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.

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)).

Git User's Manual

Table of Contents

Preface

1. Git Quick Start

Creating a new repository
Managing branches
Exploring history
Making changes
Merging
Sharing your changes
Repository maintenance

2. Repositories and Branches

How to get a git repository

How to check out a different version of a project

Understanding History: Commits

Understanding history: commits, parents, and reachability
Undestanding history: History diagrams
Understanding history: What is a branch?

Manipulating branches

Examining branches from a remote repository

Naming branches, tags, and other references

Updating a repository with git fetch

Fetching branches from other repositories

3. Exploring git history

How to use bisect to find a regression

Naming commits

Creating tags

Browsing revisions

Generating diffs

Viewing old file versions

Examples

Check whether two branches point at the same history
Find first tagged version including a given fix

4. Developing with git

Telling git your name

Creating a new repository

how to make a commit

creating good commit messages

how to merge

Resolving a merge

undoing a merge

Fast-forward merges

Fixing mistakes

Fixing a mistake with a new commit
Fixing a mistake by editing history
Checking out an old version of a file

Ensuring good performance

Ensuring reliability

Checking the repository for corruption
Recovering lost changes

5. Sharing development with others

Getting updates with git pull
Submitting patches to a project
Importing patches to a project
Setting up a public repository
Exporting a git repository via http
Exporting a git repository via the git protocol
Pushing changes to a public repository
Setting up a shared repository
Allow web browsing of a repository
Examples

6. Rewriting history and maintaining patch series

Creating the perfect patch series
Keeping a patch series up to date using git-rebase
Reordering or selecting from a patch series
Other tools
Problems with rewriting history

7. Advanced branch management

Fetching individual branches
Understanding git history: fast-forwards
Forcing git fetch to do non-fast-forward updates
Configuring remote branches

8. Git internals

The Object Database

The "index" aka "Current Directory Cache"

The Workflow

working directory -> index
index -> object database
object database -> index
index -> working directory
Tying it all together

Examining the data

Merging multiple trees

Merging multiple trees, continued

How git stores objects efficiently: pack files

Dangling objects

9. Glossary of git terms

10. Notes and todo list for this manual

Preface

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

Chapter 1 gives a brief overview of git commands, without any +explanation; you may prefer to skip to chapter 2 on a first reading.

Chapters 2 and 3 explain how to fetch and study a project using +git—the tools you'd need to build and test a particular version of a +software project, to search for regressions, and so on.

Chapter 4 explains how to do development with git, and chapter 5 how +to share that development with others.

Further chapters cover more specialized topics.

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

$ man git-clone

Chapter 1. Git Quick Start

Table of Contents

Creating a new repository
Managing branches
Exploring history
Making changes
Merging
Sharing your changes
Repository maintenance

This is a quick summary of the major commands; the following chapters +will explain how these work in more detail.

Creating a new repository

From a tarball:

$ tar xzf project.tar.gz
+$ cd project
+$ git init
+Initialized empty Git repository in .git/
+$ git add .
+$ git commit

From a remote repository:

$ git clone git://example.com/pub/project.git
+$ cd project

Managing branches

$ git branch         # list all branches in this repo
+$ git checkout test  # switch working directory to branch "test"
+$ git branch new     # create branch "new" starting at current HEAD
+$ git branch -d new  # delete branch "new"

Instead of basing new branch on current HEAD (the default), use:

$ git branch new test    # branch named "test"
+$ git branch new v2.6.15 # tag named v2.6.15
+$ git branch new HEAD^   # commit before the most recent
+$ git branch new HEAD^^  # commit before that
+$ git branch new test~10 # ten commits before tip of branch "test"

Create and switch to a new branch at the same time:

$ git checkout -b new v2.6.15

Update and examine branches from the repository you cloned from:

$ git fetch             # update
+$ git branch -r         # list
+  origin/master
+  origin/next
+  ...
+$ git branch checkout -b masterwork origin/master

Fetch a branch from a different repository, and give it a new +name in your repository:

$ git fetch git://example.com/project.git theirbranch:mybranch
+$ git fetch git://example.com/project.git v2.6.15:mybranch

Keep a list of repositories you work with regularly:

$ git remote add example git://example.com/project.git
+$ git remote                    # list remote repositories
+example
+origin
+$ git remote show example       # get details
+* remote example
+  URL: git://example.com/project.git
+  Tracked remote branches
+    master next ...
+$ git fetch example             # update branches from example
+$ git branch -r                 # list all remote branches

Exploring history

$ gitk                      # visualize and browse history
+$ git log                   # list all commits
+$ git log src/              # ...modifying src/
+$ git log v2.6.15..v2.6.16  # ...in v2.6.16, not in v2.6.15
+$ git log master..test      # ...in branch test, not in branch master
+$ git log test..master      # ...in branch master, but not in test
+$ git log test...master     # ...in one branch, not in both
+$ git log -S'foo()'         # ...where difference contain "foo()"
+$ git log --since="2 weeks ago"
+$ git log -p                # show patches as well
+$ git show                  # most recent commit
+$ git diff v2.6.15..v2.6.16 # diff between two tagged versions
+$ git diff v2.6.15..HEAD    # diff with current head
+$ git grep "foo()"          # search working directory for "foo()"
+$ git grep v2.6.15 "foo()"  # search old tree for "foo()"
+$ git show v2.6.15:a.txt    # look at old version of a.txt

Search for regressions:

$ git bisect start
+$ git bisect bad                # current version is bad
+$ git bisect good v2.6.13-rc2   # last known good revision
+Bisecting: 675 revisions left to test after this
+                                # test here, then:
+$ git bisect good               # if this revision is good, or
+$ git bisect bad                # if this revision is bad.
+                                # repeat until done.

Making changes

Make sure git knows who to blame:

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

Select file contents to include in the next commit, then make the +commit:

$ git add a.txt    # updated file
+$ git add b.txt    # new file
+$ git rm c.txt     # old file
+$ git commit

Or, prepare and create the commit in one step:

$ git commit d.txt # use latest content only of d.txt
+$ git commit -a # use latest content of all tracked files

Merging

$ git merge test   # merge branch "test" into the current branch
+$ git pull git://example.com/project.git master
+                   # fetch and merge in remote branch
+$ git pull . test  # equivalent to git merge test

Sharing your changes

Importing or exporting patches:

$ git format-patch origin..HEAD # format a patch for each commit
+ # in HEAD but not in origin
+$ git-am mbox # import patches from the mailbox "mbox"

Fetch a branch in a different git repository, then merge into the +current branch:

$ git pull git://example.com/project.git theirbranch

Store the fetched branch into a local branch before merging into the +current branch:

$ git pull git://example.com/project.git theirbranch:mybranch

After creating commits on a local branch, update the remote +branch with your commits:

$ git push ssh://example.com/project.git mybranch:theirbranch

When remote and local branch are both named "test":

$ git push ssh://example.com/project.git test

Shortcut version for a frequently used remote repository:

$ git remote add example ssh://example.com/project.git
+$ git push example test

Repository maintenance

Check for corruption:

$ git fsck

Recompress, remove unused cruft:

$ git gc

Chapter 2. Repositories and Branches

Table of Contents

How to get a git repository

How to check out a different version of a project

Understanding History: Commits

Understanding history: commits, parents, and reachability
Undestanding history: History diagrams
Understanding history: What is a branch?

