throw away your old index contents, so you should
make sure that you’ve committed those—​in fact you would normally
always do a merge against your last commit (which should thus match what
you have in your current index anyway).

To do the merge, do

$ git read-tree -m -u <origtree> <yourtree> <targettree>

which will do all trivial merge operations for you directly in the
index file, and you can just write the result out with
git write-tree.

Merging multiple trees, continued

Sadly, many merges aren’t trivial. If there are files that have
been added, moved or removed, or if both branches have modified the
same file, you will be left with an index tree that contains «merge
entries» in it. Such an index tree can NOT be written out to a tree
object, and you will have to resolve any such merge clashes using
other tools before you can write out the result.

You can examine such index state with git ls-files --unmerged
command. An example:

$ git read-tree -m $orig HEAD $target
$ git ls-files --unmerged
100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1	hello.c
100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2	hello.c
100644 cc44c73eb783565da5831b4d820c962954019b69 3	hello.c

Each line of the git ls-files --unmerged output begins with
the blob mode bits, blob SHA-1, stage number, and the
filename. The stage number is Git’s way to say which tree it
came from: stage 1 corresponds to the $orig tree, stage 2 to
the HEAD tree, and stage 3 to the $target tree.

Earlier we said that trivial merges are done inside
git read-tree -m. For example, if the file did not change
from $orig to HEAD or $target, or if the file changed
from $orig to HEAD and $orig to $target the same way,
obviously the final outcome is what is in HEAD. What the
above example shows is that file hello.c was changed from
$orig to HEAD and $orig to $target in a different way.
You could resolve this by running your favorite 3-way merge
program, e.g. diff3, merge, or Git’s own merge-file, on
the blob objects from these three stages yourself, like this:

$ git cat-file blob 263414f >hello.c~1
$ git cat-file blob 06fa6a2 >hello.c~2
$ git cat-file blob cc44c73 >hello.c~3
$ git merge-file hello.c~2 hello.c~1 hello.c~3

This would leave the merge result in hello.c~2 file, along
with conflict markers if there are conflicts. After verifying
the merge result makes sense, you can tell Git what the final
merge result for this file is by:

$ mv -f hello.c~2 hello.c
$ git update-index hello.c

When a path is in the «unmerged» state, running git update-index for
that path tells Git to mark the path resolved.

The above is the description of a Git merge at the lowest level,
to help you understand what conceptually happens under the hood.
In practice, nobody, not even Git itself, runs git cat-file three times
for this. There is a git merge-index program that extracts the
stages to temporary files and calls a «merge» script on it:

$ git merge-index git-merge-one-file hello.c

and that is what higher level git merge -s resolve is implemented with.

Object storage format

All objects have a statically determined «type» which identifies the
format of the object (i.e. how it is used, and how it can refer to other
objects). There are currently four different object types: «blob»,
«tree», «commit», and «tag».

Regardless of object type, all objects share the following
characteristics: they are all deflated with zlib, and have a header
that not only specifies their type, but also provides size information
about the data in the object. It’s worth noting that the SHA-1 hash
that is used to name the object is the hash of the original data
plus this header, so sha1sum file does not match the object name
for file.

As a result, the general consistency of an object can always be tested
independently of the contents or the type of the object: all objects can
be validated by verifying that (a) their hashes match the content of the
file and (b) the object successfully inflates to a stream of bytes that
forms a sequence of
<ascii type without space> + <space> + <ascii decimal size> +
<byte> + <binary object data>.

The structured objects can further have their structure and
connectivity to other objects verified. This is generally done with
the git fsck program, which generates a full dependency graph
of all objects, and verifies their internal consistency (in addition
to just verifying their superficial consistency through the hash).

A birds-eye view of Git’s source code

It is not always easy for new developers to find their way through Git’s
source code. This section gives you a little guidance to show where to
start.

A good place to start is with the contents of the initial commit, with:

$ git switch --detach e83c5163

The initial revision lays the foundation for almost everything Git has
today, but is small enough to read in one sitting.

Note that terminology has changed since that revision. For example, the
README in that revision uses the word «changeset» to describe what we
now call a commit.

Also, we do not call it «cache» any more, but rather «index»; however, the
file is still called cache.h. Remark: Not much reason to change it now,
especially since there is no good single name for it anyway, because it is
basically the header file which is included by all of Git’s C sources.

