KERNEL HACKERS' GUIDE TO GIT

Kernel Hackers' Guide to git
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This tutorial is a cookbook of recipes getting up and running with Linus's source code
management (SCM) software, "git." Its targetted mainly at Linux kernel hackers, though 
others may find it useful.
Table of Contents
 
    * Getting Started
          o Installing git
          o First kernel tree download
    * Basic tasks
          o Download remote tree updates
          o Undo all working dir modifications
          o Undo recent commits
          o Check in changes
          o Generate diff of working dir changes
          o Generate summary of working dir changes
          o List all changeset descriptions
          o List all changeset descriptions belonging to a specific file
    * Branches
          o List all branches
          o Switch working dir to new branch
          o Create new branch
          o List current branch
          o Diff against master branch
          o List changes present only on local branch
          o Merge changes from one branch into another
    * Misc. debris
          o Check out an older kernel version
          o Apply mailbox full of patches
          o Download tags periodically
          o Create a tag
    * Further reading
 
Getting Started
Installing git
 
git requires bootstrapping, since you must have git installed in order to check out git.git 
(git repository), and linux-2.6.git (kernel repository). You may find that your distribution
 already provides a usable version of git. If so, try that first.
 
    * Fedora 7 and later: The git-core package is available through the standard package 
repositories. Fedora Core 3 through 6: git-core package is in Fedora Extras.
 
      yum install git-core
 
If your distro does not package git, you may download the latest stable release from:
http://www.kernel.org/pub/software/scm/git/
 
tarball build-deps: zlib, libcurl, libcrypto (openssl)
 
install tarball:
 
unpack && make prefix=/usr/local && sudo make prefix=/usr/local install
 
After reading the rest of this document, come back and update your copy of git to the 
latest: git://git.kernel.org/pub/scm/git/git.git
Download a linux kernel tree for the very first time
 
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git linux-2.6
 
NOTE: The kernel tree is very large. This constitutes downloading just over 300 megabytes of
 compressed data (as of Jun 2008).
Basic Tasks
Update local kernel tree to latest 2.6.x upstream ("fast-forward merge")
 
$ cd linux-2.6
$ git pull git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git
 
or more simply, to pull from the location from which you cloned:
 
$ cd linux-2.6
$ git pull
 
to pull from the origin repository from which you originally cloned the tree.
Undo all local modifications:
 
$ git checkout -f
 
Check in your own modifications (e.g. do some hacking, or apply a patch)
 
# go to repository
$ cd linux-2.6
 
# make some modifications
$ vi drivers/net/sk98lin/skdim.c
 
# NOTE: Run 'git add' and 'git rm' if adding or removing files.
 
# check in all modifications
$ git commit -a
 
Undo recent commits:
 
Sometimes you have made a few commits, or just pulled a change, and simply want those 
commits to go away.
 
$ cd my-kernel-tree-2.6
$ git reset HEAD~2	# make last 2 commits disappear
 
will "disappear" the top two commits. DO NOT do this, if anyone has downloaded a tree 
containing the commits you just eliminated.
 
Note that this is quite different from git revert, which applies a reversed patch as an 
additional commit.
List all changes in working dir, in diff format.
 
Display changes since last 'git add' or 'git rm':
 
$ git diff
 
Display changes since last commit:
 
$ git diff HEAD
 
Obtain summary of all changes in working dir
 
$ git status
 
List all changeset descriptions
 
$ git log
 
The 'git log' option "-p" shows diffs in addition to changeset text. The option "--stat" shows the diffstat in addition to the changeset text.
List all changesets belonging to a specific file
(in this case, net/ieee80211/ieee80211_module.c)
 
$ git log net/ieee80211/ieee80211_module.c
 
Branches
List all branches
 
$ git branch
 
Make desired branch current in working directory
 
$ git checkout $branch
 
Create a new branch, and make it current
 
$ git checkout -b my-new-branch-name master
 
Examine which branch is current
 
$ git status
 
('git branch' also shows you the current branch, using a "*" to indicate this)
Obtain a diff between current branch, and master branch
 
In most trees with branches, .git/refs/heads/master contains the current 'vanilla' upstream
 tree, for easy diffing and merging. (in trees without branches, 'master' simply contains 
your latest changes)
 
$ git diff master..HEAD
 
(this is equivalent to git diff HEAD, when used with HEAD branch)
Obtain a list of changes between current branch, and master branch
 
$ git log master..HEAD
 
(this is equivalent to git log, when used with HEAD)
or rather than full changeset descriptions, obtain a one-line summary of each changes:
 
$ git shortlog master..HEAD
 
Merge changes from one branch into another
Let us suppose that you do work on branch A and branch B, and after work on those two 
branches is complete, you merge the work into mainline branch M.
 
$ git checkout M	# switch to branch M
$ git merge A		# merge A into M
$ git merge B		# merge B into M
 
Misc. Debris
Optimize your repository
 
git is heavily optimized for fast storage and retrieval on a per-command basis. However, 
over a long period of time, it can be useful to perform further optimizations, including 
packing all git objects into single "packfile" for fast retrieval and less wasted disk space.
 
$ cd my-kernel-tree-2.6
$ git gc
 
will optimize your repository. You don't need to run this frequently â   git is quite fast 
 even without it. See the 'git gc' man page for more details.
Check out an older kernel version
 
$ cd my-kernel-tree-2.6
$ git checkout -b tmp v2.6.22
 
This creates a temporary branch 'tmp', with the contents of kernel version 2.6.22.
Apply all patches in a Berkeley mbox-format file
 
First, make sure that the tools subdirectory of the git-core repository is in your PATH.
 
$ cd my-kernel-tree-2.6
$ git am --utf8 --signoff /path/to/mbox
 
The file /path/to/mbox is a Berkeley mbox file, containing one or more patches to be 
committed to the git repository. The --signoff option indicates that 'git am' should append 
the
 
Signed-off-by: Your Name <[email protected]>
 
line that is common to almost all kernel submissions. The name and email address are taken
 from the GIT_COMMITTER_NAME and GIT_COMMITTER_EMAIL environment variables (I recommend 
setting these in your .bash_profile or similar file).
Don't forget to download tags from time to time.
 
git pull only downloads sha1-indexed object data, and the requested remote head. This misses
 updates to the .git/refs/tags/ and .git/refs/heads/ directories. For tags, run git fetch 
--tags $URL.
Tag a particular commit.
 
For your own repositories, you may wish to give interesting or significant commits a name, 
known as a tag. The Linux kernel uses tags to for each kernel version: "v2.6.21", "v2.6.22",
 etc.
 
$ cd my-kernel-tree-2.6
$ git tag my-tag
 
creates a new tag named "my-tag", based on the current commit. You can do a lot more with 
tagging, including GPG-signing, so read the man page for more details.
Further reading
 
Another good introduction is the official git tutorial, followed by the more in-depth git 
man page documentation.