Manipulating branches

Examining branches from a remote repository

Naming branches, tags, and other references

Updating a repository with git fetch

Fetching branches from other repositories

How to get a git repository

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

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

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

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

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

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

How to check out a different version of a project

Git is best thought of as a tool for storing the history of a +collection of files. It stores the history as a compressed +collection of interrelated snapshots (versions) of the project's +contents.

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

$ git branch
+* master

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

Most projects also use tags. Tags, like branches, are references +into the project's history, and can be listed using the +git-tag(1) command:

$ git tag -l
+v2.6.11
+v2.6.11-tree
+v2.6.12
+v2.6.12-rc2
+v2.6.12-rc3
+v2.6.12-rc4
+v2.6.12-rc5
+v2.6.12-rc6
+v2.6.13
+...

Tags are expected to always point at the same version of a project, +while branches are expected to advance as development progresses.

Create a new branch pointing to one of these versions and check it +out using git-checkout(1):

$ git checkout -b new v2.6.13

The working directory then reflects the contents that the project had +when it was tagged v2.6.13, and git-branch(1) shows two +branches, with an asterisk marking the currently checked-out branch:

$ git branch
+ master
+* new

If you decide that you'd rather see version 2.6.17, you can modify +the current branch to point at v2.6.17 instead, with

$ git reset --hard v2.6.17

Note that if the current branch was your only reference to a +particular point in history, then resetting that branch may leave you +with no way to find the history it used to point to; so use this +command carefully.

Understanding History: Commits

Every change in the history of a project is represented by a commit. +The git-show(1) command shows the most recent commit on the +current branch:

$ git show
+commit 2b5f6dcce5bf94b9b119e9ed8d537098ec61c3d2
+Author: Jamal Hadi Salim <hadi@cyberus.ca>
+Date:   Sat Dec 2 22:22:25 2006 -0800
+
+    [XFRM]: Fix aevent structuring to be more complete.
+
+    aevents can not uniquely identify an SA. We break the ABI with this
+    patch, but consensus is that since it is not yet utilized by any
+    (known) application then it is fine (better do it now than later).
+
+    Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
+    Signed-off-by: David S. Miller <davem@davemloft.net>
+
+diff --git a/Documentation/networking/xfrm_sync.txt b/Documentation/networking/xfrm_sync.txt
+index 8be626f..d7aac9d 100644
+--- a/Documentation/networking/xfrm_sync.txt
++++ b/Documentation/networking/xfrm_sync.txt
+@@ -47,10 +47,13 @@ aevent_id structure looks like:
+
+    struct xfrm_aevent_id {
+              struct xfrm_usersa_id           sa_id;
++             xfrm_address_t                  saddr;
+              __u32                           flags;
++             __u32                           reqid;
+    };
+...

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

Every commit has a 40-hexdigit id, sometimes called the "object name" +or the "SHA1 id", shown on the first line of the "git show" output. +You can usually refer to a commit by a shorter name, such as a tag or a +branch name, but this longer name can also be useful. Most +importantly, it is a globally unique name for this commit: so if you +tell somebody else the object name (for example in email), then you are +guaranteed that name will refer to the same commit in their repository +that you it does in yours (assuming their repository has that commit at +all).

Understanding history: commits, parents, and reachability

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

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

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

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

Undestanding history: History diagrams

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

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

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

Understanding history: What is a branch?

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

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

Manipulating branches

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

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

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

Examining branches from a remote repository

The "master" branch that was created at the time you cloned is a copy +of the HEAD in the repository that you cloned from. That repository +may also have had other branches, though, and your local repository +keeps branches which track each of those remote branches, which you +can view using the "-r" option to git-branch(1):

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

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

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

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

Naming branches, tags, and other references

Branches, remote-tracking branches, and tags are all references to +commits. All references are named with a slash-separated path name +starting with "refs"; the names we've been using so far are actually +shorthand:

+The branch "test" is short for "refs/heads/test". +
+The tag "v2.6.18" is short for "refs/tags/v2.6.18". +
+"origin/master" is short for "refs/remotes/origin/master". +

The full name is occasionally useful if, for example, there ever +exists a tag and a branch with the same name.

As another useful shortcut, if the repository "origin" posesses only +a single branch, you can refer to that branch as just "origin".

More generally, if you have defined a remote repository named +"example", you can refer to the branch in that repository as +"example". And for a repository with multiple branches, this will +refer to the branch designated as the "HEAD" branch.

For the complete list of paths which git checks for references, and +the order it uses to decide which to choose when there are multiple +references with the same shorthand name, see the "SPECIFYING +REVISIONS" section of git-rev-parse(1).

Updating a repository with git fetch

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

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

Fetching branches from other repositories

You can also track branches from repositories other than the one you +cloned from, using git-remote(1):

$ git remote add linux-nfs git://linux-nfs.org/pub/nfs-2.6.git
+$ git fetch
+* refs/remotes/linux-nfs/master: storing branch 'master' ...
+ commit: bf81b46

New remote-tracking branches will be stored under the shorthand name +that you gave "git remote add", in this case linux-nfs:

$ git branch -r
+linux-nfs/master
+origin/master

If you run "git fetch <remote>" later, the tracking branches for the +named <remote> will be updated.

If you examine the file .git/config, you will see that git has added +a new stanza:

$ cat .git/config
+...
+[remote "linux-nfs"]
+ url = git://linux-nfs.org/~bfields/git.git
+ fetch = +refs/heads/*:refs/remotes/linux-nfs-read/*
+...

This is what causes git to track the remote's branches; you may modify +or delete these configuration options by editing .git/config with a +text editor. (See the "CONFIGURATION FILE" section of +git-config(1) for details.)

Chapter 3. Exploring git history

Table of Contents

How to use bisect to find a regression

Naming commits

Creating tags

Browsing revisions

Generating diffs

Viewing old file versions

Examples

Check whether two branches point at the same history
Find first tagged version including a given fix

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

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

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

How to use bisect to find a regression

Suppose version 2.6.18 of your project worked, but the version at +"master" crashes. Sometimes the best way to find the cause of such a +regression is to perform a brute-force search through the project's +history to find the particular commit that caused the problem. The +git-bisect(1) command can help you do this:

$ git bisect start
+$ git bisect good v2.6.18
+$ git bisect bad master
+Bisecting: 3537 revisions left to test after this
+[65934a9a028b88e83e2b0f8b36618fe503349f8e] BLOCK: Make USB storage depend on SCSI rather than selecting it [try #6]

If you run "git branch" at this point, you'll see that git has +temporarily moved you to a new branch named "bisect". This branch +points to a commit (with commit id 65934…) that is reachable from +v2.6.19 but not from v2.6.18. Compile and test it, and see whether +it crashes. Assume it does crash. Then:

$ git bisect bad
+Bisecting: 1769 revisions left to test after this
+[7eff82c8b1511017ae605f0c99ac275a7e21b867] i2c-core: Drop useless bitmaskings

checks out an older version. Continue like this, telling git at each +stage whether the version it gives you is good or bad, and notice +that the number of revisions left to test is cut approximately in +half each time.

