This manual is designed to be readable by someone with basic unix
commandline skills, but no previous knowledge of git.
-Chapters 1 and 2 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 1 gives a brief overview of git commands, without any
+explanation; you may prefer to skip to chapter 2 on a first reading.
-Chapter 3 explains how to do development with git and share your progress
-with others.
+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.
$ man git-clone
------------------------------------------------
+Git Quick Start
+===============
+
+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
+-----------------------------------------------
+
Repositories and Branches
=========================
together with a special top-level directory named ".git", which
contains all the information about the history of the project.
-In the following, examples will be taken from one of the two
+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
...
------------------------------------------------
+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 gitlink:git-checkout[1]:
As you can see, a commit shows who made the latest change, what they
did, and why.
-Every commit has a 20-digit id, sometimes called 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 id can also be useful. In particular, it is a globally unique
-name for this commit: so if you tell somebody else the SHA1 id (for
-example in email), then you are guaranteed they will see the same
-commit in their repository that you do in yours.
+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 it does in yours (assuming their repository has that commit at
+all).
Understanding history: commits, parents, and reachability
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
that Y is a descendent of X, or that there is a chain of parents
leading from commit Y to commit X.
-Undestanding history: History diagrams
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Understanding 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
to refer to the repository that you cloned from.
[[how-git-stores-references]]
-How git stores references
--------------------------
+Naming branches, tags, and other references
+-------------------------------------------
Branches, remote-tracking branches, and tags are all references to
-commits. Git stores these references in the ".git" directory. Most
-of them are stored in .git/refs/:
+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:
- - branches are stored in .git/refs/heads
- - tags are stored in .git/refs/tags
- - remote-tracking branches for "origin" are stored in
- .git/refs/remotes/origin/
+ - 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".
-If you look at one of these files you will see that they usually
-contain just the SHA1 id of a commit:
+The full name is occasionally useful if, for example, there ever
+exists a tag and a branch with the same name.
-------------------------------------------------
-$ ls .git/refs/heads/
-master
-$ cat .git/refs/heads/master
-c0f982dcf188d55db9d932a39d4ea7becaa55fed
-------------------------------------------------
+As another useful shortcut, if the repository "origin" posesses only
+a single branch, you can refer to that branch as just "origin".
-You can refer to a reference by its path relative to the .git
-directory. However, we've seen above that git will also accept
-shorter names; for example, "master" is an acceptable shortcut for
-"refs/heads/master", and "origin/master" is a shortcut for
-"refs/remotes/origin/master".
-
-As another useful shortcut, you can also refer to the "HEAD" of
-"origin" (or any other remote), using just the name of the remote.
+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
-how it decides which to choose when there are multiple references
-with the same name, see the "SPECIFYING REVISIONS" section of
-gitlink:git-rev-parse[1].
+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 gitlink:git-rev-parse[1].
[[Updating-a-repository-with-git-fetch]]
Updating a repository with git fetch
repository. It will not touch any of your own branches--not even the
"master" branch that was created for you on clone.
-Fetching individual branches
-----------------------------
-
-You can also choose to update just one branch at a time:
-
--------------------------------------------------
-$ git fetch origin todo:refs/remotes/origin/todo
--------------------------------------------------
-
-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/remotes/origin/todo; as we saw
-above, remote-tracking branches are stored under
-refs/remotes/<name-of-repository>/<name-of-branch>.
-
-You can also fetch branches from other repositories; so
-
--------------------------------------------------
-$ git fetch git://example.com/proj.git master:refs/remotes/example/master
--------------------------------------------------
-
-will create a new reference named "refs/remotes/example/master" and
-store in it the branch named "master" from the repository at the
-given URL. If you already have a branch named
-"refs/remotes/example/master", it will attempt to "fast-forward" to
-the commit given by example.com's master branch. So next we explain
-what a fast-forward is:
-
-[[fast-forwards]]
-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
-gitlink:git-repo-config[1]:
-
--------------------------------------------------
-$ git-repo-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 repo-config remote.example.url=git://example.com/proj.git
--------------------------------------------------
+Fetching branches from other repositories
+-----------------------------------------
-then the following two commands will do the same thing:
+You can also track branches from repositories other than the one you
+cloned from, using gitlink:git-remote[1]:
-------------------------------------------------
-$ git fetch git://example.com/proj.git master:refs/remotes/example/master
-$ git fetch example master:refs/remotes/example/master
+$ 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
-------------------------------------------------
-Even better, if you add one more option:
+New remote-tracking branches will be stored under the shorthand name
+that you gave "git remote add", in this case linux-nfs:
-------------------------------------------------
-$ git repo-config remote.example.fetch=master:refs/remotes/example/master
+$ git branch -r
+linux-nfs/master
+origin/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
--------------------------------------------------
+If you run "git fetch <remote>" later, the tracking branches for the
+named <remote> will be updated.
