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git - the stupid content tracker
git [--version] [--exec-path[=GIT_EXEC_PATH]] [-p|--paginate]
[--bare] [--git-dir=GIT_DIR] [--help] COMMAND [ARGS]
Git is a fast, scalable, distributed revision control system with an
unusually rich command set that provides both high-level operations and
full access to internals.
See this tutorial to get started, then see Everyday Git for a
useful minimum set of commands, and "man git-commandname" for
documentation of each command. CVS users may also want to read CVS
migration. See Git User’s Manual for a more in-depth
The COMMAND is either a name of a Git command (see below) or an alias
as defined in the configuration file (see git-config(1)).
Formatted and hyperlinked version of the latest git documentation can
be viewed at http://www.kernel.org/pub/software/scm/git/docs/.
Prints the git suite version that the git program came from.
Prints the synopsis and a list of the most commonly used commands.
If a git command is named this option will bring up the man-page
for that command. If the option --all or -a is given then all
available commands are printed.
Path to wherever your core git programs are installed. This can
also be controlled by setting the GIT_EXEC_PATH environment
variable. If no path is given git will print the current setting
and then exit.
Pipe all output into less (or if set, $PAGER).
Set the path to the repository. This can also be controlled by
setting the GIT_DIR environment variable.
Same as --git-dir=pwd.
See the references above to get started using git. The following is
probably more detail than necessary for a first-time user.
The Discussion section below and the Core tutorial both provide
introductions to the underlying git architecture.
See also the howto documents for some useful examples.
We divide git into high level ("porcelain") commands and low level
We separate the porcelain commands into the main commands and some
ancillary user utilities.
Main porcelain commands
Add file contents to the changeset to be committed next.
Apply a series of patches from a mailbox.
Create an archive of files from a named tree.
Find the change that introduced a bug by binary search.
List, create, or delete branches.
Move objects and refs by archive.
Checkout and switch to a branch.
Apply the change introduced by an existing commit.
Remove untracked files from the working tree.
Clone a repository into a new directory.
Record changes to the repository.
Show the most recent tag that is reachable from a commit.
Show changes between commits, commit and working tree, etc.
Download objects and refs from another repository.
Prepare patches for e-mail submission.
Cleanup unnecessary files and optimize the local repository.
Print lines matching a pattern.
Create an empty git repository or reinitialize an existing one.
The git repository browser.
Show commit logs.
Join two or more development histories together.
Move or rename a file, a directory, or a symlink.
Fetch from and merge with another repository or a local branch.
Update remote refs along with associated objects.
Forward-port local commits to the updated upstream head.
Reset current HEAD to the specified state.
Revert an existing commit.
Remove files from the working tree and from the index.
Summarize git log output.
Show various types of objects.
Show the working tree status.
Create, list, delete or verify a tag object signed with GPG.
Converts old-style git repository.
Backend for fast Git data importers.
Recover lost refs that luckily have not yet been pruned.
Run merge conflict resolution tools to resolve merge conflicts.
Pack heads and tags for efficient repository access.
Prune all unreachable objects from the object database.
Manage reflog information.
Hardlink common objects in local repositories.
Pack unpacked objects in a repository.
Get and set repository or global options.
manage set of tracked repositories.
Annotate file lines with commit info.
Apply a series of patches in a mailbox.
Show what revision and author last modified each line of a file.
Find commits not merged upstream.
Count unpacked number of objects and their disk consumption.
Verifies the connectivity and validity of the objects in the
Extract commit ID from an archive created using git-tar-tree.
Instantly browse your working repository in gitweb.
Show three-way merge without touching index.
Reuse recorded resolution of conflicted merges.
Pick out and massage parameters.
A helper for git-status and git-commit.
Show branches and their commits.
Check the GPG signature of tag.
Show logs with difference each commit introduces.
Interacting with Others
These commands are to interact with foreign SCM and with other people
via patch over e-mail.
Import an Arch repository into git.
