Provided by:
git-core_1.1.3-1ubuntu1_i386 
NAME
git - the stupid content tracker
SYNOPSIS
git [--version] [--exec-path[=GIT_EXEC_PATH]] [--help] COMMAND [ARGS]
DESCRIPTION
git is both a program and a directory content tracker system. The
program git is just a wrapper to reach the core git programs (or a
potty if you like, as it’s not exactly porcelain but still brings your
stuff to the plumbing).
OPTIONS
--version
prints the git suite version that the git program came from.
--help prints the synopsis and a list of available commands. If a git
command is named this option will bring up the man-page for that
command.
--exec-path
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.
NOT LEARNING CORE GIT COMMANDS
This manual is intended to give complete background information and
internal workings of git, which may be too much for most people. The
[xref to anchor] section below contains much useful definition and
clarification - read that first.
If you are interested in using git to manage (version control)
projects, use Everyday GIT: everyday.html as a guide to the minimum set
of commands you need to know for day-to-day work. Most likely, that
will get you started, and you can go a long way without knowing the low
level details too much.
The tutorial: tutorial.html document covers how things internally work.
If you are migrating from CVS, cvs migration: cvs-migration.html
document may be helpful after you finish the tutorial.
After you get the general feel from the tutorial and this overview
page, you may want to take a look at the howto: howto-index.html
documents.
CORE GIT COMMANDS
If you are writing your own Porcelain, you need to be familiar with
most of the low level commands --- I suggest starting from
git-update-index(1) and git-read-tree(1).
COMMANDS OVERVIEW
The git commands can helpfully be split into those that manipulate the
repository, the index and the files in the working tree, those that
interrogate and compare them, and those that moves objects and
references between repositories.
In addition, git itself comes with a spartan set of porcelain commands.
They are usable but are not meant to compete with real Porcelains.
There are also some ancillary programs that can be viewed as useful
aids for using the core commands but which are unlikely to be used by
SCMs layered over git.
Manipulation commands
git-apply(1)
Reads a "diff -up1" or git generated patch file and applies it
to the working tree.
git-checkout-index(1)
Copy files from the index to the working tree.
git-commit-tree(1)
Creates a new commit object.
git-hash-object(1)
Computes the object ID from a file.
git-index-pack(1)
Build pack idx file for an existing packed archive.
git-init-db(1)
Creates an empty git object database, or reinitialize an
existing one.
git-merge-index(1)
Runs a merge for files needing merging.
git-mktag(1)
Creates a tag object.
git-pack-objects(1)
Creates a packed archive of objects.
git-prune-packed(1)
Remove extra objects that are already in pack files.
git-read-tree(1)
Reads tree information into the index.
git-repo-config(1)
Get and set options in .git/config.
git-unpack-objects(1)
Unpacks objects out of a packed archive.
git-update-index(1)
Registers files in the working tree to the index.
git-write-tree(1)
Creates a tree from the index.
Interrogation commands
git-cat-file(1)
Provide content or type/size information for repository objects.
git-describe(1)
Show the most recent tag that is reachable from a commit.
git-diff-index(1)
Compares content and mode of blobs between the index and
repository.
git-diff-files(1)
Compares files in the working tree and the index.
git-diff-stages(1)
Compares two "merge stages" in the index.
git-diff-tree(1)
Compares the content and mode of blobs found via two tree
objects.
git-fsck-objects(1)
Verifies the connectivity and validity of the objects in the
database.
git-ls-files(1)
Information about files in the index and the working tree.
git-ls-tree(1)
Displays a tree object in human readable form.
git-merge-base(1)
Finds as good common ancestors as possible for a merge.
git-name-rev(1)
Find symbolic names for given revs.
git-pack-redundant(1)
Find redundant pack files.
git-rev-list(1)
Lists commit objects in reverse chronological order.
git-show-index(1)
Displays contents of a pack idx file.
git-tar-tree(1)
Creates a tar archive of the files in the named tree object.
git-unpack-file(1)
Creates a temporary file with a blob’s contents.
git-var(1)
Displays a git logical variable.
git-verify-pack(1)
Validates packed git archive files.
In general, the interrogate commands do not touch the files in
the working tree.