-You can also add a "+" to force the update each time:
+If you examine the file .git/config, you will see that git has added
+a new stanza:
-------------------------------------------------
-$ git repo-config +master:ref/remotes/example/master
+$ cat .git/config
+...
+[remote "linux-nfs"]
+ url = git://linux-nfs.org/~bfields/git.git
+ fetch = +refs/heads/*:refs/remotes/linux-nfs-read/*
+...
-------------------------------------------------
-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
-gitlink:git-repo-config[1].
-
-See gitlink:git-repo-config[1] for more details on the configuration
-options mentioned above.
+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
+gitlink:git-config[1] for details.)
Exploring git history
=====================
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
+We start with one specialized tool that is useful for finding the
commit that introduced a bug into a project.
How to use bisect to find a regression
We have seen several ways of naming commits already:
- - 20-digit SHA1 id
+ - 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
<<how-git-stores-references,references>>).
- HEAD: refers to the head of the current branch
-There are many more; see the "SPECIFYING REVISION" section of the
-gitlink:git-rev-list[1] man page for the complete list of ways to
+There are many more; see the "SPECIFYING REVISIONS" section of the
+gitlink: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 SHA1 id
+$ 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
which refers to the other branch that we're merging in to the current
branch.
+The gitlink: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
-------------
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
+multiple independent lines of development, the particular order that
commits are listed in may be somewhat arbitrary.
Generating diffs
Before the colon may be anything that names a commit, and after it
may be any path to a file tracked by git.
-Developing with git
-===================
+Examples
+--------
-Telling git your name
----------------------
+Check whether two branches point at the same history
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Before creating any commits, you should introduce yourself to git. The
-easiest way to do so is:
+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 gitlink: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 gitlink: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 gitlink: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 gitlink: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.
+
+
+Developing with git
+===================
+
+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
-------------------------------------------------
+$ cat >~/.gitconfig <<\EOF
+[user]
+ name = Your Name Comes Here
+ email = you@yourdomain.example.com
+EOF
+------------------------------------------------
+
+(See the "CONFIGURATION FILE" section of gitlink:git-config[1] for
+details on the configuration file.)
Creating a new repository
-------------------------------------------------
$ mkdir project
$ cd project
-$ git init-db
+$ git init
-------------------------------------------------
If you have some initial content (say, a tarball):
-------------------------------------------------
$ tar -xzvf project.tar.gz
$ cd project
-$ git init-db
+$ git init
$ git add . # include everything below ./ in the first commit:
$ git commit
-------------------------------------------------
at step 3, git maintains a snapshot of the tree's contents in a
special staging area called "the index."
-By default, the content of the index is identical to that of the
-HEAD. The command "git diff --cached" shows the difference between
-HEAD and the index, so you should no output from that command.
+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:
$ git add path/to/file
-------------------------------------------------
-To remove a file from the index that you've removed from the working
-tree,
+To remove a file from the index and from the working tree,
-------------------------------------------------
$ git rm path/to/file
-------------------------------------------------
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
+commit. Check to make sure it looks like what you expected with
-------------------------------------------------
$ git show
information from taking up to much space on disk or in memory.
This compression is not performed automatically. Therefore you
-should occasionally run
+should occasionally run gitlink:git-gc[1]:
-------------------------------------------------
$ git gc
-------------------------------------------------
-to recompress the archive and to prune any commits which are no
-longer referred to anywhere. This can be very time-consuming, and
-you should not modify the repository while it is working, so you
-should run it while you are not working.
+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 gitlink: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 gitlink:git-prune[1] or the --prune
+option to gitlink: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 <<dangling-objects>>.
+
+
+Recovering lost changes
+~~~~~~~~~~~~~~~~~~~~~~~
+
+Reflogs
+^^^^^^^
+
+Say you modify a branch with gitlink: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 gitlink:git-reflog[1] and gitlink:git-gc[1] to learn
+how to control this pruning, and see the "SPECIFYING REVISIONS"
+section of gitlink: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
+...
+-------------------------------------------------
+
+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
+------------------------------------------------
+
Sharing development with others
===============================
-------------------------------------------------
See the descriptions of the branch.<name>.remote and
-branch.<name>.merge options in gitlink:git-repo-config[1] to learn
+branch.<name>.merge options in gitlink: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
<<fast-forwards,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 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
-------------------------------------------------
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 gitlink:git-format-patches[1]; for example:
+First, use gitlink:git-format-patch[1]; for example:
-------------------------------------------------
-$ git format-patches origin
+$ git format-patch origin
-------------------------------------------------
will produce a numbered series of files in the current directory, one
single mailbox file, say "patches.mbox", then run
-------------------------------------------------
-$ git am patches.mbox
+$ 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,Resolving a merge>>". Once the index is updated
-with the results of the conflict resolution, instead of creating a
-new commit, just run
+"<<resolving-a-merge,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
-------------------------------------------------
See the explanations of the remote.<name>.url, branch.<name>.remote,
-and remote.<name>.push options in gitlink:git-repo-config[1] for
+and remote.<name>.push options in gitlink:git-config[1] for
details.
