Git User's Manual
_________________

This manual is designed to be readable by someone with basic unix
commandline skills, but no previous knowledge of git.

Chapters 1 and 2 explain how to fetch and study a project using git--the
tools you'd need to build and test a particular version of a software
project, to search for regressions, and so on.

Chapter 3 explains how to do development with git and share your progress
with others.

Further chapters cover more specialized topics.

Comprehensive reference documentation is available through the man
pages.  For a command such as "git clone", just use

------------------------------------------------
$ man git-clone
------------------------------------------------

Repositories and Branches
=========================

How to get a git repository
---------------------------

It will be useful to have a git repository to experiment with as you
read this manual.

The best way to get one is by using the gitlink:git-clone[1] command
to download a copy of an existing repository for a project that you
are interested in.  If you don't already have a project in mind, here
are some interesting examples:

------------------------------------------------
        # git itself (approx. 10MB download):
$ git clone git://git.kernel.org/pub/scm/git/git.git
        # the linux kernel (approx. 150MB download):
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git
------------------------------------------------

The initial clone may be time-consuming for a large project, but you
will only need to clone once.

The clone command creates a new directory named after the project
("git" or "linux-2.6" in the examples above).  After you cd into this
directory, you will see that it contains a copy of the project files,
together with a special top-level directory named ".git", which
contains all the information about the history of the project.

In the following, examples will be taken from one of the two
repositories above.

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 (versions) of the project's
contents.

A single git repository may contain multiple branches.  Each branch
is a bookmark referencing a particular point in the project history.
The gitlink:git-branch[1] command shows you the list of branches:

------------------------------------------------
$ git branch
* master
------------------------------------------------

A freshly cloned repository contains a single branch, named "master",
and the working directory contains the version of the project
referred to by the master branch.

Most projects also use tags.  Tags, like branches, are references
into the project's history, and can be listed using the
gitlink: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
...
------------------------------------------------

Create a new branch pointing to one of these versions and check it
out using gitlink:git-checkout[1]:

------------------------------------------------
$ git checkout -b new v2.6.13
------------------------------------------------

The working directory then reflects the contents that the project had
when it was tagged v2.6.13, and gitlink: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 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 gitlink:git-show[1] command shows the most recent commit on the
current branch:

------------------------------------------------
$ git show
commit 2b5f6dcce5bf94b9b119e9ed8d537098ec61c3d2
Author: Jamal Hadi Salim <hadi@cyberus.ca>
Date:   Sat Dec 2 22:22:25 2006 -0800

    [XFRM]: Fix aevent structuring to be more complete.
    
    aevents can not uniquely identify an SA. We break the ABI with this
    patch, but consensus is that since it is not yet utilized by any
    (known) application then it is fine (better do it now than later).
    
    Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
    Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/Documentation/networking/xfrm_sync.txt b/Documentation/networking/xfrm_sync.txt
index 8be626f..d7aac9d 100644
--- a/Documentation/networking/xfrm_sync.txt
+++ b/Documentation/networking/xfrm_sync.txt
@@ -47,10 +47,13 @@ aevent_id structure looks like:
 
    struct xfrm_aevent_id {
              struct xfrm_usersa_id           sa_id;
+             xfrm_address_t                  saddr;
              __u32                           flags;
+             __u32                           reqid;
    };
...
------------------------------------------------

As you can see, a commit shows who made the latest change, what they
did, and why.

Every commit has a 20-digit id, sometimes called the "SHA1 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 id can also be useful.  In particular, it is a globally unique
name for this commit: so if you tell somebody else the SHA1 id (for
example in email), then you are guaranteed they will see the same
commit in their repository that you do in yours.

Understanding history: commits, parents, and reachability
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Every commit (except the very first commit in a project) also has a
parent commit which shows what happened before this commit.
Following the chain of parents will eventually take you back to the
beginning of the project.

However, the commits do not form a simple list; git allows lines of
development to diverge and then reconverge, and the point where two
lines of development reconverge is called a "merge".  The commit
representing a merge can therefore have more than one parent, with
each parent representing the most recent commit on one of the lines
of development leading to that point.

