-Git User's Manual
-_________________
+Git User's Manual (for version 1.5.1 or newer)
+______________________________________________
This manual is designed to be readable by someone with basic unix
command-line skills, but no previous knowledge of git.
$ man git-clone
------------------------------------------------
+[[git-quick-start]]
Git Quick Start
===============
This is a quick summary of the major commands; the following chapters
will explain how these work in more detail.
+[[quick-creating-a-new-repository]]
Creating a new repository
-------------------------
$ cd project
-----------------------------------------------
+[[managing-branches]]
Managing branches
-----------------
-----------------------------------------------
-$ git branch # list all branches in this repo
+$ git branch # list all local branches in this repo
$ git checkout test # switch working directory to branch "test"
$ git branch new # create branch "new" starting at current HEAD
$ git branch -d new # delete branch "new"
origin/master
origin/next
...
-$ git branch checkout -b masterwork origin/master
+$ git checkout -b masterwork origin/master
-----------------------------------------------
Fetch a branch from a different repository, and give it a new
-----------------------------------------------
+[[exploring-history]]
Exploring history
-----------------
# repeat until done.
-----------------------------------------------
+[[making-changes]]
Making changes
--------------
Make sure git knows who to blame:
------------------------------------------------
-$ cat >~/.gitconfig <<\EOF
+$ cat >>~/.gitconfig <<\EOF
[user]
-name = Your Name Comes Here
-email = you@yourdomain.example.com
+ name = Your Name Comes Here
+ email = you@yourdomain.example.com
EOF
------------------------------------------------
$ git commit -a # use latest content of all tracked files
-----------------------------------------------
+[[merging]]
Merging
-------
$ git pull . test # equivalent to git merge test
-----------------------------------------------
+[[sharing-your-changes]]
Sharing your changes
--------------------
-----------------------------------------------
$ 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"
+$ git am mbox # import patches from the mailbox "mbox"
-----------------------------------------------
Fetch a branch in a different git repository, then merge into the
$ git push example test
-----------------------------------------------
+[[repository-maintenance]]
Repository maintenance
----------------------
$ git gc
-----------------------------------------------
+[[repositories-and-branches]]
Repositories and Branches
=========================
+[[how-to-get-a-git-repository]]
How to get a git repository
---------------------------
In most of the following, examples will be taken from one of the two
repositories above.
+[[how-to-check-out]]
How to check out a different version of a project
-------------------------------------------------
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:
+A single git repository may contain multiple branches. It keeps track
+of them by keeping a list of <<def_head,heads>> which reference the
+latest version on each branch; the gitlink:git-branch[1] command shows
+you the list of branch heads:
------------------------------------------------
$ 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.
+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 branches, are references
-into the project's history, and can be listed using the
+Most projects also use <<def_tag,tags>>. Tags, like heads, are
+references into the project's history, and can be listed using the
gitlink:git-tag[1] command:
------------------------------------------------
------------------------------------------------
Tags are expected to always point at the same version of a project,
-while branches are expected to advance as development progresses.
+while heads are expected to advance as development progresses.
-Create a new branch pointing to one of these versions and check it
+Create a new branch head pointing to one of these versions and check it
out using gitlink:git-checkout[1]:
------------------------------------------------
$ git reset --hard v2.6.17
------------------------------------------------
-Note that if the current branch was your only reference to a
+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.
+with no way to find the history it used to point to; so use this command
+carefully.
+[[understanding-commits]]
Understanding History: Commits
------------------------------
history, including file data and directory contents, is stored in an object
with a name that is a hash of its contents.
+[[understanding-reachability]]
Understanding history: commits, parents, and reachability
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
that Y is a descendent of X, or that there is a chain of parents
leading from commit Y to commit X.
+[[history-diagrams]]
Understanding history: History diagrams
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.
+[[what-is-a-branch]]
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
+When we need to be precise, we will use the word "branch" to mean a line
+of development, and "branch head" (or just "head") to mean a reference
+to the most recent commit on a branch. In the example above, the branch
+head named "A" is a pointer to one particular commit, but we refer to
+the line of three commits leading up to that point as all being part of
"branch A".