After about 13 tests (in this case), it will output the commit id of +the guilty commit. You can then examine the commit with +git-show(1), find out who wrote it, and mail them your bug +report with the commit id. Finally, run

$ git bisect reset

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

Note that the version which git-bisect checks out for you at each +point is just a suggestion, and you're free to try a different +version if you think it would be a good idea. For example, +occasionally you may land on a commit that broke something unrelated; +run

$ git bisect-visualize

which will run gitk and label the commit it chose with a marker that +says "bisect". Chose a safe-looking commit nearby, note its commit +id, and check it out with:

$ git reset --hard fb47ddb2db...

then test, run "bisect good" or "bisect bad" as appropriate, and +continue.

Naming commits

We have seen several ways of naming commits already:

+40-hexdigit object name +
+branch name: refers to the commit at the head of the given + branch +
+tag name: refers to the commit pointed to by the given tag + (we've seen branches and tags are special cases of + references). +
+HEAD: refers to the head of the current branch +

There are many more; see the "SPECIFYING REVISIONS" section of the +git-rev-parse(1) man page for the complete list of ways to +name revisions. Some examples:

$ git show fb47ddb2 # the first few characters of the object name
+                    # are usually enough to specify it uniquely
+$ git show HEAD^    # the parent of the HEAD commit
+$ git show HEAD^^   # the grandparent
+$ git show HEAD~4   # the great-great-grandparent

Recall that merge commits may have more than one parent; by default, +^ and ~ follow the first parent listed in the commit, but you can +also choose:

$ git show HEAD^1 # show the first parent of HEAD
+$ git show HEAD^2 # show the second parent of HEAD

In addition to HEAD, there are several other special names for +commits:

Merges (to be discussed later), as well as operations such as +git-reset, which change the currently checked-out commit, generally +set ORIG_HEAD to the value HEAD had before the current operation.

The git-fetch operation always stores the head of the last fetched +branch in FETCH_HEAD. For example, if you run git fetch without +specifying a local branch as the target of the operation

$ git fetch git://example.com/proj.git theirbranch

the fetched commits will still be available from FETCH_HEAD.

When we discuss merges we'll also see the special name MERGE_HEAD, +which refers to the other branch that we're merging in to the current +branch.

The git-rev-parse(1) command is a low-level command that is +occasionally useful for translating some name for a commit to the object +name for that commit:

$ git rev-parse origin
+e05db0fd4f31dde7005f075a84f96b360d05984b

Creating tags

We can also create a tag to refer to a particular commit; after +running

$ git-tag stable-1 1b2e1d63ff

You can use stable-1 to refer to the commit 1b2e1d63ff.

This creates a "lightweight" tag. If the tag is a tag you wish to +share with others, and possibly sign cryptographically, then you +should create a tag object instead; see the git-tag(1) man +page for details.

Browsing revisions

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

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

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

$ git log v2.5.. Makefile fs/

You can also ask git log to show patches:

$ git log -p

See the "—pretty" option in the git-log(1) man page for more +display options.

Note that git log starts with the most recent commit and works +backwards through the parents; however, since git history can contain +multiple independant lines of development, the particular order that +commits are listed in may be somewhat arbitrary.

Generating diffs

You can generate diffs between any two versions using +git-diff(1):

$ git diff master..test

Sometimes what you want instead is a set of patches:

$ git format-patch master..test

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

Viewing old file versions

You can always view an old version of a file by just checking out the +correct revision first. But sometimes it is more convenient to be +able to view an old version of a single file without checking +anything out; this command does that:

$ git show v2.5:fs/locks.c

Before the colon may be anything that names a commit, and after it +may be any path to a file tracked by git.

Examples

Check whether two branches point at the same history

Suppose you want to check whether two branches point at the same point +in history.

$ git diff origin..master

will tell you whether the contents of the project are the same at the +two branches; in theory, however, it's possible that the same project +contents could have been arrived at by two different historical +routes. You could compare the object names:

$ git rev-list origin
+e05db0fd4f31dde7005f075a84f96b360d05984b
+$ git rev-list master
+e05db0fd4f31dde7005f075a84f96b360d05984b

Or you could recall that the … operator selects all commits +contained reachable from either one reference or the other but not +both: so

$ git log origin...master

will return no commits when the two branches are equal.

Find first tagged version including a given fix

Suppose you know that the commit e05db0fd fixed a certain problem. +You'd like to find the earliest tagged release that contains that +fix.

Of course, there may be more than one answer—if the history branched +after commit e05db0fd, then there could be multiple "earliest" tagged +releases.

You could just visually inspect the commits since e05db0fd:

$ gitk e05db0fd..

Or you can use git-name-rev(1), which will give the commit a +name based on any tag it finds pointing to one of the commit's +descendants:

$ git name-rev e05db0fd
+e05db0fd tags/v1.5.0-rc1^0~23

The git-describe(1) command does the opposite, naming the +revision using a tag on which the given commit is based:

$ git describe e05db0fd
+v1.5.0-rc0-ge05db0f

but that may sometimes help you guess which tags might come after the +given commit.

If you just want to verify whether a given tagged version contains a +given commit, you could use git-merge-base(1):

$ git merge-base e05db0fd v1.5.0-rc1
+e05db0fd4f31dde7005f075a84f96b360d05984b

The merge-base command finds a common ancestor of the given commits, +and always returns one or the other in the case where one is a +descendant of the other; so the above output shows that e05db0fd +actually is an ancestor of v1.5.0-rc1.

Alternatively, note that

$ git log v1.5.0-rc1..e05db0fd

will produce empty output if and only if v1.5.0-rc1 includes e05db0fd, +because it outputs only commits that are not reachable from v1.5.0-rc1.

As yet another alternative, the git-show-branch(1) command lists +the commits reachable from its arguments with a display on the left-hand +side that indicates which arguments that commit is reachable from. So, +you can run something like

$ git show-branch e05db0fd v1.5.0-rc0 v1.5.0-rc1 v1.5.0-rc2
+! [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
+available
+ ! [v1.5.0-rc0] GIT v1.5.0 preview
+ ! [v1.5.0-rc1] GIT v1.5.0-rc1
+ ! [v1.5.0-rc2] GIT v1.5.0-rc2
+...

then search for a line that looks like

+ ++ [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if
+available

Which shows that e05db0fd is reachable from itself, from v1.5.0-rc1, and +from v1.5.0-rc2, but not from v1.5.0-rc0.