Synching repositories
git-clone-pack(1)
Clones a repository into the current repository (engine for ssh
and local transport).
git-fetch-pack(1)
Updates from a remote repository (engine for ssh and local
transport).
git-http-fetch(1)
Downloads a remote git repository via HTTP by walking commit
chain.
git-local-fetch(1)
Duplicates another git repository on a local system by walking
commit chain.
git-peek-remote(1)
Lists references on a remote repository using upload-pack
protocol (engine for ssh and local transport).
git-receive-pack(1)
Invoked by git-send-pack to receive what is pushed to it.
git-send-pack(1)
Pushes to a remote repository, intelligently.
git-http-push(1)
Push missing objects using HTTP/DAV.
git-shell(1)
Restricted shell for GIT-only SSH access.
git-ssh-fetch(1)
Pulls from a remote repository over ssh connection by walking
commit chain.
git-ssh-upload(1)
Helper "server-side" program used by git-ssh-fetch.
git-update-server-info(1)
Updates auxiliary information on a dumb server to help clients
discover references and packs on it.
git-upload-pack(1)
Invoked by git-clone-pack and git-fetch-pack to push what are
asked for.
PORCELAIN-ISH COMMANDS
git-add(1)
Add paths to the index.
git-am(1)
Apply patches from a mailbox, but cooler.
git-applymbox(1)
Apply patches from a mailbox, original version by Linus.
git-bisect(1)
Find the change that introduced a bug by binary search.
git-branch(1)
Create and Show branches.
git-checkout(1)
Checkout and switch to a branch.
git-cherry-pick(1)
Cherry-pick the effect of an existing commit.
git-clone(1)
Clones a repository into a new directory.
git-commit(1)
Record changes to the repository.
git-diff(1)
Show changes between commits, commit and working tree, etc.
git-fetch(1)
Download from a remote repository via various protocols.
git-format-patch(1)
Prepare patches for e-mail submission.
git-grep(1)
Print lines matching a pattern.
git-log(1)
Shows commit logs.
git-ls-remote(1)
Shows references in a remote or local repository.
git-merge(1)
Grand unified merge driver.
git-mv(1)
Move or rename a file, a directory, or a symlink.
git-pull(1)
Fetch from and merge with a remote repository.
git-push(1)
Update remote refs along with associated objects.
git-rebase(1)
Rebase local commits to the updated upstream head.
git-repack(1)
Pack unpacked objects in a repository.
git-reset(1)
Reset current HEAD to the specified state.
git-resolve(1)
Merge two commits.
git-revert(1)
Revert an existing commit.
git-shortlog(1)
Summarizes git log output.
git-show-branch(1)
Show branches and their commits.
git-status(1)
Shows the working tree status.
git-verify-tag(1)
Check the GPG signature of tag.
git-whatchanged(1)
Shows commit logs and differences they introduce.
ANCILLARY COMMANDS
Manipulators:
git-applypatch(1)
Apply one patch extracted from an e-mail.
git-archimport(1)
Import an arch repository into git.
git-convert-objects(1)
Converts old-style git repository.
git-cvsimport(1)
Salvage your data out of another SCM people love to hate.
git-cvsexportcommit(1)
Export a single commit to a CVS checkout.
git-lost-found(1)
Recover lost refs that luckily have not yet been pruned.
git-merge-one-file(1)
The standard helper program to use with git-merge-index.
git-prune(1)
Prunes all unreachable objects from the object database.
git-relink(1)
Hardlink common objects in local repositories.
git-svnimport(1)
Import a SVN repository into git.
git-sh-setup(1)
Common git shell script setup code.
git-symbolic-ref(1)
Read and modify symbolic refs.
git-tag(1)
An example script to create a tag object signed with GPG.
git-update-ref(1)
Update the object name stored in a ref safely.
Interrogators:
git-check-ref-format(1)
Make sure ref name is well formed.
git-cherry(1)
Find commits not merged upstream.
git-count-objects(1)
Count unpacked number of objects and their disk consumption.
git-daemon(1)
A really simple server for git repositories.
git-get-tar-commit-id(1)
Extract commit ID from an archive created using git-tar-tree.
git-mailinfo(1)
Extracts patch and authorship information from a single e-mail
message, optionally transliterating the commit message into
utf-8.
git-mailsplit(1)
A stupid program to split UNIX mbox format mailbox into
individual pieces of e-mail.
git-patch-id(1)
Compute unique ID for a patch.
git-parse-remote(1)
Routines to help parsing $GIT_DIR/remotes/ files.
git-request-pull(1)
git-request-pull.
git-rev-parse(1)
Pick out and massage parameters.
git-send-email(1)
Send patch e-mails out of "format-patch --mbox" output.
git-symbolic-ref(1)
Read and modify symbolic refs.
git-stripspace(1)
Filter out empty lines.