-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.
+However, when no confusion will result, we often just use the term
+"branch" both for branches and for branch heads.
+[[manipulating-branches]]
Manipulating branches
---------------------
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.
+ points to a commit which is not reachable from the current
+ branch, this command will fail with a warning.
git branch -D <branch>::
even if the branch points to a commit not reachable
from the current branch, you may know that that commit
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.
+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
+------------------------------------------------
+
+[[detached-head]]
+Examining an old version without creating a new branch
+------------------------------------------------------
+
+The git-checkout command normally expects a branch head, but will also
+accept an arbitrary commit; for example, you can check out the commit
+referenced by a tag:
+
+------------------------------------------------
+$ git checkout v2.6.17
+Note: moving to "v2.6.17" which isn't a local branch
+If you want to create a new branch from this checkout, you may do so
+(now or later) by using -b with the checkout command again. Example:
+ git checkout -b <new_branch_name>
+HEAD is now at 427abfa... Linux v2.6.17
+------------------------------------------------
+
+The HEAD then refers to the SHA1 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
+* (no branch)
+ 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-remote-branches]]
Examining branches from a remote repository
-------------------------------------------
The full name is occasionally useful if, for example, there ever
exists a tag and a branch with the same name.
-As another useful shortcut, if the repository "origin" posesses only
-a single branch, you can refer to that branch as just "origin".
-
-More generally, if you have defined a remote repository named
-"example", you can refer to the branch in that repository as
-"example". And for a repository with multiple branches, this will
-refer to the branch designated as the "HEAD" branch.
+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
repository. It will not touch any of your own branches--not even the
"master" branch that was created for you on clone.
+[[fetching-branches]]
Fetching branches from other repositories
-----------------------------------------
-------------------------------------------------
$ git remote add linux-nfs git://linux-nfs.org/pub/nfs-2.6.git
-$ git fetch
+$ git fetch linux-nfs
* refs/remotes/linux-nfs/master: storing branch 'master' ...
commit: bf81b46
-------------------------------------------------
$ cat .git/config
...
[remote "linux-nfs"]
- url = git://linux-nfs.org/~bfields/git.git
- fetch = +refs/heads/*:refs/remotes/linux-nfs-read/*
+ url = git://linux-nfs.org/pub/nfs-2.6.git
+ fetch = +refs/heads/*:refs/remotes/linux-nfs/*
...
-------------------------------------------------
text editor. (See the "CONFIGURATION FILE" section of
gitlink:git-config[1] for details.)
+[[exploring-git-history]]
Exploring git history
=====================
We start with one specialized tool that is useful for finding the
commit that introduced a bug into a project.
+[[using-bisect]]
How to use bisect to find a regression
--------------------------------------
run
-------------------------------------------------
-$ git bisect-visualize
+$ git bisect visualize
-------------------------------------------------
which will run gitk and label the commit it chose with a marker that
then test, run "bisect good" or "bisect bad" as appropriate, and
continue.
+[[naming-commits]]
Naming commits
--------------
e05db0fd4f31dde7005f075a84f96b360d05984b
-------------------------------------------------
+[[creating-tags]]
Creating tags
-------------
running
-------------------------------------------------
-$ git-tag stable-1 1b2e1d63ff
+$ 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.
+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 gitlink:git-tag[1] man page
+for details.
+[[browsing-revisions]]
Browsing revisions
------------------
multiple independent lines of development, the particular order that
commits are listed in may be somewhat arbitrary.
+[[generating-diffs]]
Generating diffs
----------------
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]]
Viewing old file versions
-------------------------
Before the colon may be anything that names a commit, and after it
may be any path to a file tracked by git.
+[[history-examples]]
Examples
--------
+[[checking-for-equal-branches]]
Check whether two branches point at the same history
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
will return no commits when the two branches are equal.