Chapter 4. Developing with git

Table of Contents

Telling git your name

Creating a new repository

how to make a commit

creating good commit messages

how to merge

Resolving a merge

undoing a merge

Fast-forward merges

Fixing mistakes

Fixing a mistake with a new commit
Fixing a mistake by editing history
Checking out an old version of a file

Ensuring good performance

Ensuring reliability

Checking the repository for corruption
Recovering lost changes

Telling git your name

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

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

(See the "CONFIGURATION FILE" section of git-config(1) for +details on the configuration file.)

Creating a new repository

Creating a new repository from scratch is very easy:

$ mkdir project
+$ cd project
+$ git init

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

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

how to make a commit

Creating a new commit takes three steps:

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

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

At the beginning, the content of the index will be identical to +that of the HEAD. The command "git diff —cached", which shows +the difference between the HEAD and the index, should therefore +produce no output at that point.

Modifying the index is easy:

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

$ git add path/to/file

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

$ git add path/to/file

To remove a file from the index and from the working tree,

$ git rm path/to/file

After each step you can verify that

$ git diff --cached

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

$ git diff

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

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

When you're ready, just run

$ git commit

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

$ git show

As a special shortcut,

$ git commit -a

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

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

$ git diff --cached # difference between HEAD and the index; what
+                    # would be commited if you ran "commit" now.
+$ git diff          # difference between the index file and your
+                    # working directory; changes that would not
+                    # be included if you ran "commit" now.
+$ git status        # a brief per-file summary of the above.

creating good commit messages

Though not required, it's a good idea to begin the commit message +with a single short (less than 50 character) line summarizing the +change, followed by a blank line and then a more thorough +description. Tools that turn commits into email, for example, use +the first line on the Subject line and the rest of the commit in the +body.

how to merge

You can rejoin two diverging branches of development using +git-merge(1):

$ git merge branchname

merges the development in the branch "branchname" into the current +branch. If there are conflicts—for example, if the same file is +modified in two different ways in the remote branch and the local +branch—then you are warned; the output may look something like this:

$ git pull . next
+Trying really trivial in-index merge...
+fatal: Merge requires file-level merging
+Nope.
+Merging HEAD with 77976da35a11db4580b80ae27e8d65caf5208086
+Merging:
+15e2162 world
+77976da goodbye
+found 1 common ancestor(s):
+d122ed4 initial
+Auto-merging file.txt
+CONFLICT (content): Merge conflict in file.txt
+Automatic merge failed; fix conflicts and then commit the result.

Conflict markers are left in the problematic files, and after +you resolve the conflicts manually, you can update the index +with the contents and run git commit, as you normally would when +creating a new file.

If you examine the resulting commit using gitk, you will see that it +has two parents, one pointing to the top of the current branch, and +one to the top of the other branch.

In more detail:

Resolving a merge

When a merge isn't resolved automatically, git leaves the index and +the working tree in a special state that gives you all the +information you need to help resolve the merge.

Files with conflicts are marked specially in the index, so until you +resolve the problem and update the index, git commit will fail:

$ git commit
+file.txt: needs merge

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

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

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

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

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

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

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

The git-log(1) command also provides special help for merges:

$ git log --merge

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

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

$ git add file.txt
+$ git commit

Note that the commit message will already be filled in for you with +some information about the merge. Normally you can just use this +default message unchanged, but you may add additional commentary of +your own if desired.

undoing a merge

If you get stuck and decide to just give up and throw the whole mess +away, you can always return to the pre-merge state with

$ git reset --hard HEAD

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

$ git reset --hard HEAD^

However, this last command can be dangerous in some cases—never +throw away a commit you have already committed if that commit may +itself have been merged into another branch, as doing so may confuse +further merges.

Fast-forward merges

There is one special case not mentioned above, which is treated +differently. Normally, a merge results in a merge commit, with two +parents, one pointing at each of the two lines of development that +were merged.

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

Fixing mistakes

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

$ git reset --hard HEAD

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

+You can create a new commit that undoes whatever was done + by the previous commit. This is the correct thing if your + mistake has already been made public. +
+You can go back and modify the old commit. You should + never do this if you have already made the history public; + git does not normally expect the "history" of a project to + change, and cannot correctly perform repeated merges from + a branch that has had its history changed. +

Fixing a mistake with a new commit

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

$ git revert HEAD

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

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

$ git revert HEAD^

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

Fixing a mistake by editing history

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

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

$ git commit --amend

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

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

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

Checking out an old version of a file

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

$ git checkout HEAD^ path/to/file

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

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

$ git show HEAD^ path/to/file

which will display the given version of the file.

Ensuring good performance

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

This compression is not performed automatically. Therefore you +should occasionally run git-gc(1):

$ git gc

to recompress the archive. This can be very time-consuming, so +you may prefer to run git-gc when you are not doing other work.

Ensuring reliability

Checking the repository for corruption

The git-fsck(1) command runs a number of self-consistency checks +on the repository, and reports on any problems. This may take some +time. The most common warning by far is about "dangling" objects:

$ git fsck
+dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
+dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
+dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
+dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb
+dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f
+dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e
+dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085
+dangling tree b24c2473f1fd3d91352a624795be026d64c8841f
+...

Dangling objects are objects that are harmless, but also unnecessary; +you can remove them at any time with git-prune(1) or the —prune +option to git-gc(1):

$ git gc --prune

This may be time-consuming. Unlike most other git operations (including +git-gc when run without any options), it is not safe to prune while +other git operations are in progress in the same repository.

For more about dangling objects, see the section called “Dangling objects”.

Recovering lost changes

Reflogs

Say you modify a branch with git-reset(1) —hard, and then +realize that the branch was the only reference you had to that point in +history.

Fortunately, git also keeps a log, called a "reflog", of all the +previous values of each branch. So in this case you can still find the +old history using, for example,

$ git log master@{1}

This lists the commits reachable from the previous version of the head. +This syntax can be used to with any git command that accepts a commit, +not just with git log. Some other examples:

$ git show master@{2}           # See where the branch pointed 2,
+$ git show master@{3}           # 3, ... changes ago.
+$ gitk master@{yesterday}       # See where it pointed yesterday,
+$ gitk master@{"1 week ago"}    # ... or last week

The reflogs are kept by default for 30 days, after which they may be +pruned. See git-reflog(1) and git-gc(1) to learn +how to control this pruning, and see the "SPECIFYING REVISIONS" +section of git-rev-parse(1) for details.

Note that the reflog history is very different from normal git history. +While normal history is shared by every repository that works on the +same project, the reflog history is not shared: it tells you only about +how the branches in your local repository have changed over time.

Examining dangling objects

In some situations the reflog may not be able to save you. For +example, suppose you delete a branch, then realize you need the history +it pointed you. The reflog is also deleted; however, if you have not +yet pruned the repository, then you may still be able to find +the lost commits; run git-fsck and watch for output that mentions +"dangling commits":

$ git fsck
+dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
+dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
+dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
+...

and watch for output that mentions "dangling commits". You can examine +one of those dangling commits with, for example,

$ gitk 7281251ddd --not --all

which does what it sounds like: it says that you want to see the commit +history that is described by the dangling commit(s), but not the +history that is described by all your existing branches and tags. Thus +you get exactly the history reachable from that commit that is lost. +(And notice that it might not be just one commit: we only report the +"tip of the line" as being dangling, but there might be a whole deep +and complex commit history that was gotten dropped.)