If you grasp the ideas in that initial commit, you should check out a
more recent version and skim cache.h, object.h and commit.h.

In the early days, Git (in the tradition of UNIX) was a bunch of programs
which were extremely simple, and which you used in scripts, piping the
output of one into another. This turned out to be good for initial
development, since it was easier to test new things. However, recently
many of these parts have become builtins, and some of the core has been
«libified», i.e. put into libgit.a for performance, portability reasons,
and to avoid code duplication.

By now, you know what the index is (and find the corresponding data
structures in cache.h), and that there are just a couple of object types
(blobs, trees, commits and tags) which inherit their common structure from
struct object, which is their first member (and thus, you can cast e.g.
(struct object *)commit to achieve the same as &commit->object, i.e.
get at the object name and flags).

Now is a good point to take a break to let this information sink in.

Next step: get familiar with the object naming. Read Naming commits.
There are quite a few ways to name an object (and not only revisions!).
All of these are handled in sha1_name.c. Just have a quick look at
the function get_sha1(). A lot of the special handling is done by
functions like get_sha1_basic() or the likes.

This is just to get you into the groove for the most libified part of Git:
the revision walker.

Basically, the initial version of git log was a shell script:

$ git-rev-list --pretty $(git-rev-parse --default HEAD "$@") | 
	LESS=-S ${PAGER:-less}

What does this mean?

git rev-list is the original version of the revision walker, which
always printed a list of revisions to stdout. It is still functional,
and needs to, since most new Git commands start out as scripts using
git rev-list.

git rev-parse is not as important any more; it was only used to filter out
options that were relevant for the different plumbing commands that were
called by the script.

Most of what git rev-list did is contained in revision.c and
revision.h. It wraps the options in a struct named rev_info, which
controls how and what revisions are walked, and more.

The original job of git rev-parse is now taken by the function
setup_revisions(), which parses the revisions and the common command-line
options for the revision walker. This information is stored in the struct
rev_info for later consumption. You can do your own command-line option
parsing after calling setup_revisions(). After that, you have to call
prepare_revision_walk() for initialization, and then you can get the
commits one by one with the function get_revision().

If you are interested in more details of the revision walking process,
just have a look at the first implementation of cmd_log(); call
git show v1.3.0~155^2~4 and scroll down to that function (note that you
no longer need to call setup_pager() directly).

Nowadays, git log is a builtin, which means that it is contained in the
command git. The source side of a builtin is

Sometimes, more than one builtin is contained in one source file. For
example, cmd_whatchanged() and cmd_log() both reside in builtin/log.c,
since they share quite a bit of code. In that case, the commands which are
not named like the .c file in which they live have to be listed in
BUILT_INS in the Makefile.

git log looks more complicated in C than it does in the original script,
but that allows for a much greater flexibility and performance.

Here again it is a good point to take a pause.

Lesson three is: study the code. Really, it is the best way to learn about
the organization of Git (after you know the basic concepts).

So, think about something which you are interested in, say, «how can I
access a blob just knowing the object name of it?». The first step is to
find a Git command with which you can do it. In this example, it is either
git show or git cat-file.

For the sake of clarity, let’s stay with git cat-file, because it

So, look into builtin/cat-file.c, search for cmd_cat_file() and look what
it does.

        git_config(git_default_config);
        if (argc != 3)
		usage("git cat-file [-t|-s|-e|-p|<type>] <sha1>");
        if (get_sha1(argv[2], sha1))
                die("Not a valid object name %s", argv[2]);

Let’s skip over the obvious details; the only really interesting part
here is the call to get_sha1(). It tries to interpret argv[2] as an
object name, and if it refers to an object which is present in the current
repository, it writes the resulting SHA-1 into the variable sha1.

Two things are interesting here:

You will see both of these things throughout the code.

Now, for the meat:

        case 0:
                buf = read_object_with_reference(sha1, argv[1], &size, NULL);

This is how you read a blob (actually, not only a blob, but any type of
object). To know how the function read_object_with_reference() actually
works, find the source code for it (something like git grep
read_object_with | grep ":[a-z]" in the Git repository), and read
the source.