The best way to see how this works is using the gitlink:gitk[1]
command; running gitk now on a git repository and looking for merge
commits will help understand how the git organizes history.

In the following, we say that commit X is "reachable" from commit Y
if commit X is an ancestor of commit Y.  Equivalently, you could say
that Y is a descendent of X, or that there is a chain of parents
leading from commit Y to commit X.

Undestanding history: History diagrams
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We will sometimes represent git history using diagrams like the one
below.  Commits are shown as "o", and the links between them with
lines drawn with - / and \.  Time goes left to right:

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

If we need to talk about a particular commit, the character "o" may
be replaced with another letter or number.

Understanding history: What is a branch?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Though we've been using the word "branch" to mean a kind of reference
to a particular commit, the word branch is also commonly used to
refer to the line of commits leading up to that point.  In the
example above, git may think of the branch named "A" as just a
pointer to one particular commit, but we may refer informally to the
line of three commits leading up to that point as all being part of
"branch A".

If we need to make it clear that we're just talking about the most
recent commit on the branch, we may refer to that commit as the
"head" of the branch.

Manipulating branches
---------------------

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

git branch::
        list all branches
git branch <branch>::
        create a new branch named <branch>, referencing the same
        point in history as the current branch
git branch <branch> <start-point>::
        create a new branch named <branch>, referencing
        <start-point>, which may be specified any way you like,
        including using a branch name or a tag name
git branch -d <branch>::
        delete the branch <branch>; if the branch you are deleting
        points to a commit which is not reachable from this branch,
        this command will fail with a warning.
git branch -D <branch>::
        even if the branch points to a commit not reachable
        from the current branch, you may know that that commit
        is still reachable from some other branch or tag.  In that
        case it is safe to use this command to force git to delete
        the branch.
git checkout <branch>::
        make the current branch <branch>, updating the working
        directory to reflect the version referenced by <branch>
git checkout -b <new> <start-point>::
        create a new branch <new> referencing <start-point>, and
        check it out.

It is also useful to know that the special symbol "HEAD" can always
be used to refer to the current branch.

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, which you
can view using the "-r" option to gitlink:git-branch[1]:

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

You cannot check out these remote-tracking branches, but you can
examine them on a branch of your own, just as you would a tag:

------------------------------------------------
$ git checkout -b my-todo-copy origin/todo
------------------------------------------------

Note that the name "origin" is just the name that git uses by default
to refer to the repository that you cloned from.

[[how-git-stores-references]]
How git stores references
-------------------------

Branches, remote-tracking branches, and tags are all references to
commits.  Git stores these references in the ".git" directory.  Most
of them are stored in .git/refs/:

        - branches are stored in .git/refs/heads
        - tags are stored in .git/refs/tags
        - remote-tracking branches for "origin" are stored in
          .git/refs/remotes/origin/

If you look at one of these files you will see that they usually
contain just the SHA1 id of a commit:

------------------------------------------------
$ ls .git/refs/heads/
master
$ cat .git/refs/heads/master
c0f982dcf188d55db9d932a39d4ea7becaa55fed
------------------------------------------------

You can refer to a reference by its path relative to the .git
directory.  However, we've seen above that git will also accept
shorter names; for example, "master" is an acceptable shortcut for
"refs/heads/master", and "origin/master" is a shortcut for
"refs/remotes/origin/master".

As another useful shortcut, you can also refer to the "HEAD" of
"origin" (or any other remote), using just the name of the remote.

For the complete list of paths which git checks for references, and
how it decides which to choose when there are multiple references
with the same name, see the "SPECIFYING REVISIONS" section of
gitlink:git-rev-parse[1].

[[Updating-a-repository-with-git-fetch]]
Updating a repository with git fetch
------------------------------------

Eventually the developer cloned from will do additional work in her
repository, creating new commits and advancing the branches to point
at the new commits.

The command "git fetch", with no arguments, will update all of the
remote-tracking branches to the latest version found in her
repository.  It will not touch any of your own branches--not even the
"master" branch that was created for you on clone.

Fetching individual branches
----------------------------

You can also choose to update just one branch at a time:

-------------------------------------------------
$ git fetch origin todo:refs/remotes/origin/todo
-------------------------------------------------

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/remotes/origin/todo; as we saw
above, remote-tracking branches are stored under
refs/remotes/<name-of-repository>/<name-of-branch>.