+[[finding-tagged-descendants]]
Find first tagged version including a given fix
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
descendants:
-------------------------------------------------
-$ git name-rev e05db0fd
+$ git name-rev --tags e05db0fd
e05db0fd tags/v1.5.0-rc1^0~23
-------------------------------------------------
-------------------------------------------------
$ git describe e05db0fd
-v1.5.0-rc0-ge05db0f
+v1.5.0-rc0-260-ge05db0f
-------------------------------------------------
but that may sometimes help you guess which tags might come after the
from v1.5.0-rc2, but not from v1.5.0-rc0.
+[[Developing-with-git]]
Developing with git
===================
+[[telling-git-your-name]]
Telling git your name
---------------------
Before creating any commits, you should introduce yourself to git. The
-easiest way to do so is:
+easiest way to do so is to make sure the following lines appear in a
+file named .gitconfig in your home directory:
------------------------------------------------
-$ cat >~/.gitconfig <<\EOF
[user]
name = Your Name Comes Here
email = you@yourdomain.example.com
-EOF
------------------------------------------------
(See the "CONFIGURATION FILE" section of gitlink:git-config[1] for
details on the configuration file.)
+[[creating-a-new-repository]]
Creating a new repository
-------------------------
-------------------------------------------------
[[how-to-make-a-commit]]
-how to make a commit
+How to make a commit
--------------------
Creating a new commit takes three steps:
$ 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.
-------------------------------------------------
-creating good commit messages
+[[creating-good-commit-messages]]
+Creating good commit messages
-----------------------------
Though not required, it's a good idea to begin the commit message
the first line on the Subject line and the rest of the commit in the
body.
-how to merge
+[[how-to-merge]]
+How to merge
------------
You can rejoin two diverging branches of development using
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
+$ 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.
-------------------------------------------------
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
-----------------
The above is all you need to know to resolve a simple merge. But git
also provides more information to help resolve conflicts:
+[[conflict-resolution]]
Getting conflict-resolution help during a merge
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
These will display all commits which exist only on HEAD or on
MERGE_HEAD, and which touch an unmerged file.
+You may also use gitlink:git-mergetool, which lets you merge the
+unmerged files using external tools such as emacs or kdiff3.
+
Each time you resolve the conflicts in a file and update the index:
-------------------------------------------------
git-diff will (by default) no longer show diffs for that file.
[[undoing-a-merge]]
-undoing a merge
+Undoing a merge
---------------
If you get stuck and decide to just give up and throw the whole mess
itself have been merged into another branch, as doing so may confuse
further merges.
+[[fast-forwards]]
Fast-forward merges
-------------------
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.
+However, if the current branch is a descendant of the other--so every
+commit present in the one is already contained in the other--then git
+just performs a "fast forward"; the head of the current branch is moved
+forward to point at the head of the merged-in branch, without any new
+commits being created.
+[[fixing-mistakes]]
Fixing mistakes
---------------
change, and cannot correctly perform repeated merges from
a branch that has had its history changed.
+[[reverting-a-commit]]
Fixing a mistake with a new commit
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
this is an advanced topic to be left for
<<cleaning-up-history,another chapter>>.
+[[checkout-of-path]]
Checking out an old version of a file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gitlink:git-show[1]:
-------------------------------------------------
-$ git show HEAD^ path/to/file
+$ git show HEAD^:path/to/file
-------------------------------------------------
which will display the given version of the file.
+[[ensuring-good-performance]]
Ensuring good performance
-------------------------
to recompress the archive. This can be very time-consuming, so
you may prefer to run git-gc when you are not doing other work.
+
+[[ensuring-reliability]]
Ensuring reliability
--------------------
+[[checking-for-corruption]]
Checking the repository for corruption
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
...
-------------------------------------------------
-Dangling objects are objects that are harmless, but also unnecessary;
-you can remove them at any time with gitlink:git-prune[1] or the --prune
+Dangling objects are not a problem. At worst they may take up a little
+extra disk space. They can sometimes provide a last-resort method of
+recovery lost work--see <<dangling-objects>> for details. However, if
+you want, you may remove them with gitlink:git-prune[1] or the --prune
option to gitlink:git-gc[1]:
-------------------------------------------------
git-gc when run without any options), it is not safe to prune while
other git operations are in progress in the same repository.