To find out how the result can be used, just read on in cmd_cat_file():

        write_or_die(1, buf, size);

Sometimes, you do not know where to look for a feature. In many such cases,
it helps to search through the output of git log, and then git show the
corresponding commit.

Example: If you know that there was some test case for git bundle, but
do not remember where it was (yes, you could git grep bundle t/, but that
does not illustrate the point!):

In the pager (less), just search for «bundle», go a few lines back,
and see that it is in commit 18449ab0. Now just copy this object name,
and paste it into the command line

Voila.

Another example: Find out what to do in order to make some script a
builtin:

$ git log --no-merges --diff-filter=A builtin/*.c

You see, Git is actually the best tool to find out about the source of Git
itself!

alternate object database

Via the alternates mechanism, a repository
can inherit part of its object database
from another object database, which is called an «alternate».

bare repository

A bare repository is normally an appropriately
named directory with a .git suffix that does not
have a locally checked-out copy of any of the files under
revision control. That is, all of the Git
administrative and control files that would normally be present in the
hidden .git sub-directory are directly present in the
repository.git directory instead,
and no other files are present and checked out. Usually publishers of
public repositories make bare repositories available.

blob object

Untyped object, e.g. the contents of a file.

branch

A «branch» is a line of development. The most recent
commit on a branch is referred to as the tip of
that branch. The tip of the branch is referenced by a branch
head, which moves forward as additional development
is done on the branch. A single Git
repository can track an arbitrary number of
branches, but your working tree is
associated with just one of them (the «current» or «checked out»
branch), and HEAD points to that branch.

cache

Obsolete for: index.

chain

A list of objects, where each object in the list contains
a reference to its successor (for example, the successor of a
commit could be one of its parents).

changeset

BitKeeper/cvsps speak for «commit». Since Git does not
store changes, but states, it really does not make sense to use the term
«changesets» with Git.

checkout

The action of updating all or part of the
working tree with a tree object
or blob from the
object database, and updating the
index and HEAD if the whole working tree has
been pointed at a new branch.

cherry-picking

In SCM jargon, «cherry pick» means to choose a subset of
changes out of a series of changes (typically commits) and record them
as a new series of changes on top of a different codebase. In Git, this is
performed by the «git cherry-pick» command to extract the change introduced
by an existing commit and to record it based on the tip
of the current branch as a new commit.

clean

A working tree is clean, if it
corresponds to the revision referenced by the current
head. Also see «dirty».

commit

As a noun: A single point in the
Git history; the entire history of a project is represented as a
set of interrelated commits. The word «commit» is often
used by Git in the same places other revision control systems
use the words «revision» or «version». Also used as a short
hand for commit object.

As a verb: The action of storing a new snapshot of the project’s
state in the Git history, by creating a new commit representing the current
state of the index and advancing HEAD
to point at the new commit.

commit graph concept, representations and usage

A synonym for the DAG structure formed by the commits
in the object database, referenced by branch tips,
using their chain of linked commits.
This structure is the definitive commit graph. The
graph can be represented in other ways, e.g. the
«commit-graph» file.

commit-graph file

The «commit-graph» (normally hyphenated) file is a supplemental
representation of the commit graph
which accelerates commit graph walks. The «commit-graph» file is
stored either in the .git/objects/info directory or in the info
directory of an alternate object database.

commit object

An object which contains the information about a
particular revision, such as parents, committer,
author, date and the tree object which corresponds
to the top directory of the stored
revision.

commit-ish (also committish)

A commit object or an
object that can be recursively dereferenced to
a commit object.
The following are all commit-ishes:
a commit object,
a tag object that points to a commit
object,
a tag object that points to a tag object that points to a
commit object,
etc.

core Git

Fundamental data structures and utilities of Git. Exposes only limited
source code management tools.

DAG

Directed acyclic graph. The commit objects form a
directed acyclic graph, because they have parents (directed), and the
graph of commit objects is acyclic (there is no chain
which begins and ends with the same object).

dangling object

An unreachable object which is not
reachable even from other unreachable objects; a
dangling object has no references to it from any
reference or object in the repository.

detached HEAD

Normally the HEAD stores the name of a
branch, and commands that operate on the
history HEAD represents operate on the history leading to the
tip of the branch the HEAD points at. However, Git also
allows you to check out an arbitrary
commit that isn’t necessarily the tip of any
particular branch. The HEAD in such a state is called
«detached».