You can also fetch branches from other repositories; so

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

will create a new reference named "refs/remotes/example/master" and
store in it the branch named "master" from the repository at the
given URL.  If you already have a branch named
"refs/remotes/example/master", it will attempt to "fast-forward" to
the commit given by example.com's master branch.  So next we explain
what a fast-forward is:

[[fast-forwards]]
Understanding git history: 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 she made a serious mistake, 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.  Be aware that commits which the
old version of example/master pointed at may be lost, as we saw in
the previous section.

Configuring remote branches
---------------------------

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

-------------------------------------------------
$ git-repo-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,
after

-------------------------------------------------
$ git repo-config remote.example.url=git://example.com/proj.git
-------------------------------------------------

then the following two commands will do the same thing:

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

Even better, if you add one more option:

-------------------------------------------------
$ git repo-config remote.example.fetch=master:refs/remotes/example/master
-------------------------------------------------

then the following commands will all do the same thing:

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

You can also add a "+" to force the update each time:

-------------------------------------------------
$ git repo-config +master:ref/remotes/example/master
-------------------------------------------------

Don't do this unless you're sure you won't mind "git fetch" possibly
throwing away commits on mybranch.

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

See gitlink:git-repo-config[1] for more details on the configuration
options mentioned above.

Exploring git history
=====================

Git is best thought of as a tool for storing the history of a
collection of files.  It does this by storing compressed snapshots of
the contents of a file heirarchy, together with "commits" which show
the relationships between these snapshots.

Git provides extremely flexible and fast tools for exploring the
history of a project.

We start with one specialized tool which is useful for finding the
commit that introduced a bug into a project.

How to use bisect to find a regression
--------------------------------------

Suppose version 2.6.18 of your project worked, but the version at
"master" crashes.  Sometimes the best way to find the cause of such a
regression is to perform a brute-force search through the project's
history to find the particular commit that caused the problem.  The
gitlink: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 to a new branch named "bisect".  This branch
points to a commit (with commit id 65934...) that is reachable from
v2.6.19 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
gitlink:git-show[1], find out who wrote it, and mail them your bug
report with the commit id.  Finally, run

-------------------------------------------------
$ git bisect reset
-------------------------------------------------

to return you to the branch you were on before and delete the
temporary "bisect" branch.

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

-------------------------------------------------
$ git bisect-visualize
-------------------------------------------------

which will run gitk and label the commit it chose with a marker that
says "bisect".  Chose 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.

Naming commits
--------------

We have seen several ways of naming commits already:

        - 20-digit SHA1 id
        - branch name: refers to the commit at the head of the given
          branch
        - tag name: refers to the commit pointed to by the given tag
          (we've seen branches and tags are special cases of
          <<how-git-stores-references,references>>).
        - HEAD: refers to the head of the current branch

There are many more; see the "SPECIFYING REVISION" section of the
gitlink:git-rev-list[1] man page for the complete list of ways to
name revisions.  Some examples:

-------------------------------------------------
$ git show fb47ddb2 # the first few characters of the SHA1 id
                    # 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.

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 the tag is a tag you wish to
share with others, and possibly sign cryptographically, then you
should create a tag object instead; see the gitlink:git-tag[1] man
page for details.

Browsing revisions
------------------

The gitlink:git-log[1] command can show lists of commits.  On its
own, it shows all commits reachable from the parent commit; but you
can also make more specific requests:

-------------------------------------------------
$ git log v2.5..        # commits since (not reachable from) v2.5
$ git log test..master  # commits reachable from master but not test
$ git log master..test  # ...reachable from test but not master
$ git log master...test # ...reachable from either test or master,
                        #    but not both
$ git log --since="2 weeks ago" # commits from the last 2 weeks
$ git log Makefile      # commits which modify Makefile
$ git log fs/           # ... which modify any file under fs/
$ git log -S'foo()'     # commits which add or remove any file data
                        # matching the string 'foo()'
-------------------------------------------------

And of course you can combine all of these; the following finds
commits since v2.5 which touch the Makefile or any file under fs:

