585 lines
19 KiB
Plaintext
585 lines
19 KiB
Plaintext
A tutorial introduction to git (for version 1.5.1 or newer)
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===========================================================
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This tutorial explains how to import a new project into git, make
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changes to it, and share changes with other developers.
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If you are instead primarily interested in using git to fetch a project,
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for example, to test the latest version, you may prefer to start with
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the first two chapters of link:user-manual.html[The Git User's Manual].
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First, note that you can get documentation for a command such as "git
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diff" with:
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------------------------------------------------
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$ man git-diff
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------------------------------------------------
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It is a good idea to introduce yourself to git with your name and
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public email address before doing any operation. The easiest
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way to do so is:
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------------------------------------------------
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$ git config --global user.name "Your Name Comes Here"
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$ git config --global user.email you@yourdomain.example.com
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------------------------------------------------
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Importing a new project
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-----------------------
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Assume you have a tarball project.tar.gz with your initial work. You
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can place it under git revision control as follows.
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------------------------------------------------
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$ tar xzf project.tar.gz
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$ cd project
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$ git init
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------------------------------------------------
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Git will reply
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------------------------------------------------
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Initialized empty Git repository in .git/
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------------------------------------------------
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You've now initialized the working directory--you may notice a new
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directory created, named ".git".
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Next, tell git to take a snapshot of the contents of all files under the
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current directory (note the '.'), with gitlink:git-add[1]:
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------------------------------------------------
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$ git add .
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------------------------------------------------
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This snapshot is now stored in a temporary staging area which git calls
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the "index". You can permanently store the contents of the index in the
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repository with gitlink:git-commit[1]:
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------------------------------------------------
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$ git commit
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------------------------------------------------
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This will prompt you for a commit message. You've now stored the first
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version of your project in git.
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Making changes
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--------------
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Modify some files, then add their updated contents to the index:
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------------------------------------------------
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$ git add file1 file2 file3
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------------------------------------------------
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You are now ready to commit. You can see what is about to be committed
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using gitlink:git-diff[1] with the --cached option:
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------------------------------------------------
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$ git diff --cached
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------------------------------------------------
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(Without --cached, gitlink:git-diff[1] will show you any changes that
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you've made but not yet added to the index.) You can also get a brief
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summary of the situation with gitlink:git-status[1]:
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------------------------------------------------
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$ git status
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# On branch master
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# Changes to be committed:
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# (use "git reset HEAD <file>..." to unstage)
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#
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# modified: file1
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# modified: file2
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# modified: file3
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#
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------------------------------------------------
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If you need to make any further adjustments, do so now, and then add any
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newly modified content to the index. Finally, commit your changes with:
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------------------------------------------------
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$ git commit
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------------------------------------------------
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This will again prompt your for a message describing the change, and then
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record a new version of the project.
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Alternatively, instead of running `git add` beforehand, you can use
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------------------------------------------------
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$ git commit -a
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------------------------------------------------
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which will automatically notice any modified (but not new) files, add
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them to the index, and commit, all in one step.
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A note on commit messages: Though not required, it's a good idea to
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begin the commit message with a single short (less than 50 character)
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line summarizing the change, followed by a blank line and then a more
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thorough description. Tools that turn commits into email, for
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example, use the first line on the Subject: line and the rest of the
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commit in the body.
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Git tracks content not files
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----------------------------
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Many revision control systems provide an "add" command that tells the
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system to start tracking changes to a new file. Git's "add" command
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does something simpler and more powerful: `git add` is used both for new
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and newly modified files, and in both cases it takes a snapshot of the
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given files and stages that content in the index, ready for inclusion in
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the next commit.