Note that commands that operate on the history of the current branch
(e.g. git commit to build a new history on top of it) still work
while the HEAD is detached. They update the HEAD to point at the tip
of the updated history without affecting any branch. Commands that
update or inquire information about the current branch (e.g. git
branch --set-upstream-to that sets what remote-tracking branch the
current branch integrates with) obviously do not work, as there is no
(real) current branch to ask about in this state.

directory

The list you get with «ls»

dirty

A working tree is said to be «dirty» if
it contains modifications which have not been committed to the current
branch.

evil merge

An evil merge is a merge that introduces changes that
do not appear in any parent.

fast-forward

A fast-forward is a special type of merge where you have a
revision and you are «merging» another
branch’s changes that happen to be a descendant of what
you have. In such a case, you do not make a new merge
commit but instead just update your branch to point at the same
revision as the branch you are merging. This will happen frequently on a
remote-tracking branch of a remote
repository.

fetch

Fetching a branch means to get the
branch’s head ref from a remote
repository, to find out which objects are
missing from the local object database,
and to get them, too. See also git-fetch[1].

file system

Linus Torvalds originally designed Git to be a user space file system,
i.e. the infrastructure to hold files and directories. That ensured the
efficiency and speed of Git.

Git archive

Synonym for repository (for arch people).

gitfile

A plain file .git at the root of a working tree that
points at the directory that is the real repository.

grafts

Grafts enables two otherwise different lines of development to be joined
together by recording fake ancestry information for commits. This way
you can make Git pretend the set of parents a commit has
is different from what was recorded when the commit was
created. Configured via the .git/info/grafts file.

Note that the grafts mechanism is outdated and can lead to problems
transferring objects between repositories; see git-replace[1]
for a more flexible and robust system to do the same thing.

hash

In Git’s context, synonym for object name.

head

A named reference to the commit at the tip of a
branch. Heads are stored in a file in
$GIT_DIR/refs/heads/ directory, except when using packed refs. (See
git-pack-refs[1].)

HEAD

The current branch. In more detail: Your working tree is normally derived from the state of the tree
referred to by HEAD. HEAD is a reference to one of the
heads in your repository, except when using a
detached HEAD, in which case it directly
references an arbitrary commit.

head ref

A synonym for head.

hook

During the normal execution of several Git commands, call-outs are made
to optional scripts that allow a developer to add functionality or
checking. Typically, the hooks allow for a command to be pre-verified
and potentially aborted, and allow for a post-notification after the
operation is done. The hook scripts are found in the
$GIT_DIR/hooks/ directory, and are enabled by simply
removing the .sample suffix from the filename. In earlier versions
of Git you had to make them executable.

index

A collection of files with stat information, whose contents are stored
as objects. The index is a stored version of your
working tree. Truth be told, it can also contain a second, and even
a third version of a working tree, which are used
when merging.

index entry

The information regarding a particular file, stored in the
index. An index entry can be unmerged, if a
merge was started, but not yet finished (i.e. if
the index contains multiple versions of that file).

master

The default development branch. Whenever you
create a Git repository, a branch named
«master» is created, and becomes the active branch. In most
cases, this contains the local development, though that is
purely by convention and is not required.

merge

As a verb: To bring the contents of another
branch (possibly from an external
repository) into the current branch. In the
case where the merged-in branch is from a different repository,
this is done by first fetching the remote branch
and then merging the result into the current branch. This
combination of fetch and merge operations is called a
pull. Merging is performed by an automatic process
that identifies changes made since the branches diverged, and
then applies all those changes together. In cases where changes
conflict, manual intervention may be required to complete the
merge.