-------------------------------------------------
$ git log v2.5.. Makefile fs/
-------------------------------------------------

You can also ask git log to show patches:

-------------------------------------------------
$ git log -p
-------------------------------------------------

See the "--pretty" option in the gitlink: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 independant 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
gitlink:git-diff[1]:

-------------------------------------------------
$ git diff master..test
-------------------------------------------------

Sometimes what you want instead is a set of patches:

-------------------------------------------------
$ git format-patch master..test
-------------------------------------------------

will generate a file with a patch for each commit reachable from test
but not from master.  Note that if master also has commits which are
not reachable from test, then the combined result of these patches
will not be the same as the diff produced by the git-diff example.

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.

Developing with git
===================

Telling git your name
---------------------

Before creating any commits, you should introduce yourself to git.  The
easiest way to do so is:

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


Creating a new repository
-------------------------

Creating a new repository from scratch is very easy:

-------------------------------------------------
$ mkdir project
$ cd project
$ git init-db
-------------------------------------------------

If you have some initial content (say, a tarball):

-------------------------------------------------
$ tar -xzvf project.tar.gz
$ cd project
$ git init-db
$ git add . # include everything below ./ in the first commit:
$ git commit
-------------------------------------------------

[[how-to-make-a-commit]]
how to make a commit
--------------------

Creating a new commit takes three steps:

        1. Making some changes to the working directory using your
           favorite editor.
        2. Telling git about your changes.
        3. Creating the commit using the content you told git about
           in step 2.

In practice, you can interleave and repeat steps 1 and 2 as many
times as you want: in order to keep track of what you want committed
at step 3, git maintains a snapshot of the tree's contents in a
special staging area called "the index."

By default, the content of the index is identical to that of the
HEAD.  The command "git diff --cached" shows the difference between
HEAD and the index, so you should no output from that command.

Modifying the index is easy:

To update the index with the new contents of a modified file, use

-------------------------------------------------
$ git add path/to/file
-------------------------------------------------

To add the contents of a new file to the index, use

-------------------------------------------------
$ git add path/to/file
-------------------------------------------------

To remove a file from the index that you've removed from the working
tree,

-------------------------------------------------
$ git rm path/to/file
-------------------------------------------------

After each step you can verify that

-------------------------------------------------
$ git diff --cached
-------------------------------------------------

always shows the difference between the HEAD and the index file--this
is what you'd commit if you created the commit now--and that

-------------------------------------------------
$ git diff
-------------------------------------------------

shows the difference between the working tree and the index file.

Note that "git add" always adds just the current contents of a file
to the index; further changes to the same file will be ignored unless
you run git-add on the file again.

When you're ready, just run

-------------------------------------------------
$ git commit
-------------------------------------------------

and git will prompt you for a commit message and then create the new
commmit.  Check to make sure it looks like what you expected with

-------------------------------------------------
$ git show
-------------------------------------------------

As a special shortcut,
                
-------------------------------------------------
$ git commit -a
-------------------------------------------------

will update the index with any files that you've modified or removed
and create a commit, all in one step.

A number of commands are useful for keeping track of what you're
about to commit:

-------------------------------------------------
$ git diff --cached # difference between HEAD and the index; what
                    # would be commited 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 status        # a brief per-file summary of the above.
-------------------------------------------------

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.  Tools that turn commits into email, for example, use
the first line on the Subject line and the rest of the commit in the
body.

how to merge
------------

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

-------------------------------------------------
$ git merge branchname
-------------------------------------------------

merges the development in the branch "branchname" into the current
branch.  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 pull . next
Trying really trivial in-index merge...
fatal: Merge requires file-level merging
Nope.
Merging HEAD with 77976da35a11db4580b80ae27e8d65caf5208086
Merging:
15e2162 world
77976da goodbye
found 1 common ancestor(s):
d122ed4 initial
Auto-merging 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.

In more detail:

[[resolving-a-merge]]
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 will fail:

-------------------------------------------------
$ git commit
file.txt: needs merge
-------------------------------------------------

Also, git status will list those files as "unmerged".