Export a single commit to a CVS checkout.
Salvage your data out of another SCM people love to hate.
A CVS server emulator for git.
Dump a mailbox from stdin into an imap folder.
Applies a quilt patchset onto the current branch.
Generates a summary of pending changes.
Send a collection of patches as emails.
Bidirectional operation between a single Subversion branch and git.
Import a SVN repository into git.
Although git includes its own porcelain layer, its low-level commands
are sufficient to support development of alternative porcelains.
Developers of such porcelains might start by reading about
git-update-index(1) and git-read-tree(1).
The interface (input, output, set of options and the semantics) to
these low-level commands are meant to be a lot more stable than
Porcelain level commands, because these commands are primarily for
scripted use. The interface to Porcelain commands on the other hand are
subject to change in order to improve the end user experience.
The following description divides the low-level commands into commands
that manipulate objects (in the repository, index, and working tree),
commands that interrogate and compare objects, and commands that move
objects and references between repositories.
Apply a patch on a git index file and a working tree.
Copy files from the index to the working tree.
Create a new commit object.
Compute object ID and optionally creates a blob from a file.
Build pack index file for an existing packed archive.
Run a three-way file merge.
Run a merge for files needing merging.
Creates a tag object.
Build a tree-object from ls-tree formatted text.
Create a packed archive of objects.
Remove extra objects that are already in pack files.
Reads tree information into the index.
Read and modify symbolic refs.
Unpack objects from a packed archive.
Register file contents in the working tree to the index.
Update the object name stored in a ref safely.
Create a tree object from the current index.
Provide content or type/size information for repository objects.
Compares files in the working tree and the index.
Compares content and mode of blobs between the index and
Compares the content and mode of blobs found via two tree objects.
Output information on each ref.
Show information about files in the index and the working tree.
List references in a remote repository.
List the contents of a tree object.
Find as good common ancestors as possible for a merge.
Find symbolic names for given revs.
Find redundant pack files.
Lists commit objects in reverse chronological order.
Show packed archive index.
List references in a local repository.
Create a tar archive of the files in the named tree object.
Creates a temporary file with a blob’s contents.
Show a git logical variable.
Validate packed git archive files.
In general, the interrogate commands do not touch the files in the
A really simple server for git repositories.
Receive missing objects from another repository.
Duplicate another git repository on a local system.
Push objects over git protocol to another repository.
Fetch from a remote repository over ssh connection.
Push to a remote repository over ssh connection.
Update auxiliary info file to help dumb servers.
The following are helper programs used by the above; end users
typically do not use them directly.
Download from a remote git repository via HTTP.
Push objects over HTTP/DAV to another repository.
Routines to help parsing remote repository access parameters.
Receive what is pushed into the repository.
Restricted login shell for GIT-only SSH access.
Send archive back to git-archive.
Send objects packed back to git-fetch-pack.
Internal helper commands
These are internal helper commands used by other commands; end users
typically do not use them directly.
Apply one patch extracted from an e-mail.
Display gitattributes information..
Make sure ref name is well formed.
Produce a merge commit message.
Extracts patch and authorship from a single e-mail message.
Simple UNIX mbox splitter program.
The standard helper program to use with git-merge-index.
Compute unique ID for a patch.
List the references in a remote repository.
Common git shell script setup code.
Filter out empty lines.
Starting from 0.99.9 (actually mid 0.99.8.GIT), .git/config file is
used to hold per-repository configuration options. It is a simple text
file modeled after .ini format familiar to some people. Here is an
# A ’#’ or ’;’ character indicates a comment.
; core variables
; Don’t trust file modes
filemode = false
; user identity
name = "Junio C Hamano"
email = "email@example.com"
Various commands read from the configuration file and adjust their
Indicates the object name for any type of object.
Indicates a blob object name.
Indicates a tree object name.
Indicates a commit object name.