Setting up a shared repository
Allow web browsing of a repository
----------------------------------
-TODO: Brief setup-instructions for gitweb
+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, ?
-Working with other version control systems
-==========================================
-
-TODO: CVS, Subversion, series-of-release-tarballs, ?
-
[[cleaning-up-history]]
Rewriting history and maintaining patch series
==============================================
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 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.
4. 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.
+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:
+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
$ 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:
+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 as described in
-"<<resolving-a-merge,Resolving a merge>>". Once the index is updated with
-the results of the conflict resolution, instead of creating a new commit,
-just run
+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
Reordering or selecting from a patch series
-------------------------------------------
-Given one existing commit, the gitlink:git-cherry-pick[1] command allows
-you to apply the change introduced by that commit and create a new commit
-that records it.
+Given one existing commit, the gitlink: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.
-This can be useful for modifying a patch series.
+Another technique is to use git-format-patch to create a series of
+patches, then reset the state to before the patches:
-TODO: elaborate
+-------------------------------------------------
+$ git format-patch origin
+$ git reset --hard origin
+-------------------------------------------------
+
+Then modify, reorder, or eliminate patches as preferred before applying
+them again with gitlink:git-am[1].
Other tools
-----------
-There are numerous other tools, such as stgit, which exist for the purpose
-of maintianing a patch series. These are out of the scope of this manual.
+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.
+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.
+
+Advanced branch management
+==========================
+
+Fetching individual branches
+----------------------------
+
+Instead of using gitlink: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:
+
+[[fast-forwards]]
+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
+gitlink: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:
-TODO: elaborate
+-------------------------------------------------
+$ 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
+gitlink:git-config[1].
+
+See gitlink:git-config[1] for more details on the configuration
+options mentioned above.
Git internals
=============
-Architectural overview
+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 gitlink:git-update-index[1]
+is run, and its data can be accessed by gitlink: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 gitlink:git-write-tree[1] and
+its data can be accessed by gitlink:git-ls-tree[1].
+Two trees can be compared with gitlink: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 gitlink:git-commit-tree[1] and
+its data can be accessed by gitlink: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 gitlink:git-mktag[1],
+its data can be accessed by gitlink:git-cat-file[1],
+and the signature can be verified by
+gitlink: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 gitlink: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.
+
+------------
+
+ commit-tree
+ commit obj
+ +----+
+ | |
+ | |
+ V V
+ +-----------+
+ | Object DB |
+ | Backing |
+ | Store |
+ +-----------+
+ ^
+ write-tree | |
+ tree obj | |
+ | | read-tree
+ | | tree obj
+ V
+ +-----------+
+ | Index |
+ | "cache" |
+ +-----------+
+ update-index ^
+ blob obj | |
+ | |
+ checkout-index -u | | checkout-index
+ stat | | blob obj
+ V
+ +-----------+
+ | Working |
+ | Directory |
+ +-----------+
+
+------------
+
+
+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
+gitlink: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
----------------------
-TODO: Sources, README, core-tutorial, tutorial-2.txt, technical/
+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 link:technical/pack-format.txt[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 gitlink:git-gc[1] command performs packing, pruning, and more for
+you, so is normally the only high-level command you need.
+
+[[dangling-objects]]
+Dangling objects
+----------------
+
+The gitlink: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
+<<cleaning-up-history>>. In that case, the old head of the original
+branch still exists, as does obviously everything it pointed to. The
+branch pointer itself just doesn't, since you replaced it with another
+one.
+
+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).
Glossary of git terms
=====================
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.
+ - 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.
+
+Say something about .gitignore.
Scan Documentation/ for other stuff left out; in particular:
howto's
- README
some of technical/?
hooks
- etc.
+ list of commands in gitlink:git[1]
Scan email archives for other stuff left out
Scan man pages to see if any assume more background than this manual
provides.
-Update git fetch discussion to use "git remote", move most of branch
-discussion till later.
-
-Can also simplify beginning by suggesting disconnected head instead
-of temporary branch creation.
+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. Also what's easiest way to see common merge base?
+"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?
+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.
-To document:
- reflogs, git reflog expire
- shallow clones?? See draft 1.5.0 release notes for some documentation.
+Document shallow clones? See draft 1.5.0 release notes for some
+documentation.
+
+Add a section 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.