git(7)

NAME

git - the stupid content tracker

SYNOPSIS

git      [--version]      [--exec-path[=GIT_EXEC_PATH]]
[-p|--paginate]
    [--bare]   [--git-dir=GIT_DIR]   [--help]   COMMAND
[ARGS]

DESCRIPTION

Git is a fast, scalable, distributed revision control sys

tem with an unusually rich command set that provides both high

level operations and full access to internals.

See this [1]tutorial to get started, then see [2]Everyday

Git for a useful minimum set of commands, and "man git-command

name" for documentation of each command. CVS users may also want

to read [3]CVS migration.

The COMMAND is either a name of a Git command (see below)

or an alias as defined in the configuration file (see

git-repo-config(1)).

OPTIONS

--version

Prints the git suite version that the git program came

from.

--help

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.

--exec-path

Path to wherever your core git programs are installed.

This can also be controlled by setting the GIT_EXEC_PATH environ

ment variable. If no path is given git will print the current

setting and then exit.

-p|--paginate

Pipe all output into less (or if set, $PAGER).

--git-dir=<path>

Set the path to the repository. This can also be con

trolled by setting the GIT_DIR environment variable.

--bare

Same as --git-dir=pwd.

FURTHER DOCUMENTATION

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 [4]Core tutorial both

provide introductions to the underlying git architecture.

See also the [5]howto documents for some useful examples.

GIT COMMANDS

We divide git into high level ("porcelain") commands and

low level ("plumbing") commands.

HIGH-LEVEL COMMANDS (PORCELAIN)

We separate the porcelain commands into the main commands

and some ancillary user utilities.

Main porcelain 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 Li

nus.

git-archive(1)

Creates an archive of files from a named tree.

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-clean(1)

Remove untracked files from the working tree.

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 proto

cols.

git-format-patch(1)

Prepare patches for e-mail submission.

git-grep(1)

Print lines matching a pattern.

gitk(1)

The git repository browser.

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-pack-refs(1)

Pack heads and tags for efficient repository access.

git-pull(1)

Fetch from and merge with a remote repository or a lo

cal branch.

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-rerere(1)

Reuse recorded resolution of conflicted merges.

git-reset(1)

Reset current HEAD to the specified state.

git-resolve(1)

Merge two commits.

git-revert(1)

Revert an existing commit.

git-rm(1)

Remove files from the working tree and from the index.

git-shortlog(1)

Summarizes git log output.

git-show(1)

Show one commit log and its diff.

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-cvsserver(1)

A CVS server emulator for git.

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-in

dex.

git-prune(1)

Prunes all unreachable objects from the object

database.

git-quiltimport(1)

Applies a quilt patchset onto the current branch.

git-relink(1)

Hardlink common objects in local repositories.

git-svn(1)

Bidirectional operation between a single Subversion

branch and git.

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-annotate(1)

Annotate file lines with commit info.

git-blame(1)

Find out where each line in a file came from.

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 con

sumption.

git-daemon(1)

A really simple server for git repositories.

git-fmt-merge-msg(1)

Produce a merge commit message.

git-get-tar-commit-id(1)

Extract commit ID from an archive created using git

tar-tree.

git-imap-send(1)

Dump a mailbox from stdin into an imap folder.

git-instaweb(1)

Instantly browse your working repository in gitweb.

git-mailinfo(1)

Extracts patch and authorship information from a single

e-mail message, optionally transliterating the commit message in

to utf-8.

git-mailsplit(1)

A stupid program to split UNIX mbox format mailbox into

individual pieces of e-mail.

git-merge-tree(1)

Show three-way merge without touching index.

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-runstatus(1)

A helper for git-status and git-commit.

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.

LOW-LEVEL COMMANDS (PLUMBING)

Although git includes its own porcelain layer, its low

level commands are sufficient to support development of alterna

tive porcelains. Developers of such porcelains might start by

reading about git-update-index(1) and git-read-tree(1).

We divide the low-level commands into commands that manip

ulate objects (in the repository, index, and working tree), com

mands that interrogate and compare objects, and commands that

move objects and references between repositories.

Manipulation commands

git-apply(1)

Reads a "diff -up1" or git generated patch file and ap

plies 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-mktree(1)

Build a tree-object from ls-tree formatted text.

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 com

mit.

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-for-each-ref(1)

Output information on each ref.

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-show-ref(1)

List references in a local repository.