As a noun: unless it is a fast-forward, a
successful merge results in the creation of a new commit
representing the result of the merge, and having as
parents the tips of the merged branches.
This commit is referred to as a «merge commit», or sometimes just a
«merge».

object

The unit of storage in Git. It is uniquely identified by the
SHA-1 of its contents. Consequently, an
object cannot be changed.

object database

Stores a set of «objects», and an individual object is
identified by its object name. The objects usually
live in $GIT_DIR/objects/.

object identifier (oid)

Synonym for object name.

object name

The unique identifier of an object. The
object name is usually represented by a 40 character
hexadecimal string. Also colloquially called SHA-1.

object type

One of the identifiers «commit»,
«tree», «tag» or
«blob» describing the type of an
object.

octopus

To merge more than two branches.

origin

The default upstream repository. Most projects have
at least one upstream project which they track. By default
origin is used for that purpose. New upstream updates
will be fetched into remote-tracking branches named
origin/name-of-upstream-branch, which you can see using
git branch -r.

overlay

Only update and add files to the working directory, but don’t
delete them, similar to how cp -R would update the contents
in the destination directory. This is the default mode in a
checkout when checking out files from the
index or a tree-ish. In
contrast, no-overlay mode also deletes tracked files not
present in the source, similar to rsync —delete.

pack

A set of objects which have been compressed into one file (to save space
or to transmit them efficiently).

pack index

The list of identifiers, and other information, of the objects in a
pack, to assist in efficiently accessing the contents of a
pack.

pathspec

Pattern used to limit paths in Git commands.

Pathspecs are used on the command line of «git ls-files», «git
ls-tree», «git add», «git grep», «git diff», «git checkout»,
and many other commands to
limit the scope of operations to some subset of the tree or
working tree. See the documentation of each command for whether
paths are relative to the current directory or toplevel. The
pathspec syntax is as follows:

• any path matches itself

• the pathspec up to the last slash represents a
  directory prefix. The scope of that pathspec is
  limited to that subtree.

• the rest of the pathspec is a pattern for the remainder
  of the pathname. Paths relative to the directory
  prefix will be matched against that pattern using fnmatch(3);
  in particular, * and ? can match directory separators.

For example, Documentation/*.jpg will match all .jpg files
in the Documentation subtree,
including Documentation/chapter_1/figure_1.jpg.

A pathspec that begins with a colon : has special meaning. In the
short form, the leading colon : is followed by zero or more «magic
signature» letters (which optionally is terminated by another colon :),
and the remainder is the pattern to match against the path.
The «magic signature» consists of ASCII symbols that are neither
alphanumeric, glob, regex special characters nor colon.
The optional colon that terminates the «magic signature» can be
omitted if the pattern begins with a character that does not belong to
«magic signature» symbol set and is not a colon.

In the long form, the leading colon : is followed by an open
parenthesis (, a comma-separated list of zero or more «magic words»,
and a close parentheses ), and the remainder is the pattern to match
against the path.

A pathspec with only a colon means «there is no pathspec». This form
should not be combined with other pathspec.

top

The magic word top (magic signature: /) makes the pattern
match from the root of the working tree, even when you are
running the command from inside a subdirectory.

literal

Wildcards in the pattern such as * or ? are treated
as literal characters.

icase

Case insensitive match.

glob

Git treats the pattern as a shell glob suitable for
consumption by fnmatch(3) with the FNM_PATHNAME flag:
wildcards in the pattern will not match a / in the pathname.
For example, «Documentation/*.html» matches
«Documentation/git.html» but not «Documentation/ppc/ppc.html»
or «tools/perf/Documentation/perf.html».

Two consecutive asterisks («**«) in patterns matched against
full pathname may have special meaning:

• A leading «**» followed by a slash means match in all
  directories. For example, «**/foo» matches file or directory
  «foo» anywhere, the same as pattern «foo«. «**/foo/bar»
  matches file or directory «bar» anywhere that is directly
  under directory «foo«.

• A trailing «/**» matches everything inside. For example,
  «abc/**» matches all files inside directory «abc», relative
  to the location of the .gitignore file, with infinite depth.

• A slash followed by two consecutive asterisks then a slash
  matches zero or more directories. For example, «a/**/b»
  matches «a/b«, «a/x/b«, «a/x/y/b» and so on.

• Other consecutive asterisks are considered invalid.

  Glob magic is incompatible with literal magic.

attr

After attr: comes a space separated list of «attribute
requirements», all of which must be met in order for the
path to be considered a match; this is in addition to the
usual non-magic pathspec pattern matching.
See gitattributes[5].

Each of the attribute requirements for the path takes one of
these forms:

• «ATTR» requires that the attribute ATTR be set.

• «-ATTR» requires that the attribute ATTR be unset.

• «ATTR=VALUE» requires that the attribute ATTR be
  set to the string VALUE.

• «!ATTR» requires that the attribute ATTR be
  unspecified.