If you decide you want the history back, you can always create a new +reference pointing to it, for example, a new branch:

$ git branch recovered-branch 7281251ddd

Chapter 5. Sharing development with others

Table of Contents

Getting updates with git pull
Submitting patches to a project
Importing patches to a project
Setting up a public repository
Exporting a git repository via http
Exporting a git repository via the git protocol
Pushing changes to a public repository
Setting up a shared repository
Allow web browsing of a repository
Examples

Getting updates with git pull

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

We have already seen how to keep remote tracking branches up to date with git-fetch(1), +and how to merge two branches. So you can merge in changes from the +original repository's master branch with:

$ git fetch
+$ git merge origin/master

However, the git-pull(1) command provides a way to do this in +one step:

$ git pull origin master

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

$ git pull

See the descriptions of the branch.<name>.remote and +branch.<name>.merge options in git-config(1) to learn +how to control these defaults depending on the current branch.

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

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

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

$ git pull . branch
+$ git merge branch

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

Submitting patches to a project

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

First, use git-format-patch(1); for example:

$ git format-patch origin

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

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

Importing patches to a project

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

$ git am -3 patches.mbox

Git will apply each patch in order; if any conflicts are found, it +will stop, and you can fix the conflicts as described in +"Resolving a merge". (The "-3" option tells +git to perform a merge; if you would prefer it just to abort and +leave your tree and index untouched, you may omit that option.)

Once the index is updated with the results of the conflict +resolution, instead of creating a new commit, just run

$ git am --resolved

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

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

Setting up a public repository

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

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

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

If this sort of setup is inconvenient or impossible, another (more +common) option is to set up a public repository on a public server. +This also allows you to cleanly separate private work in progress +from publicly visible work.

You will continue to do your day-to-day work in your personal +repository, but periodically "push" changes from your personal +repository into your public repository, allowing other developers to +pull from that repository. So the flow of changes, in a situation +where there is one other developer with a public repository, looks +like this:

                      you push
+your personal repo ------------------> your public repo
+      ^                                     |
+      |                                     |
+      | you pull                            | they pull
+      |                                     |
+      |                                     |
+      |               they push             V
+their public repo <------------------- their repo

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

$ git clone --bare proj-clone.git

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

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

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

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

Exporting a git repository via http

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

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

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

(For an explanation of the last two lines, see +git-update-server-info(1), and the documentation +Hooks used by git.)

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

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

(See also +setup-git-server-over-http +for a slightly more sophisticated setup using WebDAV which also +allows pushing over http.)

Exporting a git repository via the git protocol

This is the preferred method.

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

Pushing changes to a public repository

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

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

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

or just

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

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

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

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

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

you should be able to perform the above push with just

$ git push public-repo master

See the explanations of the remote.<name>.url, branch.<name>.remote, +and remote.<name>.push options in git-config(1) for +details.

Setting up a shared repository

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

Allow web browsing of a repository

The gitweb cgi script provides users an easy way to browse your +project's files and history without having to install git; see the file +gitweb/README in the git source tree for instructions on setting it up.

Examples

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

Chapter 6. Rewriting history and maintaining patch series

Table of Contents

Creating the perfect patch series
Keeping a patch series up to date using git-rebase
Reordering or selecting from a patch series
Other tools
Problems with rewriting history

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

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

Creating the perfect patch series

Suppose you are a contributor to a large project, and you want to add a +complicated feature, and to present it to the other developers in a way +that makes it easy for them to read your changes, verify that they are +correct, and understand why you made each change.

If you present all of your changes as a single patch (or commit), they +may find it is too much to digest all at once.

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

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

+Each patch can be applied in order. +
+Each patch includes a single logical change, together with a + message explaining the change. +
+No patch introduces a regression: after applying any initial + part of the series, the resulting project still compiles and + works, and has no bugs that it didn't have before. +
+The complete series produces the same end result as your own + (probably much messier!) development process did. +

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

Keeping a patch series up to date using git-rebase

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

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

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

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

o--o--o <-- origin
+                o--o--o <-- mywork

Some more interesting work has been done in the upstream project, and +"origin" has advanced:

o--o--O--o--o--o <-- origin
+                a--b--c <-- mywork

At this point, you could use "pull" to merge your changes back in; +the result would create a new merge commit, like this:

o--o--O--o--o--o <-- origin
+       \                 a--b--c--m <-- mywork

However, if you prefer to keep the history in mywork a simple series of +commits without any merges, you may instead choose to use +git-rebase(1):

$ git checkout mywork
+$ git rebase origin

This will remove each of your commits from mywork, temporarily saving +them as patches (in a directory named ".dotest"), update mywork to +point at the latest version of origin, then apply each of the saved +patches to the new mywork. The result will look like:

o--o--O--o--o--o <-- origin
+                                  a'--b'--c' <-- mywork

In the process, it may discover conflicts. In that case it will stop +and allow you to fix the conflicts; after fixing conflicts, use "git +add" to update the index with those contents, and then, instead of +running git-commit, just run

$ git rebase --continue

and git will continue applying the rest of the patches.

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

$ git rebase --abort

Reordering or selecting from a patch series

Given one existing commit, the git-cherry-pick(1) command +allows you to apply the change introduced by that commit and create a +new commit that records it. So, for example, if "mywork" points to a +series of patches on top of "origin", you might do something like:

$ git checkout -b mywork-new origin
+$ gitk origin..mywork &

And browse through the list of patches in the mywork branch using gitk, +applying them (possibly in a different order) to mywork-new using +cherry-pick, and possibly modifying them as you go using commit +—amend.

Another technique is to use git-format-patch to create a series of +patches, then reset the state to before the patches:

$ git format-patch origin
+$ git reset --hard origin

Then modify, reorder, or eliminate patches as preferred before applying +them again with git-am(1).

Other tools

There are numerous other tools, such as stgit, which exist for the +purpose of maintaining a patch series. These are out of the scope of +this manual.

Problems with rewriting history

The primary problem with rewriting the history of a branch has to do +with merging. Suppose somebody fetches your branch and merges it into +their branch, with a result something like this:

o--o--O--o--o--o <-- origin
+       \                 t--t--t--m <-- their branch:

Then suppose you modify the last three commits:

        o--o--o <-- new head of origin
+       /
+o--o--O--o--o--o <-- old head of origin

If we examined all this history together in one repository, it will +look like:

        o--o--o <-- new head of origin
+       /
+o--o--O--o--o--o <-- old head of origin
+       \                 t--t--t--m <-- their branch:

Git has no way of knowing that the new head is an updated version of +the old head; it treats this situation exactly the same as it would if +two developers had independently done the work on the old and new heads +in parallel. At this point, if someone attempts to merge the new head +in to their branch, git will attempt to merge together the two (old and +new) lines of development, instead of trying to replace the old by the +new. The results are likely to be unexpected.