COMMANDS NOT YET DOCUMENTED
gitk(1)
The gitk repository browser.
CONFIGURATION MECHANISM
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 modelled after .ini format familiar to some people. Here is an
example:
# # A ’#’ or ’;’ character indicates a comment. #
; core variables [core]
; Don’t trust file modes
filemode = false
; user identity [user]
name = "Junio C Hamano"
email = "junkio@twinsun.com"
Various commands read from the configuration file and adjust
their operation accordingly.
IDENTIFIER TERMINOLOGY
<object>
Indicates the object name for any type of object.
<blob> Indicates a blob object name.
<tree> Indicates a tree object name.
<commit>
Indicates a commit object name.
<tree-ish>
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>.
<type> Indicates that an object type is required. Currently one of:
blob, tree, commit, or tag.
<file> Indicates a filename - almost always relative to the root of the
tree structure GIT_INDEX_FILE describes.
SYMBOLIC IDENTIFIERS
Any git command accepting any <object> can also use the following
symbolic notation:
HEAD indicates the head of the current branch (i.e. the contents of
$GIT_DIR/HEAD).
<tag> a valid tag name (i.e. the contents of
$GIT_DIR/refs/tags/<tag>).
<head> a valid head name (i.e. the contents of
$GIT_DIR/refs/heads/<head>).
<snap> a valid snapshot name (i.e. the contents of
$GIT_DIR/refs/snap/<snap>).
FILE/DIRECTORY STRUCTURE
Please see repository layout: repository-layout.html document.
Higher level SCMs may provide and manage additional information in the
$GIT_DIR.
TERMINOLOGY
Please see glossary: glossary.html document.
ENVIRONMENT VARIABLES
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.
GIT_INDEX_FILE
This environment allows the specification of an alternate index
file. If not specified, the default of $GIT_DIR/index is used.
GIT_OBJECT_DIRECTORY
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.
GIT_ALTERNATE_OBJECT_DIRECTORIES
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.
GIT_DIR
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 Commits
GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE, GIT_COMMITTER_NAME,
GIT_COMMITTER_EMAIL
see git-commit-tree(1)
git Diffs
GIT_DIFF_OPTS, GIT_EXTERNAL_DIFF
see the "generating patches" section in : git-diff-index(1);
git-diff-files(1); git-diff-tree(1)
DISCUSSION
"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 room.
· "goddamn idiotic truckload of sh*t": when it breaks
This is a stupid (but extremely fast) directory content manager. It
doesn’t do a whole lot, 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-objects 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:
Blob Object
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) is run, and its
data can be accessed by git-cat-file(1).
Tree Object
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
tree.
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).
Commit Object
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,
and why.
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
manager.
A commit is created with git-commit-tree(1) and its data can be
accessed by git-cat-file(1).
Trust
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...
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
git-verify-tag(1).
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
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
state.
(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.
THE WORKFLOW
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
combinations:
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
so:
git-update-index filename
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
git-write-tree
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
direction:
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
with
git-checkout-index filename
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
the checkout.
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
history.
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.
commit-tree
commit obj
+----+
| |
| |
V V
+-----------+
| Object DB |
| Backing |
| Store |
+-----------+
^
write-tree | |
tree obj | |
| | read-tree
| | tree obj
V
+-----------+
| Index |
| "cache" |
+-----------+
update-index ^
blob obj | |
| |
checkout-index -u | | checkout-index
stat | | blob obj
V
+-----------+
| Working |
| Directory |
+-----------+
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
can do
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
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 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.
An example:
$ git-read-tree -m $orig HEAD $target $ git-ls-files --unmerged
100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello.c
100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello.c
100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello.c
Each line of the git-ls-files --unmerged output begins with the
blob mode bits, blob 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
git-update-index 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 resolve is implemented with.
AUTHORS
· git’s founding father is Linus Torvalds <torvalds@osdl.org>.
· The current git nurse is Junio C Hamano <junkio@cox.net>.
· The git potty was written by Andres Ericsson <ae@op5.se>.
· General upbringing is handled by the git-list <git@vger.kernel.org>.
DOCUMENTATION
The documentation for git suite was started by David Greaves
<david@dgreaves.com>, and later enhanced greatly by the contributors on
the git-list <git@vger.kernel.org>.
GIT
Part of the git(7) suite
GIT(7)