  Note that when matching against a tree object, attributes are still
  obtained from working tree, not from the given tree object.

exclude

After a path matches any non-exclude pathspec, it will be run
through all exclude pathspecs (magic signature: ! or its
synonym ^). If it matches, the path is ignored. When there
is no non-exclude pathspec, the exclusion is applied to the
result set as if invoked without any pathspec.

parent

A commit object contains a (possibly empty) list
of the logical predecessor(s) in the line of development, i.e. its
parents.

pickaxe

The term pickaxe refers to an option to the diffcore
routines that help select changes that add or delete a given text
string. With the --pickaxe-all option, it can be used to view the full
changeset that introduced or removed, say, a
particular line of text. See git-diff[1].

plumbing

Cute name for core Git.

porcelain

Cute name for programs and program suites depending on
core Git, presenting a high level access to
core Git. Porcelains expose more of a SCM
interface than the plumbing.

per-worktree ref

Refs that are per-worktree, rather than
global. This is presently only HEAD and any refs
that start with refs/bisect/, but might later include other
unusual refs.

pseudoref

Pseudorefs are a class of files under $GIT_DIR which behave
like refs for the purposes of rev-parse, but which are treated
specially by git. Pseudorefs both have names that are all-caps,
and always start with a line consisting of a
SHA-1 followed by whitespace. So, HEAD is not a
pseudoref, because it is sometimes a symbolic ref. They might
optionally contain some additional data. MERGE_HEAD and
CHERRY_PICK_HEAD are examples. Unlike
per-worktree refs, these files cannot
be symbolic refs, and never have reflogs. They also cannot be
updated through the normal ref update machinery. Instead,
they are updated by directly writing to the files. However,
they can be read as if they were refs, so git rev-parse
MERGE_HEAD will work.

pull

Pulling a branch means to fetch it and
merge it. See also git-pull[1].

push

Pushing a branch means to get the branch’s
head ref from a remote repository,
find out if it is an ancestor to the branch’s local
head ref, and in that case, putting all
objects, which are reachable from the local
head ref, and which are missing from the remote
repository, into the remote
object database, and updating the remote
head ref. If the remote head is not an
ancestor to the local head, the push fails.

reachable

All of the ancestors of a given commit are said to be
«reachable» from that commit. More
generally, one object is reachable from
another if we can reach the one from the other by a chain
that follows tags to whatever they tag,
commits to their parents or trees, and
trees to the trees or blobs
that they contain.

reachability bitmaps

Reachability bitmaps store information about the
reachability of a selected set of commits in
a packfile, or a multi-pack index (MIDX), to speed up object search.
The bitmaps are stored in a «.bitmap» file. A repository may have at
most one bitmap file in use. The bitmap file may belong to either one
pack, or the repository’s multi-pack index (if it exists).

rebase

To reapply a series of changes from a branch to a
different base, and reset the head of that branch
to the result.

ref

A name that begins with refs/ (e.g. refs/heads/master)
that points to an object name or another
ref (the latter is called a symbolic ref).
For convenience, a ref can sometimes be abbreviated when used
as an argument to a Git command; see gitrevisions[7]
for details.
Refs are stored in the repository.

The ref namespace is hierarchical.
Different subhierarchies are used for different purposes (e.g. the
refs/heads/ hierarchy is used to represent local branches).

There are a few special-purpose refs that do not begin with refs/.
The most notable example is HEAD.

reflog

A reflog shows the local «history» of a ref. In other words,
it can tell you what the 3rd last revision in this repository
was, and what was the current state in this repository,
yesterday 9:14pm. See git-reflog[1] for details.

refspec

A «refspec» is used by fetch and
push to describe the mapping between remote
ref and local ref.

remote repository

A repository which is used to track the same
project but resides somewhere else. To communicate with remotes,
see fetch or push.

remote-tracking branch

A ref that is used to follow changes from another
repository. It typically looks like
refs/remotes/foo/bar (indicating that it tracks a branch named
bar in a remote named foo), and matches the right-hand-side of
a configured fetch refspec. A remote-tracking
branch should not contain direct modifications or have local
commits made to it.

repository

A collection of refs together with an
object database containing all objects
which are reachable from the refs, possibly
accompanied by meta data from one or more porcelains. A
repository can share an object database with other repositories
via alternates mechanism.

resolve

The action of fixing up manually what a failed automatic
merge left behind.

revision

Synonym for commit (the noun).

rewind

To throw away part of the development, i.e. to assign the
head to an earlier revision.