You may still choose to publish branches whose history is rewritten, +and it may be useful for others to be able to fetch those branches in +order to examine or test them, but they should not attempt to pull such +branches into their own work.

For true distributed development that supports proper merging, +published branches should never be rewritten.

Chapter 7. Advanced branch management

Table of Contents

Fetching individual branches
Understanding git history: fast-forwards
Forcing git fetch to do non-fast-forward updates
Configuring remote branches

Fetching individual branches

Instead of using git-remote(1), you can also choose just +to update one branch at a time, and to store it locally under an +arbitrary name:

$ git fetch origin todo:my-todo-work

The first argument, "origin", just tells git to fetch from the +repository you originally cloned from. The second argument tells git +to fetch the branch named "todo" from the remote repository, and to +store it locally under the name refs/heads/my-todo-work.

You can also fetch branches from other repositories; so

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

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

Understanding git history: fast-forwards

In the previous example, when updating an existing branch, "git +fetch" checks to make sure that the most recent commit on the remote +branch is a descendant of the most recent commit on your copy of the +branch before updating your copy of the branch to point at the new +commit. Git calls this process a "fast forward".

A fast forward looks something like this:

o--o--o--o <-- old head of the branch
+                      o--o--o <-- new head of the branch

In some cases it is possible that the new head will not actually be +a descendant of the old head. For example, the developer may have +realized she made a serious mistake, and decided to backtrack, +resulting in a situation like:

o--o--o--o--a--b <-- old head of the branch
+                      o--o--o <-- new head of the branch

In this case, "git fetch" will fail, and print out a warning.

In that case, you can still force git to update to the new head, as +described in the following section. However, note that in the +situation above this may mean losing the commits labeled "a" and "b", +unless you've already created a reference of your own pointing to +them.

Forcing git fetch to do non-fast-forward updates

If git fetch fails because the new head of a branch is not a +descendant of the old head, you may force the update with:

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

Note the addition of the "+" sign. Be aware that commits which the +old version of example/master pointed at may be lost, as we saw in +the previous section.

Configuring remote branches

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

$ git config -l
+core.repositoryformatversion=0
+core.filemode=true
+core.logallrefupdates=true
+remote.origin.url=git://git.kernel.org/pub/scm/git/git.git
+remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
+branch.master.remote=origin
+branch.master.merge=refs/heads/master

If there are other repositories that you also use frequently, you can +create similar configuration options to save typing; for example, +after

$ git config remote.example.url git://example.com/proj.git

then the following two commands will do the same thing:

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

Even better, if you add one more option:

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

then the following commands will all do the same thing:

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

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

$ git config remote.example.fetch +master:ref/remotes/example/master

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

Also note that all of the above configuration can be performed by +directly editing the file .git/config instead of using +git-config(1).

See git-config(1) for more details on the configuration +options mentioned above.

Chapter 8. Git internals

Table of Contents

The Object Database

The "index" aka "Current Directory Cache"

The Workflow

working directory -> index
index -> object database
object database -> index
index -> working directory
Tying it all together

Examining the data

Merging multiple trees

Merging multiple trees, continued

How git stores objects efficiently: pack files

Dangling objects

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:

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:

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.

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:

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.

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:

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.

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.

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.

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.

How git stores objects efficiently: pack files

We've seen how git stores each object in a file named after the +object's SHA1 hash.

Unfortunately this system becomes inefficient once a project has a +lot of objects. Try this on an old project:

$ git count-objects
+6930 objects, 47620 kilobytes

The first number is the number of objects which are kept in +individual files. The second is the amount of space taken up by +those "loose" objects.

You can save space and make git faster by moving these loose objects in +to a "pack file", which stores a group of objects in an efficient +compressed format; the details of how pack files are formatted can be +found in technical/pack-format.txt.

To put the loose objects into a pack, just run git repack:

$ git repack
+Generating pack...
+Done counting 6020 objects.
+Deltifying 6020 objects.
+ 100% (6020/6020) done
+Writing 6020 objects.
+ 100% (6020/6020) done
+Total 6020, written 6020 (delta 4070), reused 0 (delta 0)
+Pack pack-3e54ad29d5b2e05838c75df582c65257b8d08e1c created.

You can then run

$ git prune

to remove any of the "loose" objects that are now contained in the +pack. This will also remove any unreferenced objects (which may be +created when, for example, you use "git reset" to remove a commit). +You can verify that the loose objects are gone by looking at the +.git/objects directory or by running

$ git count-objects
+0 objects, 0 kilobytes

Although the object files are gone, any commands that refer to those +objects will work exactly as they did before.

The git-gc(1) command performs packing, pruning, and more for +you, so is normally the only high-level command you need.

Dangling objects

The git-fsck(1) command will sometimes complain about dangling +objects. They are not a problem.

The most common cause of dangling objects is that you've rebased a +branch, or you have pulled from somebody else who rebased a branch—see +Chapter 6, Rewriting history and maintaining patch series. In that case, the old head of the original +branch still exists, as does obviously everything it pointed to. The +branch pointer itself just doesn't, since you replaced it with another +one.

There are also other situations too that cause dangling objects. For +example, a "dangling blob" may arise because you did a "git add" of a +file, but then, before you actually committed it and made it part of the +bigger picture, you changed something else in that file and committed +that updated thing - the old state that you added originally ends up +not being pointed to by any commit or tree, so it's now a dangling blob +object.

Similarly, when the "recursive" merge strategy runs, and finds that +there are criss-cross merges and thus more than one merge base (which is +fairly unusual, but it does happen), it will generate one temporary +midway tree (or possibly even more, if you had lots of criss-crossing +merges and more than two merge bases) as a temporary internal merge +base, and again, those are real objects, but the end result will not end +up pointing to them, so they end up "dangling" in your repository.

Generally, dangling objects aren't anything to worry about. They can +even be very useful: if you screw something up, the dangling objects can +be how you recover your old tree (say, you did a rebase, and realized +that you really didn't want to - you can look at what dangling objects +you have, and decide to reset your head to some old dangling state).

For commits, the most useful thing to do with dangling objects tends to +be to do a simple

$ gitk <dangling-commit-sha-goes-here> --not --all

For blobs and trees, you can't do the same, but you can examine them. +You can just do

$ git show <dangling-blob/tree-sha-goes-here>

to show what the contents of the blob were (or, for a tree, basically +what the "ls" for that directory was), and that may give you some idea +of what the operation was that left that dangling object.

Usually, dangling blobs and trees aren't very interesting. They're +almost always the result of either being a half-way mergebase (the blob +will often even have the conflict markers from a merge in it, if you +have had conflicting merges that you fixed up by hand), or simply +because you interrupted a "git fetch" with ^C or something like that, +leaving _some_ of the new objects in the object database, but just +dangling and useless.

Anyway, once you are sure that you're not interested in any dangling +state, you can just prune all unreachable objects:

$ git prune

and they'll be gone. But you should only run "git prune" on a quiescent +repository - it's kind of like doing a filesystem fsck recovery: you +don't want to do that while the filesystem is mounted.