Indicates a tree, commit or tag object name. A command that takes a
<tree-ish> argument ultimately wants to operate on a <tree> object
but automatically dereferences <commit> and <tag> objects that
point at a <tree>.
Indicates a commit or tag object name. A command that takes a
<commit-ish> argument ultimately wants to operate on a <commit>
object but automatically dereferences <tag> objects that point at a
Indicates that an object type is required. Currently one of: blob,
tree, commit, or tag.
Indicates a filename - almost always relative to the root of the
tree structure GIT_INDEX_FILE describes.
Any git command accepting any <object> can also use the following
indicates the head of the current branch (i.e. the contents of
a valid tag name (i.e. the contents of $GIT_DIR/refs/tags/<tag>).
a valid head name (i.e. the contents of
For a more complete list of ways to spell object names, see "SPECIFYING
REVISIONS" section in git-rev-parse(1).
Please see repository layout document.
Read hooks for more details about each hook.
Higher level SCMs may provide and manage additional information in the
Please see glossary document.
Various git commands use the following environment variables:
The git Repository
These environment variables apply to all core git commands. Nb: it is
worth noting that they may be used/overridden by SCMS sitting above git
so take care if using Cogito etc.
This environment allows the specification of an alternate index
file. If not specified, the default of $GIT_DIR/index is used.
If the object storage directory is specified via this environment
variable then the sha1 directories are created underneath -
otherwise the default $GIT_DIR/objects directory is used.
Due to the immutable nature of git objects, old objects can be
archived into shared, read-only directories. This variable
specifies a ":" separated list of git object directories which can
be used to search for git objects. New objects will not be written
to these directories.
If the GIT_DIR environment variable is set then it specifies a path
to use instead of the default .git for the base of the repository.
GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE, GIT_COMMITTER_NAME,
GIT_COMMITTER_EMAIL, GIT_COMMITTER_DATE, EMAIL
Only valid setting is "--unified=??" or "-u??" to set the number of
context lines shown when a unified diff is created. This takes
precedence over any "-U" or "--unified" option value passed on the
git diff command line.
When the environment variable GIT_EXTERNAL_DIFF is set, the program
named by it is called, instead of the diff invocation described
above. For a path that is added, removed, or modified,
GIT_EXTERNAL_DIFF is called with 7 parameters:
path old-file old-hex old-mode new-file new-hex new-mode
<old|new>-file are files
GIT_EXTERNAL_DIFF can use
to read the contents of
<old|new>-hex are the 40-hexdigit SHA1
<old|new>-mode are the octal
representation of the file
The file parameters can point at the user’s working file (e.g.
new-file in "git-diff-files"), /dev/null (e.g. old-file when a new
file is added), or a temporary file (e.g. old-file in the index).
GIT_EXTERNAL_DIFF should not worry about unlinking the temporary
file --- it is removed when GIT_EXTERNAL_DIFF exits.
For a path that is unmerged, GIT_EXTERNAL_DIFF is called with 1
This environment variable overrides $PAGER.
If this variable is set to "1", "2" or "true" (comparison is case
insensitive), git will print trace: messages on stderr telling
about alias expansion, built-in command execution and external
command execution. If this variable is set to an integer value
greater than 1 and lower than 10 (strictly) then git will interpret
this value as an open file descriptor and will try to write the
trace messages into this file descriptor. Alternatively, if this
variable is set to an absolute path (starting with a / character),
git will interpret this as a file path and will try to write the
trace messages into it.
"git" can mean anything, depending on your mood.
· random three-letter combination that is pronounceable, and not
actually used by any common UNIX command. The fact that it is a
mispronunciation of "get" may or may not be relevant.
· stupid. contemptible and despicable. simple. Take your pick from
the dictionary of slang.
· "global information tracker": you’re in a good mood, and it
actually works for you. Angels sing, and a light suddenly fills the
· "goddamn idiotic truckload of sh*t": when it breaks
This is a (not so) stupid but extremely fast directory content manager.