All of the changes that git was able to merge automatically are
already added to the index file, so gitlink:git-diff[1] shows only
the conflicts.  Also, it uses a somewhat unusual syntax:

-------------------------------------------------
$ git diff
diff --cc file.txt
index 802992c,2b60207..0000000
--- a/file.txt
+++ b/file.txt
@@@ -1,1 -1,1 +1,5 @@@
++<<<<<<< HEAD:file.txt
 +Hello world
++=======
+ Goodbye
++>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
-------------------------------------------------

Recall that the commit which will be commited after we resolve this
conflict will have two parents instead of the usual one: one parent
will be HEAD, the tip of the current branch; the other will be the
tip of the other branch, which is stored temporarily in MERGE_HEAD.

The diff above shows the differences between the working-tree version
of file.txt and two previous version: one version from HEAD, and one
from MERGE_HEAD.  So instead of preceding each line by a single "+"
or "-", it now uses two columns: the first column is used for
differences between the first parent and the working directory copy,
and the second for differences between the second parent and the
working directory copy.  Thus after resolving the conflict in the
obvious way, the diff will look like:

-------------------------------------------------
$ git diff
diff --cc file.txt
index 802992c,2b60207..0000000
--- a/file.txt
+++ b/file.txt
@@@ -1,1 -1,1 +1,1 @@@
- Hello world
 -Goodbye
++Goodbye world
-------------------------------------------------

This shows that our resolved version deleted "Hello world" from the
first parent, deleted "Goodbye" from the second parent, and added
"Goodbye world", which was previously absent from both.

The gitlink:git-log[1] command also provides special help for merges:

-------------------------------------------------
$ git log --merge
-------------------------------------------------

This will list all commits which exist only on HEAD or on MERGE_HEAD,
and which touch an unmerged file.

We can now add the resolved version to the index and commit:

-------------------------------------------------
$ 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.

[[undoing-a-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

-------------------------------------------------
$ git reset --hard HEAD
-------------------------------------------------

Or, if you've already commited the merge that you want to throw away,

-------------------------------------------------
$ git reset --hard 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 one of the two lines of development is completely
contained within the other--so every commit present in the one is
already contained in the other--then git just performs a
<<fast-forwards,fast forward>>; the head of the current branch is
moved forward to point at the head of the merged-in branch, without
any new commits being created.

Fixing mistakes
---------------

If you've messed up the working tree, but haven't yet committed your
mistake, you can return the entire working tree to the last committed
state with

-------------------------------------------------
$ git reset --hard HEAD
-------------------------------------------------

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

        1. You can create a new commit that undoes whatever was done
        by the previous commit.  This is the correct thing if your
        mistake has already been made public.

        2. You can go back and modify the old commit.  You should
        never do this if you have already made the history public;
        git does not normally expect the "history" of a project to
        change, and cannot correctly perform repeated merges from
        a branch that has had its history changed.

Fixing a mistake with a new commit
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Creating a new commit that reverts an earlier change is very easy;
just pass the gitlink:git-revert[1] command a reference to the bad
commit; for example, to revert the most recent commit:

-------------------------------------------------
$ git revert HEAD
-------------------------------------------------

This will create a new commit which undoes the change in HEAD.  You
will be given a chance to edit the commit message for the new commit.

You can also revert an earlier change, for example, the next-to-last:

-------------------------------------------------
$ git revert HEAD^
-------------------------------------------------

In this case git will attempt to undo the old change while leaving
intact any changes made since then.  If more recent changes overlap
with the changes to be reverted, then you will be asked to fix
conflicts manually, just as in the case of <<resolving-a-merge,
resolving a merge>>.

Fixing a mistake by editing history
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If the problematic commit is the most recent commit, and you have not
yet made that commit public, then you may just
<<undoing-a-merge,destroy it using git-reset>>.

Alternatively, you
can edit the working directory and update the index to fix your
mistake, just as if you were going to <<how-to-make-a-commit,create a
new commit>>, then run

-------------------------------------------------
$ git commit --amend
-------------------------------------------------

which will replace the old commit by a new commit incorporating your
changes, giving you a chance to edit the old commit message first.

Again, you should never do this to a commit that may already have
been merged into another branch; use gitlink:git-revert[1] instead in
that case.