(The same is true of "git-fsck" itself, btw - but since +git-fsck never actually changes the repository, it just reports +on what it found, git-fsck itself is never "dangerous" to run. +Running it while somebody is actually changing the repository can cause +confusing and scary messages, but it won't actually do anything bad. In +contrast, running "git prune" while somebody is actively changing the +repository is a BAD idea).

Chapter 9. Glossary of git terms

+alternate object database +: + Via the alternates mechanism, a repository can inherit part of its + object database from another object database, which is called + "alternate". +
+bare repository +: + A bare repository is normally an appropriately named + directory with a .git suffix that does not have a + locally checked-out copy of any of the files under revision + control. That is, all of the git administrative and + control files that would normally be present in the + hidden .git sub-directory are directly present in + the repository.git directory instead, and no other files + are present and checked out. Usually publishers of public + repositories make bare repositories available. +
+blob object +: + Untyped object, e.g. the contents of a file. +
+branch +: + A non-cyclical graph of revisions, i.e. the complete history of + a particular revision, which is called the branch head. The + branch heads are stored in $GIT_DIR/refs/heads/. +
+cache +: + Obsolete for: index. +
+chain +: + A list of objects, where each object in the list contains a + reference to its successor (for example, the successor of a commit + could be one of its parents). +
+changeset +: + BitKeeper/cvsps speak for "commit". Since git does not store + changes, but states, it really does not make sense to use + the term "changesets" with git. +
+checkout +: + The action of updating the working tree to a revision which was + stored in the object database. +
+cherry-picking +: + In SCM jargon, "cherry pick" means to choose a subset of + changes out of a series of changes (typically commits) + and record them as a new series of changes on top of + different codebase. In GIT, this is performed by + "git cherry-pick" command to extract the change + introduced by an existing commit and to record it based + on the tip of the current branch as a new commit. +
+clean +: + A working tree is clean, if it corresponds to the revision + referenced by the current head. Also see "dirty". +
+commit +: + As a verb: The action of storing the current state of the index in the + object database. The result is a revision. + As a noun: Short hand for commit object. +
+commit object +: + An object which contains the information about a particular + revision, such as parents, committer, author, date and the + tree object which corresponds to the top directory of the + stored revision. +
+core git +: + Fundamental data structures and utilities of git. Exposes only + limited source code management tools. +
+DAG +: + Directed acyclic graph. The commit objects form a directed acyclic + graph, because they have parents (directed), and the graph of commit + objects is acyclic (there is no chain which begins and ends with the + same object). +
+dircache +: + You are waaaaay behind. +
+dirty +: + A working tree is said to be dirty if it contains modifications + which have not been committed to the current branch. +
+directory +: + The list you get with "ls" :-) +
+ent +: + Favorite synonym to "tree-ish" by some total geeks. See + http://en.wikipedia.org/wiki/Ent_(Middle-earth) for an in-depth + explanation. Avoid this term, not to confuse people. +
+fast forward +: + A fast-forward is a special type of merge where you have + a revision and you are "merging" another branch's changes + that happen to be a descendant of what you have. + In such these cases, you do not make a new merge commit but + instead just update to his revision. This will happen + frequently on a tracking branch of a remote repository. +
+fetch +: + Fetching a branch means to get the branch's head ref from a + remote repository, to find out which objects are missing from + the local object database, and to get them, too. +
+file system +: + Linus Torvalds originally designed git to be a user space file + system, i.e. the infrastructure to hold files and directories. + That ensured the efficiency and speed of git. +
+git archive +: + Synonym for repository (for arch people). +
+grafts +: + Grafts enables two otherwise different lines of development to be + joined together by recording fake ancestry information for commits. + This way you can make git pretend the set of parents a commit + has is different from what was recorded when the commit was created. + Configured via the .git/info/grafts file. +
+hash +: + In git's context, synonym to object name. +
+head +: + The top of a branch. It contains a ref to the corresponding + commit object. +
+head ref +: + A ref pointing to a head. Often, this is abbreviated to "head". + Head refs are stored in $GIT_DIR/refs/heads/. +
+hook +: + During the normal execution of several git commands, + call-outs are made to optional scripts that allow + a developer to add functionality or checking. + Typically, the hooks allow for a command to be pre-verified + and potentially aborted, and allow for a post-notification + after the operation is done. + The hook scripts are found in the $GIT_DIR/hooks/ directory, + and are enabled by simply making them executable. +
+index +: + A collection of files with stat information, whose contents are + stored as objects. The index is a stored version of your working + tree. Truth be told, it can also contain a second, and even a third + version of a working tree, which are used when merging. +
+index entry +: + The information regarding a particular file, stored in the index. + An index entry can be unmerged, if a merge was started, but not + yet finished (i.e. if the index contains multiple versions of + that file). +
+master +: + The default development branch. Whenever you create a git + repository, a branch named "master" is created, and becomes + the active branch. In most cases, this contains the local + development, though that is purely conventional and not required. +
+merge +: + To merge branches means to try to accumulate the changes since a + common ancestor and apply them to the first branch. An automatic + merge uses heuristics to accomplish that. Evidently, an automatic + merge can fail. +
+object +: + The unit of storage in git. It is uniquely identified by + the SHA1 of its contents. Consequently, an object can not + be changed. +
+object database +: + Stores a set of "objects", and an individual object is identified + by its object name. The objects usually live in $GIT_DIR/objects/. +
+object identifier +: + Synonym for object name. +
+object name +: + The unique identifier of an object. The hash of the object's contents + using the Secure Hash Algorithm 1 and usually represented by the 40 + character hexadecimal encoding of the hash of the object (possibly + followed by a white space). +
+object type +: + One of the identifiers "commit","tree","tag" and "blob" describing + the type of an object. +
+octopus +: + To merge more than two branches. Also denotes an intelligent + predator. +
+origin +: + The default upstream repository. Most projects have at + least one upstream project which they track. By default + origin is used for that purpose. New upstream updates + will be fetched into remote tracking branches named + origin/name-of-upstream-branch, which you can see using + "git branch -r". +
+pack +: + A set of objects which have been compressed into one file (to save + space or to transmit them efficiently). +
+pack index +: + The list of identifiers, and other information, of the objects in a + pack, to assist in efficiently accessing the contents of a pack. +
+parent +: + A commit object contains a (possibly empty) list of the logical + predecessor(s) in the line of development, i.e. its parents. +
+pickaxe +: + The term pickaxe refers to an option to the diffcore routines + that help select changes that add or delete a given text string. + With the —pickaxe-all option, it can be used to view the + full changeset that introduced or removed, say, a particular + line of text. See git-diff(1). +
+plumbing +: + Cute name for core git. +
+porcelain +: + Cute name for programs and program suites depending on core git, + presenting a high level access to core git. Porcelains expose + more of a SCM interface than the plumbing. +
+pull +: + Pulling a branch means to fetch it and merge it. +
+push +: + Pushing a branch means to get the branch's head ref from a remote + repository, find out if it is an ancestor to the branch's local + head ref is a direct, and in that case, putting all objects, which + are reachable from the local head ref, and which are missing from + the remote repository, into the remote object database, and updating + the remote head ref. If the remote head is not an ancestor to the + local head, the push fails. +
+reachable +: + All of the ancestors of a given commit are said to be reachable from + that commit. More generally, one object is reachable from another if + we can reach the one from the other by a chain that follows tags to + whatever they tag, commits to their parents or trees, and trees to the + trees or blobs that they contain. +
+rebase +: + To clean a branch by starting from the head of the main line of + development ("master"), and reapply the (possibly cherry-picked) + changes from that branch. +
+ref +: + A 40-byte hex representation of a SHA1 or a name that denotes + a particular object. These may be stored in $GIT_DIR/refs/. +
+refspec +: + A refspec is used by fetch and push to describe the mapping + between remote ref and local ref. They are combined with + a colon in the format <src>:<dst>, preceded by an optional + plus sign, +. For example: + git fetch $URL refs/heads/master:refs/heads/origin + means "grab the master branch head from the $URL and store + it as my origin branch head". + And git push $URL refs/heads/master:refs/heads/to-upstream + means "publish my master branch head as to-upstream branch + at $URL". See also git-push(1) +
+repository +: + A collection of refs together with an object database containing + all objects, which are reachable from the refs, possibly accompanied + by meta data from one or more porcelains. A repository can + share an object database with other repositories. +
+resolve +: + The action of fixing up manually what a failed automatic merge + left behind. +
+revision +: + A particular state of files and directories which was stored in + the object database. It is referenced by a commit object. +
+rewind +: + To throw away part of the development, i.e. to assign the head to + an earlier revision. +
+SCM +: + Source code management (tool). +
+SHA1 +: + Synonym for object name. +
+shallow repository +: + A shallow repository has an incomplete history some of + whose commits have parents cauterized away (in other + words, git is told to pretend that these commits do not + have the parents, even though they are recorded in the + commit object). This is sometimes useful when you are + interested only in the recent history of a project even + though the real history recorded in the upstream is + much larger. A shallow repository is created by giving + —depth option to git-clone(1), and its + history can be later deepened with git-fetch(1). +
+symref +: + Symbolic reference: instead of containing the SHA1 id itself, it + is of the format ref: refs/some/thing and when referenced, it + recursively dereferences to this reference. HEAD is a prime + example of a symref. Symbolic references are manipulated with + the git-symbolic-ref(1) command. +
+topic branch +: + A regular git branch that is used by a developer to + identify a conceptual line of development. Since branches + are very easy and inexpensive, it is often desirable to + have several small branches that each contain very well + defined concepts or small incremental yet related changes. +
+tracking branch +: + A regular git branch that is used to follow changes from + another repository. A tracking branch should not contain + direct modifications or have local commits made to it. + A tracking branch can usually be identified as the + right-hand-side ref in a Pull: refspec. +
+tree object +: + An object containing a list of file names and modes along with refs + to the associated blob and/or tree objects. A tree is equivalent + to a directory. +
+tree +: + Either a working tree, or a tree object together with the + dependent blob and tree objects (i.e. a stored representation + of a working tree). +
+tree-ish +: + A ref pointing to either a commit object, a tree object, or a + tag object pointing to a tag or commit or tree object. +
+tag object +: + An object containing a ref pointing to another object, which can + contain a message just like a commit object. It can also + contain a (PGP) signature, in which case it is called a "signed + tag object". +
+tag +: + A ref pointing to a tag or commit object. In contrast to a head, + a tag is not changed by a commit. Tags (not tag objects) are + stored in $GIT_DIR/refs/tags/. A git tag has nothing to do with + a Lisp tag (which is called object type in git's context). + A tag is most typically used to mark a particular point in the + commit ancestry chain. +
+unmerged index +: + An index which contains unmerged index entries. +
+working tree +: + The set of files and directories currently being worked on, + i.e. you can work in your working tree without using git at all. +