SCM

Source code management (tool).

SHA-1

«Secure Hash Algorithm 1»; a cryptographic hash function.
In the context of Git used as a synonym for object name.

shallow clone

Mostly a synonym to shallow repository
but the phrase makes it more explicit that it was created by
running git clone --depth=... command.

shallow repository

A shallow repository has an incomplete
history some of whose commits have parents cauterized away (in other
words, Git is told to pretend that these commits do not have the
parents, even though they are recorded in the commit
object). This is sometimes useful when you are interested only in the
recent history of a project even though the real history recorded in the
upstream is much larger. A shallow repository
is created by giving the --depth option to git-clone[1], and
its history can be later deepened with git-fetch[1].

stash entry

An object used to temporarily store the contents of a
dirty working directory and the index for future reuse.

submodule

A repository that holds the history of a
separate project inside another repository (the latter of
which is called superproject).

superproject

A repository that references repositories
of other projects in its working tree as submodules.
The superproject knows about the names of (but does not hold
copies of) commit objects of the contained submodules.

symref

Symbolic reference: instead of containing the SHA-1
id itself, it is of the format ref: refs/some/thing and when
referenced, it recursively dereferences to this reference.
HEAD is a prime example of a symref. Symbolic
references are manipulated with the git-symbolic-ref[1]
command.

tag

A ref under refs/tags/ namespace that points to an
object of an arbitrary type (typically a tag points to either a
tag or a commit object).
In contrast to a head, a tag is not updated by
the commit command. A Git tag has nothing to do with a Lisp
tag (which would be called an object type
in Git’s context). A tag is most typically used to mark a particular
point in the commit ancestry chain.

tag object

An object containing a ref pointing to
another object, which can contain a message just like a
commit object. It can also contain a (PGP)
signature, in which case it is called a «signed tag object».

topic branch

A regular Git branch that is used by a developer to
identify a conceptual line of development. Since branches are very easy
and inexpensive, it is often desirable to have several small branches
that each contain very well defined concepts or small incremental yet
related changes.

tree

Either a working tree, or a tree
object together with the dependent blob and tree objects
(i.e. a stored representation of a working tree).

tree object

An object containing a list of file names and modes along
with refs to the associated blob and/or tree objects. A
tree is equivalent to a directory.

tree-ish (also treeish)

A tree object or an object
that can be recursively dereferenced to a tree object.
Dereferencing a commit object yields the
tree object corresponding to the revision’s
top directory.
The following are all tree-ishes:
a commit-ish,
a tree object,
a tag object that points to a tree object,
a tag object that points to a tag object that points to a tree
object,
etc.

unmerged index

An index which contains unmerged
index entries.

unreachable object

An object which is not reachable from a
branch, tag, or any other reference.

upstream branch

The default branch that is merged into the branch in
question (or the branch in question is rebased onto). It is configured
via branch.<name>.remote and branch.<name>.merge. If the upstream branch
of A is origin/B sometimes we say «A is tracking origin/B«.

working tree

The tree of actual checked out files. The working tree normally
contains the contents of the HEAD commit’s tree,
plus any local changes that you have made but not yet committed.

worktree

A repository can have zero (i.e. bare repository) or one or
more worktrees attached to it. One «worktree» consists of a
«working tree» and repository metadata, most of which are
shared among other worktrees of a single repository, and
some of which are maintained separately per worktree
(e.g. the index, HEAD and pseudorefs like MERGE_HEAD,
per-worktree refs and per-worktree configuration file).