It is also possible to edit commits further back in the history, but
this is an advanced topic to be left for
<<cleaning-up-history,another chapter>>.

Checking out an old version of a file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In the process of undoing a previous bad change, you may find it
useful to check out an older version of a particular file using
gitlink:git-checkout[1].  We've used git checkout before to switch
branches, but it has quite different behavior if it is given a path
name: the command

-------------------------------------------------
$ git checkout HEAD^ path/to/file
-------------------------------------------------

replaces path/to/file by the contents it had in the commit HEAD^, and
also updates the index to match.  It does not change branches.

If you just want to look at an old version of the file, without
modifying the working directory, you can do that with
gitlink:git-show[1]:

-------------------------------------------------
$ git show HEAD^ path/to/file
-------------------------------------------------

which will display the given version of the file.

Ensuring good performance
-------------------------

On large repositories, git depends on compression to keep the history
information from taking up to much space on disk or in memory.

This compression is not performed automatically.  Therefore you
should occasionally run

-------------------------------------------------
$ git gc
-------------------------------------------------

to recompress the archive and to prune any commits which are no
longer referred to anywhere.  This can be very time-consuming, and
you should not modify the repository while it is working, so you
should run it while you are not working.

Sharing development with others
===============================

[[getting-updates-with-git-pull]]
Getting updates with git pull
-----------------------------

After you clone a repository and make a few changes of your own, you
may wish to check the original repository for updates and merge them
into your own work.

We have already seen <<Updating-a-repository-with-git-fetch,how to
keep remote tracking branches up to date>> with gitlink: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 gitlink:git-pull[1] command provides a way to do this in
one step:

-------------------------------------------------
$ git pull origin master
-------------------------------------------------

In fact, "origin" is normally the default repository to pull from,
and the default branch is normally the HEAD of the remote repository,
so often you can accomplish the above with just

-------------------------------------------------
$ git pull
-------------------------------------------------

See the descriptions of the branch.<name>.remote and
branch.<name>.merge options in gitlink:git-repo-config[1] to learn
how to control these defaults depending on the current branch.

In addition to saving you keystrokes, "git pull" also helps you by
producing a default commit message documenting the branch and
repository that you pulled from.

(But note that no such commit will be created in the case of a
<<fast-forwards,fast forward>>; instead, your branch will just be
updated to point to the latest commit from the upstream branch).

The git-pull command can also be given "." as the "remote" repository, in
which case it just merges in a branch from the current repository; so
the commands

-------------------------------------------------
$ git pull . branch
$ git merge branch
-------------------------------------------------

are roughly equivalent.  The former is actually very commonly used.

Submitting patches to a project
-------------------------------

If you just have a few changes, the simplest way to submit them may
just be to send them as patches in email:

First, use gitlink:git-format-patches[1]; for example:

-------------------------------------------------
$ git format-patches origin
-------------------------------------------------

will produce a numbered series of files in the current directory, one
for each patch in the current branch but not in origin/HEAD.

You can then import these into your mail client and send them by
hand.  However, if you have a lot to send at once, you may prefer to
use the gitlink:git-send-email[1] script to automate the process.
Consult the mailing list for your project first to determine how they
prefer such patches be handled.

Importing patches to a project
------------------------------

Git also provides a tool called gitlink:git-am[1] (am stands for
"apply mailbox"), for importing such an emailed series of patches.
Just save all of the patch-containing messages, in order, into a
single mailbox file, say "patches.mbox", then run

-------------------------------------------------
$ git am patches.mbox
-------------------------------------------------

Git will apply each patch in order; if any conflicts are found, it
will stop, and you can fix the conflicts as described in
"<<resolving-a-merge,Resolving a merge>>".  Once the index is updated
with the results of the conflict resolution, instead of creating a
new commit, just run

-------------------------------------------------
$ git am --resolved
-------------------------------------------------

and git will create the commit for you and continue applying the
remaining patches from the mailbox.

The final result will be a series of commits, one for each patch in
the original mailbox, with authorship and commit log message each
taken from the message containing each patch.

[[setting-up-a-public-repository]]
Setting up a public repository
------------------------------

Another way to submit changes to a project is to simply tell the
maintainer of that project to pull from your repository, exactly as
you did in the section "<<getting-updates-with-git-pull, Getting
updates with git pull>>".