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-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-archive(1)

Invoked by git-archive to send a generated archive.

git-upload-pack(1)

Invoked by git-fetch-pack to push what are asked for.

CONFIGURATION MECHANISM

Starting from 0.99.9 (actually mid 0.99.8.GIT), .git/con

fig 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 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 ad

just 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 con

tents 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>).

For a more complete list of ways to spell object names,

see "SPECIFYING REVISIONS" section in git-rev-parse(1).

FILE/DIRECTORY STRUCTURE

Please see [6]repository layout document.

Read [7]hooks for more details about each hook.

Higher level SCMs may provide and manage additional infor

mation in the $GIT_DIR.

TERMINOLOGY

Please see [8]glossary document.

ENVIRONMENT VARIABLES

Various git commands use the following environment vari

ables:

The git Repository

These environment variables apply to all core git com

mands. 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 alter

nate 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 under

neath - 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)

other

GIT_PAGER

This environment variable overrides $PAGER.

GIT_TRACE

If this variable is set to "1", "2" or "true" (compari

son 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 in

teger 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. Alterna

tively, 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.

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-address

able collection of objects. All objects are named by their con

tent, which is approximated by the SHA1 hash of the object it

self. 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 re

fer 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 ob

ject 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 hier

archy.

A "commit" object ties such directory hierarchies together

into a DAG of revisions - each "commit" is associated with exact

ly one tree (the directory hierarchy at the time of the commit).

In addition, a "commit" refers to one or more "parent" commit ob

jects that describe the history of how we arrived at that direc

tory 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 anoth

er object, a symbolic name (of course!) and, optionally, a signa

ture.

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 al

ways 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 success

fully inflates to a stream of bytes that forms a sequence of

<ascii type without space> + <space> + <ascii decimal size> +

<byte > + <binary object data>.

The structured objects can further have their structure

and connectivity to other objects verified. This is generally

done with the git-fsck-objects program, which generates a full

dependency graph of all objects, and verifies their internal con

sistency (in addition to just verifying their superficial consis

tency 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 oth

er 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 associa

tions, 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. Al

ternatively, the mode data may specify a directory mode, in which

case instead of naming a blob, that name is associated with an

other TREE object.

Like the "blob" object, a tree object is uniquely deter

mined 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 re

fer to any other trees, only blobs) as well as for a whole subdi

rectory.

For that reason a "tree" object is just a pure data ab

straction: 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 com

mon 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 permis

sion 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 no

tion of history into the picture. In contrast to the other ob

jects, 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 re

sult 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 unique

ly 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 sig

nature shows others that you trust that commit, and the im

mutability 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 us

ing something like GPG/PGP.

To assist in this, git also provides the tag object...

Tag Object

Git provides the "tag" object to simplify creating, manag

ing 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 in

formation (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 han

dles content integrity; the trust framework (and signature provi

sion and verification) has to come from outside.

A tag is created with git-mktag(1), its data can be ac

cessed 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 effi

cient representation of a virtual directory content at some ran

dom time. It does so by a simple array that associates a set of

names, dates, permissions and content (aka "blob") objects to

gether. 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 op

erations 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 al

ways involves a controlled modification of the index file. In

particular, the index file can have the representation of an in

termediate 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 cur

rent 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 di

rectory with the git-update-index(1) command. You generally up

date 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 en

tries.

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 file

names 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 --re

fresh, 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-lev

el 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 pure

ly 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 com

mits.

In other words, while a "tree" represents a particular di

rectory 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 de

scribes the state at the time of the commit, and a list of par

ents:

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 prac

tice 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 ob

ject, 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 par

ticular, 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 com

mit 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 cur

rent index anyway).

To do the merge, do

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

which will do all trivial merge operations for you direct

ly 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 work

ing 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 modi

fied the same file, you will be left with an index tree that con

tains "merge entries" in it. Such an index tree can NOT be writ

ten out to a tree object, and you will have to resolve any such

merge clashes using other tools before you can write out the re

sult.

You can examine such index state with git-ls-files --un

merged 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 file

name. 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 hel

lo.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@os

dl.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

REFERENCES

1. tutorial

tutorial.html

2. Everyday Git
everyday.html

3. CVS migration
cvs-migration.html

4. Core tutorial
core-tutorial.html

5. howto
howto-index.html

6. repository layout
repository-layout.html

7. hooks
hooks.html

8. glossary
glossary.html

03/08/2007

GIT(7)