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Viewing project history
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-----------------------
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At any point you can view the history of your changes using
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$ git log
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------------------------------------------------
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If you also want to see complete diffs at each step, use
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$ git log -p
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------------------------------------------------
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Often the overview of the change is useful to get a feel of
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each step
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------------------------------------------------
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$ git log --stat --summary
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------------------------------------------------
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Managing branches
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-----------------
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A single git repository can maintain multiple branches of
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development. To create a new branch named "experimental", use
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$ git branch experimental
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------------------------------------------------
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If you now run
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------------------------------------------------
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$ git branch
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------------------------------------------------
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you'll get a list of all existing branches:
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------------------------------------------------
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experimental
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* master
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------------------------------------------------
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The "experimental" branch is the one you just created, and the
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"master" branch is a default branch that was created for you
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automatically. The asterisk marks the branch you are currently on;
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type
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------------------------------------------------
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$ git checkout experimental
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------------------------------------------------
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to switch to the experimental branch. Now edit a file, commit the
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change, and switch back to the master branch:
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------------------------------------------------
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(edit file)
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$ git commit -a
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$ git checkout master
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------------------------------------------------
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Check that the change you made is no longer visible, since it was
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made on the experimental branch and you're back on the master branch.
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You can make a different change on the master branch:
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------------------------------------------------
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(edit file)
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$ git commit -a
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------------------------------------------------
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at this point the two branches have diverged, with different changes
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made in each. To merge the changes made in experimental into master, run
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------------------------------------------------
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$ git merge experimental
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------------------------------------------------
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If the changes don't conflict, you're done. If there are conflicts,
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markers will be left in the problematic files showing the conflict;
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------------------------------------------------
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$ git diff
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------------------------------------------------
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will show this. Once you've edited the files to resolve the
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conflicts,
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------------------------------------------------
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$ git commit -a
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------------------------------------------------
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will commit the result of the merge. Finally,
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------------------------------------------------
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$ gitk
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------------------------------------------------
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will show a nice graphical representation of the resulting history.
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At this point you could delete the experimental branch with
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------------------------------------------------
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$ git branch -d experimental
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------------------------------------------------
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This command ensures that the changes in the experimental branch are
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already in the current branch.
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If you develop on a branch crazy-idea, then regret it, you can always
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delete the branch with
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-------------------------------------
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$ git branch -D crazy-idea
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-------------------------------------
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Branches are cheap and easy, so this is a good way to try something
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out.
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Using git for collaboration
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---------------------------
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Suppose that Alice has started a new project with a git repository in
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/home/alice/project, and that Bob, who has a home directory on the
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same machine, wants to contribute.
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Bob begins with:
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------------------------------------------------
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$ git clone /home/alice/project myrepo
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------------------------------------------------
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This creates a new directory "myrepo" containing a clone of Alice's
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repository. The clone is on an equal footing with the original
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project, possessing its own copy of the original project's history.
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Bob then makes some changes and commits them:
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------------------------------------------------
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(edit files)
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$ git commit -a
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(repeat as necessary)
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------------------------------------------------
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When he's ready, he tells Alice to pull changes from the repository
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at /home/bob/myrepo. She does this with:
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------------------------------------------------
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$ cd /home/alice/project
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$ git pull /home/bob/myrepo master
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------------------------------------------------
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This merges the changes from Bob's "master" branch into Alice's
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current branch. If Alice has made her own changes in the meantime,
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then she may need to manually fix any conflicts. (Note that the
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"master" argument in the above command is actually unnecessary, as it
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is the default.)
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The "pull" command thus performs two operations: it fetches changes
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from a remote branch, then merges them into the current branch.
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When you are working in a small closely knit group, it is not
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unusual to interact with the same repository over and over
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again. By defining 'remote' repository shorthand, you can make
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it easier:
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------------------------------------------------
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$ git remote add bob /home/bob/myrepo
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------------------------------------------------
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With this, Alice can perform the first operation alone using the
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"git fetch" command without merging them with her own branch,
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using:
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-------------------------------------
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$ git fetch bob
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-------------------------------------
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Unlike the longhand form, when Alice fetches from Bob using a
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remote repository shorthand set up with `git remote`, what was
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fetched is stored in a remote tracking branch, in this case
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`bob/master`. So after this:
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-------------------------------------
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$ git log -p master..bob/master
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-------------------------------------
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shows a list of all the changes that Bob made since he branched from
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Alice's master branch.