If you and maintainer both have accounts on the same machine, then
then you can just pull changes from each other's repositories
directly; note that all of the command (gitlink:git-clone[1],
git-fetch[1], git-pull[1], etc.) which accept a URL as an argument
will also accept a local file patch; so, for example, you can
use

-------------------------------------------------
$ git clone /path/to/repository
$ git pull /path/to/other/repository
-------------------------------------------------

If this sort of setup is inconvenient or impossible, another (more
common) option is to set up a public repository on a public server.
This also 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

Now, assume your personal repository is in the directory ~/proj.  We
first create a new clone of the repository:

-------------------------------------------------
$ git clone --bare proj-clone.git
-------------------------------------------------

The resulting directory proj-clone.git will contains a "bare" git
repository--it is just the contents of the ".git" directory, without
a checked-out copy of a working directory.

Next, copy proj-clone.git to the server where you plan to host the
public repository.  You can use scp, rsync, or whatever is most
convenient.

If somebody else maintains the public server, they may already have
set up a git service for you, and you may skip to the section
"<<pushing-changes-to-a-public-repository,Pushing changes to a public
repository>>", below.

Otherwise, the following sections explain how to export your newly
created public repository:

[[exporting-via-http]]
Exporting a git repository via http
-----------------------------------

The git protocol gives better performance and reliability, but on a
host with a web server set up, http exports may be simpler to set up.

All you need to do is place the newly created bare git repository in
a directory that is exported by the web server, and make some
adjustments to give web clients some extra information they need:

-------------------------------------------------
$ mv proj.git /home/you/public_html/proj.git
$ cd proj.git
$ git update-server-info
$ chmod a+x hooks/post-update
-------------------------------------------------

(For an explanation of the last two lines, see
gitlink:git-update-server-info[1], and the documentation
link:hooks.txt[Hooks used by git].)

Advertise the url of proj.git.  Anybody else should then be able to
clone or pull from that url, for example with a commandline like:

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

(See also
link:howto/setup-git-server-over-http.txt[setup-git-server-over-http]
for a slightly more sophisticated setup using WebDAV which also
allows pushing over http.)

[[exporting-via-git]]
Exporting a git repository via the git protocol
-----------------------------------------------

This is the preferred method.

For now, we refer you to the gitlink:git-daemon[1] man page for
instructions.  (See especially the examples section.)

[[pushing-changes-to-a-public-repository]]
Pushing changes to a public repository
--------------------------------------

Note that the two techniques outline above (exporting via
<<exporting-via-http,http>> or <<exporting-via-git,git>>) allow other
maintainers to fetch your latest changes, but they do not allow write
access, which you will need to update the public repository with the
latest changes created in your private repository.

The simplest way to do this is using gitlink:git-push[1] and ssh; to
update the remote branch named "master" with the latest state of your
branch named "master", run

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

or just

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

As with git-fetch, git-push will complain if this does not result in
a <<fast-forwards,fast forward>>.  Normally this is a sign of
something wrong.  However, if you are sure you know what you're
doing, you may force git-push to perform the update anyway by
proceeding the branch name by a plus sign:

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

As with git-fetch, you may also set up configuration options to
save typing; so, for example, after

-------------------------------------------------
$ cat >.git/config <<EOF
[remote "public-repo"]
        url = ssh://yourserver.com/~you/proj.git
EOF
-------------------------------------------------

you should be able to perform the above push with just

-------------------------------------------------
$ git push public-repo master
-------------------------------------------------

See the explanations of the remote.<name>.url, branch.<name>.remote,
and remote.<name>.push options in gitlink:git-repo-config[1] for
details.

Setting up a shared repository
------------------------------

Another way to collaborate is by using a model similar to that
commonly used in CVS, where several developers with special rights
all push to and pull from a single shared repository.  See
link:cvs-migration.txt[git for CVS users] for instructions on how to
set this up.

Allow web browsing of a repository
----------------------------------

TODO: Brief setup-instructions for gitweb

Examples
--------

TODO: topic branches, typical roles as in everyday.txt, ?