-For more about dangling objects, see <<dangling-objects>>.
-
-
+[[recovering-lost-changes]]
Recovering lost changes
~~~~~~~~~~~~~~~~~~~~~~~
+[[reflogs]]
Reflogs
^^^^^^^
$ git show master@{3} # 3, ... changes ago.
$ gitk master@{yesterday} # See where it pointed yesterday,
$ gitk master@{"1 week ago"} # ... or last week
+$ git log --walk-reflogs master # show reflog entries for master
-------------------------------------------------
+A separate reflog is kept for the HEAD, so
+
+-------------------------------------------------
+$ git show HEAD@{"1 week ago"}
+-------------------------------------------------
+
+will show what HEAD pointed to one week ago, not what the current branch
+pointed to one week ago. This allows you to see the history of what
+you've checked out.
+
The reflogs are kept by default for 30 days, after which they may be
pruned. See gitlink:git-reflog[1] and gitlink:git-gc[1] to learn
how to control this pruning, and see the "SPECIFYING REVISIONS"
same project, the reflog history is not shared: it tells you only about
how the branches in your local repository have changed over time.
+[[dangling-object-recovery]]
Examining dangling objects
^^^^^^^^^^^^^^^^^^^^^^^^^^
-In some situations the reflog may not be able to save you. For
-example, suppose you delete a branch, then realize you need the history
-it contained. The reflog is also deleted; however, if you have not
-yet pruned the repository, then you may still be able to find
-the lost commits; run git-fsck and watch for output that mentions
-"dangling commits":
+In some situations the reflog may not be able to save you. For example,
+suppose you delete a branch, then realize you need the history it
+contained. The reflog is also deleted; however, if you have not yet
+pruned the repository, then you may still be able to find the lost
+commits in the dangling objects that git-fsck reports. See
+<<dangling-objects>> for the details.
-------------------------------------------------
$ git fsck
$ git branch recovered-branch 7281251ddd
------------------------------------------------
+Other types of dangling objects (blobs and trees) are also possible, and
+dangling objects can arise in other situations.
+
+[[sharing-development]]
Sharing development with others
===============================
$ git pull
-------------------------------------------------
-See the descriptions of the branch.<name>.remote and
-branch.<name>.merge options in gitlink:git-config[1] to learn
-how to control these defaults depending on the current branch.
+See the descriptions of the branch.<name>.remote and branch.<name>.merge
+options in gitlink:git-config[1] to learn how to control these defaults
+depending on the current branch. Also note that the --track option to
+gitlink:git-branch[1] and gitlink:git-checkout[1] can be used to
+automatically set the default remote branch to pull from at the time
+that a branch is created:
+
+-------------------------------------------------
+$ git checkout --track -b origin/maint maint
+-------------------------------------------------
In addition to saving you keystrokes, "git pull" also helps you by
producing a default commit message documenting the branch and
are roughly equivalent. The former is actually very commonly used.
+[[submitting-patches]]
Submitting patches to a project
-------------------------------
Consult the mailing list for your project first to determine how they
prefer such patches be handled.
+[[importing-patches]]
Importing patches to a project
------------------------------
then you can just pull changes from each other's repositories
directly; note that all of the commands (gitlink:git-clone[1],
git-fetch[1], git-pull[1], etc.) that accept a URL as an argument
-will also accept a local file patch; so, for example, you can
+will also accept a local directory name; so, for example, you can
use
-------------------------------------------------
first create a new clone of the repository:
-------------------------------------------------
-$ git clone --bare proj-clone.git
+$ git clone --bare ~/proj proj.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.
+The resulting directory proj.git 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
+Next, copy proj.git to the server where you plan to host the
public repository. You can use scp, rsync, or whatever is most
convenient.