It doesn’t do a whole lot at its core, but what it does do is track
directory contents efficiently.
There are two object abstractions: the "object database", and the
"current directory cache" aka "index".
The Object Database
The object database is literally just a content-addressable collection
of objects. All objects are named by their content, which is
approximated by the SHA1 hash of the object itself. Objects may refer
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
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".
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 "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.
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
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.
Regardless of object type, all objects share the following
characteristics: they are all deflated with zlib, and have a header
that not only specifies their type, but also provides size information
about the data in the object. It’s worth noting that the SHA1 hash that
is used to name the object is the hash of the original data plus this
header, so sha1sum file does not match the object name for file.
(Historical note: in the dawn of the age of git the hash was the sha1
of the compressed object.)
As a result, the general consistency of an object can always be tested
independently of the contents or the type of the object: all objects
can be validated by verifying that (a) their hashes match the content
of the file and (b) the object successfully inflates to a stream of
bytes that forms a sequence of <ascii type without space> + <space> +
<ascii decimal size> + <byte\0> + <binary object data>.
The structured objects can further have their structure and
connectivity to other objects verified. This is generally done with the
git-fsck program, which generates a full dependency graph of all
objects, and verifies their internal consistency (in addition to just
verifying their superficial consistency through the hash).
The object types in some more detail:
A "blob" object is nothing but a binary blob of data, and doesn’t refer
to anything else. There is no signature or any other verification of
the data, so while the object is consistent (it is indexed by its sha1
hash, so the data itself is certainly correct), it has absolutely no
other attributes. No name associations, no permissions. It is purely a
blob of data (i.e. normally "file contents").
In particular, since the blob is entirely defined by its data, if two
files in a directory tree (or in multiple different versions of the
repository) have the same contents, they will share the same blob
object. The object is totally independent of its location in the
directory tree, and renaming a file does not change the object that
file is associated with in any way.
A blob is typically created when git-update-index(1) (or git-add(1)) is
run, and its data can be accessed by git-cat-file(1).
The next hierarchical object type is the "tree" object. A tree object
is a list of mode/name/blob data, sorted by name. Alternatively, the
mode data may specify a directory mode, in which case instead of naming
a blob, that name is associated with another TREE object.
Like the "blob" object, a tree object is uniquely determined by the set
contents, and so two separate but identical trees will always share the
exact same object. This is true at all levels, i.e. it’s true for a
"leaf" tree (which does not refer to any other trees, only blobs) as
well as for a whole subdirectory.
For that reason a "tree" object is just a pure data abstraction: it has
no history, no signatures, no verification of validity, except that
since the contents are again protected by the hash itself, we can trust
that the tree is immutable and its contents never change.
So you can trust the contents of a tree to be valid, the same way you
can trust the contents of a blob, but you don’t know where those
contents came from.
Side note on trees: since a "tree" object is a sorted list of
"filename+content", you can create a diff between two trees without
actually having to unpack two trees. Just ignore all common parts, and
your diff will look right. In other words, you can effectively (and
efficiently) tell the difference between any two random trees by O(n)
where "n" is the size of the difference, rather than the size of the
Side note 2 on trees: since the name of a "blob" depends entirely and
exclusively on its contents (i.e. there are no names or permissions
involved), you can see trivial renames or permission changes by
noticing that the blob stayed the same. However, renames with data
changes need a smarter "diff" implementation.
A tree is created with git-write-tree(1) and its data can be accessed
by git-ls-tree(1). Two trees can be compared with git-diff-tree(1).
The "commit" object is an object that introduces the notion of history
into the picture. In contrast to the other objects, it doesn’t just
describe the physical state of a tree, it describes how we got there,
A "commit" is defined by the tree-object that it results in, the parent
commits (zero, one or more) that led up to that point, and a comment on
what happened. Again, a commit is not trusted per se: the contents are
well-defined and "safe" due to the cryptographically strong signatures
at all levels, but there is no reason to believe that the tree is
"good" or that the merge information makes sense. 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 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 file
A commit is created with git-commit-tree(1) and its data can be
accessed by git-cat-file(1).