Creating a new repository

From a tarball:

$ tar xzf project.tar.gz
$ cd project
$ git init
Initialized empty Git repository in .git/
$ git add .
$ git commit

From a remote repository:

$ git clone git://example.com/pub/project.git
$ cd project

Managing branches

$ git branch			# list all local branches in this repo
$ git switch test	        # switch working directory to branch "test"
$ git branch new		# create branch "new" starting at current HEAD
$ git branch -d new		# delete branch "new"

Instead of basing a new branch on current HEAD (the default), use:

$ git branch new test    # branch named "test"
$ git branch new v2.6.15 # tag named v2.6.15
$ git branch new HEAD^   # commit before the most recent
$ git branch new HEAD^^  # commit before that
$ git branch new test~10 # ten commits before tip of branch "test"

Create and switch to a new branch at the same time:

$ git switch -c new v2.6.15

Update and examine branches from the repository you cloned from:

$ git fetch		# update
$ git branch -r		# list
  origin/master
  origin/next
  ...
$ git switch -c masterwork origin/master

Fetch a branch from a different repository, and give it a new
name in your repository:

$ git fetch git://example.com/project.git theirbranch:mybranch
$ git fetch git://example.com/project.git v2.6.15:mybranch

Keep a list of repositories you work with regularly:

$ git remote add example git://example.com/project.git
$ git remote			# list remote repositories
example
origin
$ git remote show example	# get details
* remote example
  URL: git://example.com/project.git
  Tracked remote branches
    master
    next
    ...
$ git fetch example		# update branches from example
$ git branch -r			# list all remote branches

Exploring history

$ gitk			    # visualize and browse history
$ git log		    # list all commits
$ git log src/		    # ...modifying src/
$ git log v2.6.15..v2.6.16  # ...in v2.6.16, not in v2.6.15
$ git log master..test	    # ...in branch test, not in branch master
$ git log test..master	    # ...in branch master, but not in test
$ git log test...master	    # ...in one branch, not in both
$ git log -S'foo()'	    # ...where difference contain "foo()"
$ git log --since="2 weeks ago"
$ git log -p		    # show patches as well
$ git show		    # most recent commit
$ git diff v2.6.15..v2.6.16 # diff between two tagged versions
$ git diff v2.6.15..HEAD    # diff with current head
$ git grep "foo()"	    # search working directory for "foo()"
$ git grep v2.6.15 "foo()"  # search old tree for "foo()"
$ git show v2.6.15:a.txt    # look at old version of a.txt

Search for regressions:

$ git bisect start
$ git bisect bad		# current version is bad
$ git bisect good v2.6.13-rc2	# last known good revision
Bisecting: 675 revisions left to test after this
				# test here, then:
$ git bisect good		# if this revision is good, or
$ git bisect bad		# if this revision is bad.
				# repeat until done.

Making changes

Make sure Git knows who to blame:

$ cat >>~/.gitconfig <<EOF
[user]
	name = Your Name Comes Here
	email = you@yourdomain.example.com
EOF

Select file contents to include in the next commit, then make the
commit:

$ git add a.txt    # updated file
$ git add b.txt    # new file
$ git rm c.txt     # old file
$ git commit

Or, prepare and create the commit in one step:

$ git commit d.txt # use latest content only of d.txt
$ git commit -a	   # use latest content of all tracked files

Merging

$ git merge test   # merge branch "test" into the current branch
$ git pull git://example.com/project.git master
		   # fetch and merge in remote branch
$ git pull . test  # equivalent to git merge test

Sharing your changes

Importing or exporting patches:

$ git format-patch origin..HEAD # format a patch for each commit
				# in HEAD but not in origin
$ git am mbox # import patches from the mailbox "mbox"

Fetch a branch in a different Git repository, then merge into the
current branch:

$ git pull git://example.com/project.git theirbranch

Store the fetched branch into a local branch before merging into the
current branch:

$ git pull git://example.com/project.git theirbranch:mybranch

After creating commits on a local branch, update the remote
branch with your commits:

$ git push ssh://example.com/project.git mybranch:theirbranch

When remote and local branch are both named «test»:

$ git push ssh://example.com/project.git test

Shortcut version for a frequently used remote repository:

$ git remote add example ssh://example.com/project.git
$ git push example test

Repository maintenance

Check for corruption:

Recompress, remove unused cruft:

• It must be readable in order, from beginning to end, by someone
  intelligent with a basic grasp of the UNIX command line, but without
  any special knowledge of Git. If necessary, any other prerequisites
  should be specifically mentioned as they arise.

• Whenever possible, section headings should clearly describe the task
  they explain how to do, in language that requires no more knowledge
  than necessary: for example, «importing patches into a project» rather
  than «the git am command»

• howto’s

• some of technical/?

• hooks

• list of commands in git[1]