-------------------------------------------------
$ mv proj.git /home/you/public_html/proj.git
$ cd proj.git
-$ git update-server-info
+$ git --bare update-server-info
$ chmod a+x hooks/post-update
-------------------------------------------------
save typing; so, for example, after
-------------------------------------------------
-$ cat >.git/config <<EOF
+$ cat >>.git/config <<EOF
[remote "public-repo"]
url = ssh://yourserver.com/~you/proj.git
EOF
and remote.<name>.push options in gitlink:git-config[1] for
details.
+[[setting-up-a-shared-repository]]
Setting up a shared repository
------------------------------
link:cvs-migration.txt[git for CVS users] for instructions on how to
set this up.
+[[setting-up-gitweb]]
Allow web browsing of a repository
----------------------------------
The gitweb cgi script provides users an easy way to browse your
project's files and history without having to install git; see the file
-gitweb/README in the git source tree for instructions on setting it up.
+gitweb/INSTALL in the git source tree for instructions on setting it up.
+[[sharing-development-examples]]
Examples
--------
However, there is a situation in which it can be useful to violate this
assumption.
+[[patch-series]]
Creating the perfect patch series
---------------------------------
use them, and then explain some of the problems that can arise because
you are rewriting history.
+[[using-git-rebase]]
Keeping a patch series up to date using git-rebase
--------------------------------------------------
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
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
$ git rebase --abort
-------------------------------------------------
+[[modifying-one-commit]]
Modifying a single commit
-------------------------
(Either gitk or git-log may be useful for finding the commit.)
-Then check out a new branch at that commit, edit it, and rebase the rest of
-the series on top of it:
+Then check out that commit, edit it, and rebase the rest of the series
+on top of it (note that we could check out the commit on a temporary
+branch, but instead we're using a <<detached-head,detached head>>):
-------------------------------------------------
-$ git checkout -b TMP bad
+$ git checkout bad
$ # make changes here and update the index
$ git commit --amend
-$ git rebase --onto TMP bad mywork
+$ git rebase --onto HEAD bad mywork
-------------------------------------------------
-When you're done, you'll be left with mywork checked out, with the top patches
-on mywork reapplied on top of the modified commit you created in TMP. You can
+When you're done, you'll be left with mywork checked out, with the top
+patches on mywork reapplied on top of your modified commit. You can
then clean up with
-------------------------------------------------
-$ git branch -d TMP
$ git tag -d bad
-------------------------------------------------
"modified" existing commits; instead, you have replaced the old commits with
new commits having new object names.
+[[reordering-patch-series]]
Reordering or selecting from a patch series
-------------------------------------------
Then modify, reorder, or eliminate patches as preferred before applying
them again with gitlink:git-am[1].
+[[patch-series-tools]]
Other tools
-----------
purpose of maintaining a patch series. These are outside of the scope of
this manual.
+[[problems-with-rewriting-history]]
Problems with rewriting history
-------------------------------
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
For true distributed development that supports proper merging,
published branches should never be rewritten.
+[[advanced-branch-management]]
Advanced branch management
==========================
+[[fetching-individual-branches]]
Fetching individual branches
----------------------------
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. So
-next we explain what a fast-forward is:
+<<fast-forwards,fast-forward>> to the commit given by example.com's
+master branch. In more detail:
-[[fast-forwards]]
-Understanding git history: fast-forwards
-----------------------------------------
+[[fetch-fast-forwards]]
+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".
+commit. Git calls this process a <<fast-forwards,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
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.
unless you've already created a reference of your own pointing to
them.
+[[forcing-fetch]]
Forcing git fetch to do non-fast-forward updates
------------------------------------------------
$ git fetch git://example.com/proj.git +master:refs/remotes/example/master
-------------------------------------------------
-Note the addition of the "+" sign. Be aware that commits that the
-old version of example/master pointed at may be lost, as we saw in
-the previous section.