An aside on the notion of "trust". Trust is really outside the scope of
"git", but it’s worth noting a few things. First off, since everything
is hashed with SHA1, you can trust that an object is intact and has not
been messed with by external sources. So the name of an object uniquely
identifies a known state - just not a state that you may want to trust.
Furthermore, since the SHA1 signature of a commit refers to the SHA1
signatures of the tree it is associated with and the signatures of the
parent, a single named commit specifies uniquely a whole set of
history, with full contents. You can’t later fake any step of the way
once you have the name of a commit.
So to introduce some real trust in the system, the only thing you need
to do is to digitally sign just one special note, which includes the
name of a top-level commit. Your digital signature shows others that
you trust that commit, and the immutability of the history of commits
tells others that they can trust the whole history.
In other words, you can easily validate a whole archive by just sending
out a single email that tells the people the name (SHA1 hash) of the
top commit, and digitally sign that email using something like GPG/PGP.
To assist in this, git also provides the tag object...
Git provides the "tag" object to simplify creating, managing and
exchanging symbolic and signed tokens. The "tag" object at its simplest
simply symbolically identifies another object by containing the sha1,
type and symbolic name.
However it can optionally contain additional signature information
(which git doesn’t care about as long as there’s less than 8k of it).
This can then be verified externally to git.
Note that despite the tag features, "git" itself only handles content
integrity; the trust framework (and signature provision and
verification) has to come from outside.
A tag is created with git-mktag(1), its data can be accessed by
git-cat-file(1), and the signature can be verified by
The index is a simple binary file, which contains an efficient
representation of a virtual directory content at some random time. 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
specific rules) at any point in time, but the cache has no long-term
meaning, and can be partially updated at any time.
In particular, the index certainly does not need to be consistent with
the current directory contents (in fact, most operations will depend on
different ways to make the index not be consistent with the directory
hierarchy), but it has three very important attributes:
(a) it can re-generate the full state it caches (not just the directory
structure: it contains pointers to the "blob" objects so that it can
regenerate the data too)
As a special case, there is a clear and unambiguous one-way mapping
from a current directory cache to a "tree object", which can be
efficiently created from just the current directory cache without
actually looking at any other data. So a directory cache at any one
time uniquely specifies one and only one "tree" object (but has
additional data to make it easy to match up that tree object with what
has happened in the directory)
(b) it has efficient methods for finding inconsistencies between that
cached state ("tree object waiting to be instantiated") and the current
(c) it can additionally efficiently represent information about merge
conflicts between different tree objects, allowing each pathname to be
associated with sufficient information about the trees involved that
you can create a three-way merge between them.
Those are the three ONLY things that the directory cache does. It’s a
cache, and the normal operation is to re-generate it completely from a
known tree object, or update/compare it with a live tree that is being
developed. If you blow the directory cache away entirely, you generally
haven’t lost any information as long as you have the name of the tree
that it described.
At the same time, the index is at the same time also the staging area
for creating new trees, and creating a new tree always involves a
controlled modification of the index file. In particular, the index
file can have the representation of an intermediate tree that has not
yet been instantiated. So the index can be thought of as a write-back
cache, which can contain dirty information that has not yet been
written back to the backing store.
Generally, all "git" operations work on the index file. Some operations
work purely on the index file (showing the current state of the index),
but most operations move data to and from the index file. Either from
the database or from the working directory. Thus there are four main
1) working directory -> index
You update the index with information from the working directory with
the git-update-index(1) command. You generally update the index
information by just specifying the filename you want to update, like
but to avoid common mistakes with filename globbing etc, the command
will not normally add totally new entries or remove old entries, i.e.
it will normally just update existing cache entries.
To tell git that yes, you really do realize that certain files no
longer exist, or that new files should be added, you should use the
--remove and --add flags respectively.