Chapter 10. Notes and todo list for this manual

This is a work in progress.

The basic requirements: + - It must be readable in order, from beginning to end, by + someone intelligent with a basic grasp of the unix + commandline, but without any special knowledge of git. If + necessary, any other prerequisites should be specifically + mentioned as they arise. + - Whenever possible, section headings should clearly describe + the task they explain how to do, in language that requires + no more knowledge than necessary: for example, "importing + patches into a project" rather than "the git-am command"

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

Scan Documentation/ for other stuff left out; in particular: + howto's + some of technical/? + hooks + list of commands in git(1)

Scan email archives for other stuff left out

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

Simplify beginning by suggesting disconnected head instead of +temporary branch creation?

Explain how to refer to file stages in the "how to resolve a merge" +section: diff -1, -2, -3, —ours, —theirs :1:/path notation. The +"git ls-files —unmerged —stage" thing is sorta useful too, +actually. And note gitk —merge.

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

Include cross-references to the glossary, where appropriate.

Document shallow clones? See draft 1.5.0 release notes for some +documentation.

Add a sectin on working with other version control systems, including +CVS, Subversion, and just imports of series of release tarballs.

More details on gitweb?

Write a chapter on using plumbing and writing scripts.

DESCRIPTION

OPTIONS

Git User's Manual

Preface

Chapter 1. Git Quick Start

Creating a new repository

Managing branches

Exploring history

Making changes

Merging

Sharing your changes

Repository maintenance

Chapter 2. Repositories and Branches

How to get a git repository

How to check out a different version of a project

Understanding History: Commits

Understanding history: commits, parents, and reachability

Undestanding history: History diagrams

Understanding history: What is a branch?

Manipulating branches

Examining branches from a remote repository

Naming branches, tags, and other references

Updating a repository with git fetch

Fetching branches from other repositories

Chapter 3. Exploring git history

How to use bisect to find a regression

Naming commits

Creating tags

Browsing revisions

Generating diffs

Viewing old file versions

Examples

Check whether two branches point at the same history

Find first tagged version including a given fix

Chapter 4. Developing with git

Telling git your name

Creating a new repository

how to make a commit

creating good commit messages

how to merge

Resolving a merge

undoing a merge

Fast-forward merges

Fixing mistakes

Fixing a mistake with a new commit

Fixing a mistake by editing history

Checking out an old version of a file

Ensuring good performance

Ensuring reliability

Checking the repository for corruption

Recovering lost changes

Reflogs

Examining dangling objects

Chapter 5. Sharing development with others

Getting updates with git pull

Submitting patches to a project

Importing patches to a project

Setting up a public repository

Exporting a git repository via http

Exporting a git repository via the git protocol

Pushing changes to a public repository

Setting up a shared repository

Allow web browsing of a repository

Examples

Chapter 6. Rewriting history and maintaining patch series

Creating the perfect patch series

Keeping a patch series up to date using git-rebase

Reordering or selecting from a patch series

Other tools

Problems with rewriting history

Chapter 7. Advanced branch management

Fetching individual branches

Understanding git history: fast-forwards

Forcing git fetch to do non-fast-forward updates

Configuring remote branches

Chapter 8. Git internals

The Object Database

Blob Object

Tree Object

Commit Object