+Note the addition of the "+" sign. Alternatively, you can use the "-f"
+flag to force updates of all the fetched branches, as in:
+
+-------------------------------------------------
+$ git fetch -f origin
+-------------------------------------------------
+Be aware that commits that the old version of example/master pointed at
+may be lost, as we saw in the previous section.
+
+[[remote-branch-configuration]]
Configuring remote branches
---------------------------
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 git://example.com/proj.git master:refs/remotes/example/master
+$ git fetch example master:refs/remotes/example/master
$ git fetch example
-------------------------------------------------
Git internals
=============
-There are two object abstractions: the "object database", and the
-"current directory cache" aka "index".
+Git depends on two fundamental abstractions: the "object database", and
+the "current directory cache" aka "index".
+[[the-object-database]]
The Object Database
-------------------
to other objects (by referencing their SHA1 hash), and so you can
build up a hierarchy of objects.
-All objects have a statically determined "type" aka "tag", which is
+All objects have a statically determined "type" which is
determined at object creation time, and 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".
+"tree", "commit", and "tag".
-A "blob" object cannot refer to any other object, and is, like the type
-implies, a pure storage object containing some user data. It is used to
-actually store the file data, i.e. a blob object is associated with some
-particular version of some file.
+A <<def_blob_object,"blob" object>> cannot refer to any other object,
+and is, as the name implies, a pure storage object containing some
+user data. It is used to actually store the file data, i.e. a blob
+object is associated with some particular version of some file.
-A "tree" object is an object that ties one or more "blob" objects into a
-directory structure. In addition, a tree object can refer to other tree
-objects, thus creating a directory hierarchy.
+A <<def_tree_object,"tree" object>> is an object that ties one or more
+"blob" objects into a directory structure. In addition, a tree object
+can refer to other tree objects, thus creating a directory hierarchy.
-A "commit" object ties such directory hierarchies together into
-a DAG of revisions - each "commit" is associated with exactly one tree
-(the directory hierarchy at the time of the commit). In addition, a
-"commit" refers to one or more "parent" commit objects that describe the
-history of how we arrived at that directory hierarchy.
+A <<def_commit_object,"commit" object>> ties such directory hierarchies
+together into a <<def_DAG,directed acyclic graph>> of revisions - each
+"commit" is associated with exactly one tree (the directory hierarchy at
+the time of the commit). In addition, a "commit" refers to one or more
+"parent" commit objects that describe the history of how we arrived at
+that directory hierarchy.
As a special case, a commit object with no parents is called the "root"
-object, and is the point of an initial project commit. Each project
+commit, and is the point of an initial project commit. Each project
must have at least one root, and while you can tie several different
root objects together into one project by creating a commit object which
has two or more separate roots as its ultimate parents, that's probably
just going to confuse people. So aim for the notion of "one root object
per project", even if git itself does not enforce that.
-A "tag" object symbolically identifies and can be used to sign other
-objects. It contains the identifier and type of another object, a
-symbolic name (of course!) and, optionally, a signature.
+A <<def_tag_object,"tag" object>> symbolically identifies and can be
+used to sign other objects. It contains the identifier and type of
+another object, a symbolic name (of course!) and, optionally, a
+signature.
Regardless of object type, all objects share the following
characteristics: they are all deflated with zlib, and have a header
The object types in some more detail:
+[[blob-object]]
Blob Object
-----------
A blob is typically created when gitlink:git-update-index[1]
is run, and its data can be accessed by gitlink:git-cat-file[1].
+[[tree-object]]
Tree Object
-----------
its data can be accessed by gitlink:git-ls-tree[1].
Two trees can be compared with gitlink:git-diff-tree[1].
+[[commit-object]]
Commit Object
-------------
The parents do not have to actually have any relationship with the
result, for example.
-Note on commits: unlike real SCM's, commits do not contain
+Note on commits: unlike some SCM's, commits do not contain
rename information or file mode change information. All of that is
implicit in the trees involved (the result tree, and the result trees
of the parents), and describing that makes no sense in this idiotic
A commit is created with gitlink:git-commit-tree[1] and
its data can be accessed by gitlink:git-cat-file[1].