NOTE! A --remove flag does not mean that subsequent filenames will
necessarily be removed: if the files still exist in your directory
structure, the index will be updated with their new status, not
removed. The only thing --remove means is that update-cache will be
considering a removed file to be a valid thing, and if the file really
does not exist any more, it will update the index accordingly.
As a special case, you can also do git-update-index --refresh, which
will refresh the "stat" information of each index to match the current
stat information. It will not update the object status itself, and it
will only update the fields that are used to quickly test whether an
object still matches its old backing store object.
2) index -> object database
You write your current index file to a "tree" object with the program
that doesn’t come with any options - it will just write out the current
index into the set of tree objects that describe that state, and it
will return the name of the resulting top-level tree. You can use that
tree to re-generate the index at any time by going in the other
3) object database -> index
You read a "tree" file from the object database, and use that to
populate (and overwrite - don’t do this if your index contains any
unsaved state that you might want to restore later!) your current
index. Normal operation is just
git-read-tree <sha1 of tree>
and your index file will now be equivalent to the tree that you saved
earlier. However, that is only your index file: your working directory
contents have not been modified.
4) index -> working directory
You update your working directory from the index by "checking out"
files. This is not a very common operation, since normally you’d just
keep your files updated, and rather than write to your working
directory, you’d tell the index files about the changes in your working
directory (i.e. git-update-index).
However, if you decide to jump to a new version, or check out somebody
else’s version, or just restore a previous tree, you’d populate your
index file with read-tree, and then you need to check out the result
or, if you want to check out all of the index, use -a.
NOTE! git-checkout-index normally refuses to overwrite old files, so if
you have an old version of the tree already checked out, you will need
to use the "-f" flag (before the "-a" flag or the filename) to force
Finally, there are a few odds and ends which are not purely moving from
one representation to the other:
5) Tying it all together
To commit a tree you have instantiated with "git-write-tree", you’d
create a "commit" object that refers to that tree and the history
behind it - most notably the "parent" commits that preceded it in
Normally a "commit" has one parent: the previous state of the tree
before a certain change was made. However, sometimes it can have two or
more parent commits, in which case we call it a "merge", due to the
fact that such a commit brings together ("merges") two or more previous
states represented by other commits.
In other words, while a "tree" represents a particular directory state
of a working directory, a "commit" represents that state in "time", and
explains how we got there.
You create a commit object by giving it the tree that describes the
state at the time of the commit, and a list of parents:
git-commit-tree <tree> -p <parent> [-p <parent2> ..]
and then giving the reason for the commit on stdin (either through
redirection from a pipe or file, or by just typing it at the tty).
git-commit-tree will return the name of the object that represents that
commit, and you should save it away for later use. Normally, you’d
commit a new HEAD state, and while git doesn’t care where you save the
note about that state, in practice we tend to just write the result to
the file pointed at by .git/HEAD, so that we can always see what the
last committed state was.
Here is an ASCII art by Jon Loeliger that illustrates how various
pieces fit together.
| Object DB |
| Backing |
| Store |
write-tree | |
tree obj | |
| | read-tree
| | tree obj
| Index |
| "cache" |
blob obj | |
checkout-index -u | | checkout-index
stat | | blob obj
| Working |
| Directory |
6) Examining the data
You can examine the data represented in the object database and the
index with various helper tools. For every object, you can use
git-cat-file(1) to examine details about the object:
git-cat-file -t <objectname>
shows the type of the object, and once you have the type (which is
usually implicit in where you find the object), you can use
git-cat-file blob|tree|commit|tag <objectname>
to show its contents. NOTE! Trees have binary content, and as a result
there is a special helper for showing that content, called git-ls-tree,
which turns the binary content into a more easily readable form.
It’s especially instructive to look at "commit" objects, since those
tend to be small and fairly self-explanatory. In particular, if you
follow the convention of having the top commit name in .git/HEAD, you
git-cat-file commit HEAD
to see what the top commit was.