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After examining those changes, Alice
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could merge the changes into her master branch:
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-------------------------------------
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$ git merge bob/master
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-------------------------------------
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This `merge` can also be done by 'pulling from her own remote
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tracking branch', like this:
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-------------------------------------
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$ git pull . remotes/bob/master
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-------------------------------------
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Note that git pull always merges into the current branch,
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regardless of what else is given on the commandline.
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Later, Bob can update his repo with Alice's latest changes using
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-------------------------------------
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$ git pull
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-------------------------------------
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Note that he doesn't need to give the path to Alice's repository;
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when Bob cloned Alice's repository, git stored the location of her
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repository in the repository configuration, and that location is
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used for pulls:
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-------------------------------------
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$ git config --get remote.origin.url
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/home/bob/myrepo
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-------------------------------------
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(The complete configuration created by git-clone is visible using
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"git config -l", and the gitlink:git-config[1] man page
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explains the meaning of each option.)
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Git also keeps a pristine copy of Alice's master branch under the
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name "origin/master":
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-------------------------------------
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$ git branch -r
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origin/master
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-------------------------------------
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If Bob later decides to work from a different host, he can still
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perform clones and pulls using the ssh protocol:
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-------------------------------------
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$ git clone alice.org:/home/alice/project myrepo
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-------------------------------------
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Alternatively, git has a native protocol, or can use rsync or http;
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see gitlink:git-pull[1] for details.
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Git can also be used in a CVS-like mode, with a central repository
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that various users push changes to; see gitlink:git-push[1] and
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link:cvs-migration.html[git for CVS users].
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Exploring history
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-----------------
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Git history is represented as a series of interrelated commits. We
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have already seen that the git log command can list those commits.
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Note that first line of each git log entry also gives a name for the
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commit:
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-------------------------------------
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$ git log
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commit c82a22c39cbc32576f64f5c6b3f24b99ea8149c7
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Author: Junio C Hamano <junkio@cox.net>
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Date: Tue May 16 17:18:22 2006 -0700
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merge-base: Clarify the comments on post processing.
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-------------------------------------
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We can give this name to git show to see the details about this
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commit.
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-------------------------------------
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$ git show c82a22c39cbc32576f64f5c6b3f24b99ea8149c7
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-------------------------------------
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But there are other ways to refer to commits. You can use any initial
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part of the name that is long enough to uniquely identify the commit:
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-------------------------------------
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$ git show c82a22c39c # the first few characters of the name are
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# usually enough
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$ git show HEAD # the tip of the current branch
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$ git show experimental # the tip of the "experimental" branch
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-------------------------------------
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Every commit usually has one "parent" commit
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which points to the previous state of the project:
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-------------------------------------
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$ git show HEAD^ # to see the parent of HEAD
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$ git show HEAD^^ # to see the grandparent of HEAD
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$ git show HEAD~4 # to see the great-great grandparent of HEAD
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-------------------------------------
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Note that merge commits may have more than one parent:
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-------------------------------------
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$ git show HEAD^1 # show the first parent of HEAD (same as HEAD^)
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$ git show HEAD^2 # show the second parent of HEAD
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-------------------------------------
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You can also give commits names of your own; after running
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-------------------------------------
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$ git-tag v2.5 1b2e1d63ff
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-------------------------------------
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you can refer to 1b2e1d63ff by the name "v2.5". If you intend to
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share this name with other people (for example, to identify a release
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version), you should create a "tag" object, and perhaps sign it; see
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gitlink:git-tag[1] for details.
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Any git command that needs to know a commit can take any of these
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names. For example:
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-------------------------------------
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$ git diff v2.5 HEAD # compare the current HEAD to v2.5
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$ git branch stable v2.5 # start a new branch named "stable" based
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# at v2.5
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$ git reset --hard HEAD^ # reset your current branch and working
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# directory to its state at HEAD^
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-------------------------------------
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Be careful with that last command: in addition to losing any changes
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in the working directory, it will also remove all later commits from
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this branch. If this branch is the only branch containing those
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commits, they will be lost. Also, don't use "git reset" on a
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publicly-visible branch that other developers pull from, as it will
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force needless merges on other developers to clean up the history.