+[[trust]]
Trust
-----
To assist in this, git also provides the tag object...
+[[tag-object]]
Tag Object
----------
gitlink:git-verify-tag[1].
+[[the-index]]
The "index" aka "Current Directory Cache"
-----------------------------------------
The index is a simple binary file, which contains an efficient
-representation of a virtual directory content at some random time. It
+representation of the contents of a virtual directory. It
does so by a simple array that associates a set of names, dates,
permissions and content (aka "blob") objects together. The cache is
always kept ordered by name, and names are unique (with a few very
+[[the-workflow]]
The Workflow
------------
from the database or from the working directory. Thus there are four
main combinations:
+[[working-directory-to-index]]
working directory -> index
~~~~~~~~~~~~~~~~~~~~~~~~~~
it will only update the fields that are used to quickly test whether
an object still matches its old backing store object.
+[[index-to-object-database]]
index -> object database
~~~~~~~~~~~~~~~~~~~~~~~~
use that tree to re-generate the index at any time by going in the
other direction:
+[[object-database-to-index]]
object database -> index
~~~~~~~~~~~~~~~~~~~~~~~~
earlier. However, that is only your 'index' file: your working
directory contents have not been modified.
+[[index-to-working-directory]]
index -> working directory
~~~~~~~~~~~~~~~~~~~~~~~~~~
Finally, there are a few odds and ends which are not purely moving
from one representation to the other:
+[[tying-it-all-together]]
Tying it all together
~~~~~~~~~~~~~~~~~~~~~
------------
+[[examining-the-data]]
Examining the data
------------------
to see what the top commit was.
+[[merging-multiple-trees]]
Merging multiple trees
----------------------
object.
Once you know the three trees you are going to merge (the one "original"
-tree, aka the common case, and the two "result" trees, aka the branches
+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
`git-write-tree`.
+[[merging-multiple-trees-2]]
Merging multiple trees, continued
---------------------------------
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` or `merge`, on the blob objects from
-these three stages yourself, like this:
+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
-$ merge hello.c~2 hello.c~1 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
and that is what higher level `git merge -s resolve` is implemented with.
+[[pack-files]]
How git stores objects efficiently: pack files
----------------------------------------------
The most common cause of dangling objects is that you've rebased a
branch, or you have pulled from somebody else who rebased a branch--see
<<cleaning-up-history>>. In that case, the old head of the original
-branch still exists, as does obviously everything it pointed to. The
-branch pointer itself just doesn't, since you replaced it with another
-one.
+branch still exists, as does everything it pointed to. The branch
+pointer itself just doesn't, since you replaced it with another one.
-There are also other situations too that cause dangling objects. For
+There are also other situations that cause dangling objects. For
example, a "dangling blob" may arise because you did a "git add" of a
file, but then, before you actually committed it and made it part of the
bigger picture, you changed something else in that file and committed
that you really didn't want to - you can look at what dangling objects
you have, and decide to reset your head to some old dangling state).
-For commits, the most useful thing to do with dangling objects tends to
-be to do a simple
+For commits, you can just use:
------------------------------------------------
$ gitk <dangling-commit-sha-goes-here> --not --all
------------------------------------------------
-For blobs and trees, you can't do the same, but you can examine them.
-You can just do
+This asks for all the history reachable from the given commit but not
+from any branch, tag, or other reference. If you decide it's something
+you want, you can always create a new reference to it, e.g.,
+
+------------------------------------------------
+$ git branch recovered-branch <dangling-commit-sha-goes-here>
+------------------------------------------------
+
+For blobs and trees, you can't do the same, but you can still examine
+them. You can just do
------------------------------------------------
$ git show <dangling-blob/tree-sha-goes-here>
contrast, running "git prune" while somebody is actively changing the
repository is a *BAD* idea).
-Glossary of git terms
-=====================
-
+[[glossary]]
include::glossary.txt[]
+[[todo]]
Notes and todo list for this manual
===================================