Working with other version control systems
==========================================

TODO: CVS, Subversion, series-of-release-tarballs, ?

[[cleaning-up-history]]
Rewriting history and maintaining patch series
==============================================

Normally commits are only added to a project, never taken away or
replaced.  Git is designed with this assumption, and violating it will
cause git's merge machinery (for example) to do the wrong thing.

However, there is a situation in which it can be useful to violate this
assumption.

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 it is too much to digest all at once.

If you present them with the entire history of your work, complete with
mistakes, corrections, and dead ends, they may be overwhelmed.

So the ideal is usually to produce a series of patches such that:

        1. Each patch can be applied in order.

        2. Each patch includes a single logical change, together with a
           message explaining the change.

        3. No patch introduces a regression: after applying any initial
           part of the series, the resulting project still compiles and
           works, and has no bugs that it didn't have before.

        4. The complete series produces the same end result as your own
           (probably much messier!) development process did.

We will introduce some tools that can help you do this, explain how to use
them, and then explain some of the problems that can arise because you are
rewriting history.

Keeping a patch series up to date using git-rebase
--------------------------------------------------

Suppose you have a series of commits in a branch "mywork", which
originally branched off from "origin".

Suppose you create a branch "mywork" on a remote-tracking branch "origin",
and created some commits on top of it:

-------------------------------------------------
$ git checkout -b mywork origin
$ vi file.txt
$ git commit
$ vi otherfile.txt
$ git commit
...
-------------------------------------------------

You have performed no merges into mywork, so it is just a simple linear
sequence of patches on top of "origin":


 o--o--o <-- origin
        \
         o--o--o <-- 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
gitlink:git-rebase[1]:

-------------------------------------------------
$ git checkout mywork
$ git rebase origin
-------------------------------------------------

This will remove each of your commits from mywork, temporarily saving them
as patches (in a directory named ".dotest"), 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 as described in
"<<resolving-a-merge,Resolving a merge>>".  Once the index is updated with
the results of the conflict resolution, instead of creating a new commit,
just run

-------------------------------------------------
$ git rebase --continue
-------------------------------------------------

and git will continue applying the rest of the patches.

At any point you may use the --abort option to abort this process and
return mywork to the state it had before you started the rebase:

-------------------------------------------------
$ git rebase --abort
-------------------------------------------------

Reordering or selecting from a patch series
-------------------------------------------

Given one existing commit, the gitlink:git-cherry-pick[1] command allows
you to apply the change introduced by that commit and create a new commit
that records it.

This can be useful for modifying a patch series.

TODO: elaborate

Other tools
-----------

There are numerous other tools, such as stgit, which exist for the purpose
of maintianing a patch series.  These are out 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.

TODO: elaborate


Git internals
=============

Architectural overview
----------------------

TODO: Sources, README, core-tutorial, tutorial-2.txt, technical/

Glossary of git terms
=====================

include::glossary.txt[]

Notes and todo list for this manual
===================================

This is a work in progress.

The basic requirements:
        - It must be readable in order, from beginning to end, by someone
          intelligent with a basic grasp of the unix commandline, 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"

Think about how to create a clear chapter dependency graph that will allow
people to get to important topics without necessarily reading everything
in between.

Scan Documentation/ for other stuff left out; in particular:
        howto's
        README
        some of technical/?
        hooks
        etc.

Scan email archives for other stuff left out

Scan man pages to see if any assume more background than this manual
provides.

Update git fetch discussion to use "git remote", move most of branch
discussion till later.

Can also simplify beginning by suggesting disconnected head instead
of temporary branch creation.

Explain how to refer to file stages in the "how to resolve a merge"
section: diff -1, -2, -3, --ours, --theirs :1:/path notation.  The
"git ls-files --unmerged --stage" thing is sorta useful too, actually.  And
note gitk --merge.  Also what's easiest way to see common merge base?

Add more good examples.  Entire sections of just cookbook examples might
be a good idea; maybe make an "advanced examples" section a standard
end-of-chapter section?

Include cross-references to the glossary, where appropriate.

To document:
        reflogs, git reflog expire
        shallow clones??  See draft 1.5.0 release notes for some documentation.