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If you need to undo changes that you have pushed, use gitlink:git-revert[1]
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instead.
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The git grep command can search for strings in any version of your
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project, so
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-------------------------------------
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$ git grep "hello" v2.5
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-------------------------------------
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searches for all occurrences of "hello" in v2.5.
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If you leave out the commit name, git grep will search any of the
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files it manages in your current directory. So
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-------------------------------------
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$ git grep "hello"
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-------------------------------------
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is a quick way to search just the files that are tracked by git.
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Many git commands also take sets of commits, which can be specified
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in a number of ways. Here are some examples with git log:
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-------------------------------------
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$ git log v2.5..v2.6 # commits between v2.5 and v2.6
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$ git log v2.5.. # commits since v2.5
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$ git log --since="2 weeks ago" # commits from the last 2 weeks
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$ git log v2.5.. Makefile # commits since v2.5 which modify
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# Makefile
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-------------------------------------
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You can also give git log a "range" of commits where the first is not
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necessarily an ancestor of the second; for example, if the tips of
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the branches "stable-release" and "master" diverged from a common
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commit some time ago, then
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-------------------------------------
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$ git log stable..experimental
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-------------------------------------
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will list commits made in the experimental branch but not in the
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stable branch, while
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-------------------------------------
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$ git log experimental..stable
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-------------------------------------
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will show the list of commits made on the stable branch but not
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the experimental branch.
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The "git log" command has a weakness: it must present commits in a
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list. When the history has lines of development that diverged and
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then merged back together, the order in which "git log" presents
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those commits is meaningless.
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Most projects with multiple contributors (such as the linux kernel,
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or git itself) have frequent merges, and gitk does a better job of
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visualizing their history. For example,
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-------------------------------------
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$ gitk --since="2 weeks ago" drivers/
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-------------------------------------
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allows you to browse any commits from the last 2 weeks of commits
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that modified files under the "drivers" directory. (Note: you can
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adjust gitk's fonts by holding down the control key while pressing
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"-" or "+".)
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Finally, most commands that take filenames will optionally allow you
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to precede any filename by a commit, to specify a particular version
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of the file:
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-------------------------------------
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$ git diff v2.5:Makefile HEAD:Makefile.in
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-------------------------------------
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You can also use "git show" to see any such file:
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-------------------------------------
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$ git show v2.5:Makefile
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-------------------------------------
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Next Steps
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----------
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This tutorial should be enough to perform basic distributed revision
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control for your projects. However, to fully understand the depth
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and power of git you need to understand two simple ideas on which it
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is based:
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* The object database is the rather elegant system used to
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store the history of your project--files, directories, and
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commits.
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* The index file is a cache of the state of a directory tree,
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used to create commits, check out working directories, and
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hold the various trees involved in a merge.
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link:tutorial-2.html[Part two of this tutorial] explains the object
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database, the index file, and a few other odds and ends that you'll
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need to make the most of git.
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If you don't want to continue with that right away, a few other
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digressions that may be interesting at this point are:
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* gitlink:git-format-patch[1], gitlink:git-am[1]: These convert
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series of git commits into emailed patches, and vice versa,
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useful for projects such as the linux kernel which rely heavily
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on emailed patches.
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* gitlink:git-bisect[1]: When there is a regression in your
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project, one way to track down the bug is by searching through
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the history to find the exact commit that's to blame. Git bisect
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can help you perform a binary search for that commit. It is
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smart enough to perform a close-to-optimal search even in the
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case of complex non-linear history with lots of merged branches.
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* link:everyday.html[Everyday GIT with 20 Commands Or So]
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* link:cvs-migration.html[git for CVS users].
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