7) Merging multiple trees
Git helps you do a three-way merge, which you can expand to n-way by
repeating the merge procedure arbitrary times until you finally
"commit" the state. The normal situation is that you’d only do one
three-way merge (two parents), and commit it, but if you like to, you
can do multiple parents in one go.
To do a three-way merge, you need the two sets of "commit" objects that
you want to merge, use those to find the closest common parent (a third
"commit" object), and then use those commit objects to find the state
of the directory ("tree" object) at these points.
To get the "base" for the merge, you first look up the common parent of
two commits with
git-merge-base <commit1> <commit2>
which will return you the commit they are both based on. You should now
look up the "tree" objects of those commits, which you can easily do
with (for example)
git-cat-file commit <commitname> | head -1
since the tree object information is always the first line in a commit
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 you want to merge), you do a "merge" read into the index.
This will complain if it has to throw away your old index contents, so
you should make sure that you’ve committed those - in fact you would
normally always do a merge against your last commit (which should thus
match what you have in your current index anyway).
To do the merge, do
git-read-tree -m -u <origtree> <yourtree> <targettree>
which will do all trivial merge operations for you directly in the
index file, and you can just write the result out with git-write-tree.
Historical note. We did not have -u facility when this section was
first written, so we used to warn that the merge is done in the index
file, not in your working tree, and your working tree will not match
your index after this step. This is no longer true. The above command,
thanks to -u option, updates your working tree with the merge results
for paths that have been trivially merged.
8) Merging multiple trees, continued
Sadly, many merges aren’t trivial. If there are files that have been
added.moved or removed, or if both branches have modified the same
file, you will be left with an index tree that contains "merge entries"
in it. Such an index tree can NOT be written out to a tree object, and
you will have to resolve any such merge clashes using other tools
before you can write out the result.
You can examine such index state with git-ls-files --unmerged command.
$ git-read-tree -m $orig HEAD $target
$ git-ls-files --unmerged
100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello.c
100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello.c
100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello.c
Each line of the git-ls-files --unmerged output begins with the blob
mode bits, blob SHA1, stage number, and the filename. The stage number
is git’s way to say which tree it came from: stage 1 corresponds to
$orig tree, stage 2 HEAD tree, and stage3 $target tree.
Earlier we said that trivial merges are done inside git-read-tree -m.
For example, if the file did not change from $orig to HEAD nor $target,
or if the file changed from $orig to HEAD and $orig to $target the same
way, obviously the final outcome is what is in HEAD. What the above
example shows is that file hello.c was changed from $orig to HEAD and
$orig to $target in a different way. You could resolve this by running
your favorite 3-way merge program, e.g. diff3 or merge, 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
This would leave the merge result in hello.c~2 file, along with
conflict markers if there are conflicts. After verifying the merge
result makes sense, you can tell git what the final merge result for
this file is by:
mv -f hello.c~2 hello.c
When a path is in unmerged state, running git-update-index for that
path tells git to mark the path resolved.
The above is the description of a git merge at the lowest level, to
help you understand what conceptually happens under the hood. In
practice, nobody, not even git itself, uses three git-cat-file for
this. There is git-merge-index program that extracts the stages to
temporary files and calls a "merge" script on it:
git-merge-index git-merge-one-file hello.c
and that is what higher level git merge -s resolve is implemented with.
· git’s founding father is Linus Torvalds <firstname.lastname@example.org>.
· The current git nurse is Junio C Hamano <email@example.com>.
· The git potty was written by Andres Ericsson <firstname.lastname@example.org>.
· General upbringing is handled by the git-list
The documentation for git suite was started by David Greaves
<email@example.com>, and later enhanced greatly by the contributors on
the git-list <firstname.lastname@example.org>.
Part of the git(7) suite
2. Everyday Git
3. CVS migration
4. Git User’s Manual
5. Core tutorial
7. repository layout