gittutorial - A tutorial introduction to Git

This tutorial explains how to import a new project into Git, make changes to it, and share changes with other developers.

If you are instead primarily interested in using Git to fetch a project, for example, to test the latest version, you may prefer to start with the first two chapters of The Git User’s Manual.

First, note that you can get documentation for a command such as git log --graph with:

or:

With the latter, you can use the manual viewer of your choice; see git-help[1] for more information.

It is a good idea to introduce yourself to Git with your name and public email address before doing any operation. The easiest way to do so is:

Assume you have a tarball project.tar.gz with your initial work. You can place it under Git revision control as follows.

Git will reply

You’ve now initialized the working directory—​you may notice a new directory created, named ".git".

Next, tell Git to take a snapshot of the contents of all files under the current directory (note the .), with git add:

This snapshot is now stored in a temporary staging area which Git calls the "index". You can permanently store the contents of the index in the repository with git commit:

This will prompt you for a commit message. You’ve now stored the first version of your project in Git.

Modify some files, then add their updated contents to the index:

You are now ready to commit. You can see what is about to be committed using git diff with the --cached option:

(Without --cached, git diff will show you any changes that you’ve made but not yet added to the index.) You can also get a brief summary of the situation with git status:

If you need to make any further adjustments, do so now, and then add any newly modified content to the index. Finally, commit your changes with:

This will again prompt you for a message describing the change, and then record a new version of the project.

Alternatively, instead of running git add beforehand, you can use

which will automatically notice any modified (but not new) files, add them to the index, and commit, all in one step.

A note on commit messages: Though not required, it’s a good idea to begin the commit message with a single short (less than 50 character) line summarizing the change, followed by a blank line and then a more thorough description. The text up to the first blank line in a commit message is treated as the commit title, and that title is used throughout Git. For example, git-format-patch[1] turns a commit into email, and it uses the title on the Subject line and the rest of the commit in the body.

Many revision control systems provide an add command that tells the system to start tracking changes to a new file. Git’s add command does something simpler and more powerful: git add is used both for new and newly modified files, and in both cases it takes a snapshot of the given files and stages that content in the index, ready for inclusion in the next commit.

At any point you can view the history of your changes using

If you also want to see complete diffs at each step, use

Often the overview of the change is useful to get a feel of each step

A single Git repository can maintain multiple branches of development. To create a new branch named "experimental", use

If you now run

you’ll get a list of all existing branches:

The "experimental" branch is the one you just created, and the "master" branch is a default branch that was created for you automatically. The asterisk marks the branch you are currently on; type

to switch to the experimental branch. Now edit a file, commit the change, and switch back to the master branch:

Check that the change you made is no longer visible, since it was made on the experimental branch and you’re back on the master branch.

You can make a different change on the master branch:

at this point the two branches have diverged, with different changes made in each. To merge the changes made in experimental into master, run

If the changes don’t conflict, you’re done. If there are conflicts, markers will be left in the problematic files showing the conflict;

will show this. Once you’ve edited the files to resolve the conflicts,

will commit the result of the merge. Finally,

will show a nice graphical representation of the resulting history.

At this point you could delete the experimental branch with

This command ensures that the changes in the experimental branch are already in the current branch.

If you develop on a branch crazy-idea, then regret it, you can always delete the branch with

Branches are cheap and easy, so this is a good way to try something out.

Suppose that Alice has started a new project with a Git repository in /home/alice/project, and that Bob, who has a home directory on the same machine, wants to contribute.

Bob begins with:

This creates a new directory "myrepo" containing a clone of Alice’s repository. The clone is on an equal footing with the original project, possessing its own copy of the original project’s history.

Bob then makes some changes and commits them:

When he’s ready, he tells Alice to pull changes from the repository at /home/bob/myrepo. She does this with:

This merges the changes from Bob’s "master" branch into Alice’s current branch. If Alice has made her own changes in the meantime, then she may need to manually fix any conflicts.

The "pull" command thus performs two operations: it fetches changes from a remote branch, then merges them into the current branch.

Note that in general, Alice would want her local changes committed before initiating this "pull". If Bob’s work conflicts with what Alice did since their histories forked, Alice will use her working tree and the index to resolve conflicts, and existing local changes will interfere with the conflict resolution process (Git will still perform the fetch but will refuse to merge --- Alice will have to get rid of her local changes in some way and pull again when this happens).

Alice can peek at what Bob did without merging first, using the "fetch" command; this allows Alice to inspect what Bob did, using a special symbol "FETCH_HEAD", in order to determine if he has anything worth pulling, like this:

This operation is safe even if Alice has uncommitted local changes. The range notation "HEAD..FETCH_HEAD" means "show everything that is reachable from the FETCH_HEAD but exclude anything that is reachable from HEAD". Alice already knows everything that leads to her current state (HEAD), and reviews what Bob has in his state (FETCH_HEAD) that she has not seen with this command.

If Alice wants to visualize what Bob did since their histories forked she can issue the following command:

This uses the same two-dot range notation we saw earlier with git log.

Alice may want to view what both of them did since they forked. She can use three-dot form instead of the two-dot form:

This means "show everything that is reachable from either one, but exclude anything that is reachable from both of them".

Please note that these range notation can be used with both gitk and "git log".

After inspecting what Bob did, if there is nothing urgent, Alice may decide to continue working without pulling from Bob. If Bob’s history does have something Alice would immediately need, Alice may choose to stash her work-in-progress first, do a "pull", and then finally unstash her work-in-progress on top of the resulting history.

When you are working in a small closely knit group, it is not unusual to interact with the same repository over and over again. By defining remote repository shorthand, you can make it easier:

With this, Alice can perform the first part of the "pull" operation alone using the git fetch command without merging them with her own branch, using:

Unlike the longhand form, when Alice fetches from Bob using a remote repository shorthand set up with git remote, what was fetched is stored in a remote-tracking branch, in this case bob/master. So after this:

shows a list of all the changes that Bob made since he branched from Alice’s master branch.

After examining those changes, Alice could merge the changes into her master branch:

This merge can also be done by pulling from her own remote-tracking branch, like this:

Note that git pull always merges into the current branch, regardless of what else is given on the command line.

Later, Bob can update his repo with Alice’s latest changes using

Note that he doesn’t need to give the path to Alice’s repository; when Bob cloned Alice’s repository, Git stored the location of her repository in the repository configuration, and that location is used for pulls:

(The complete configuration created by git clone is visible using git config -l, and the git-config[1] man page explains the meaning of each option.)

Git also keeps a pristine copy of Alice’s master branch under the name "origin/master":

If Bob later decides to work from a different host, he can still perform clones and pulls using the ssh protocol:

Alternatively, Git has a native protocol, or can use http; see git-pull[1] for details.

Git can also be used in a CVS-like mode, with a central repository that various users push changes to; see git-push[1] and gitcvs-migration[7].

Git history is represented as a series of interrelated commits. We have already seen that the git log command can list those commits. Note that first line of each git log entry also gives a name for the commit:

We can give this name to git show to see the details about this commit.

But there are other ways to refer to commits. You can use any initial part of the name that is long enough to uniquely identify the commit:

Every commit usually has one "parent" commit which points to the previous state of the project:

Note that merge commits may have more than one parent:

You can also give commits names of your own; after running

you can refer to 1b2e1d63ff by the name "v2.5". If you intend to share this name with other people (for example, to identify a release version), you should create a "tag" object, and perhaps sign it; see git-tag[1] for details.

Any Git command that needs to know a commit can take any of these names. For example:

Be careful with that last command: in addition to losing any changes in the working directory, it will also remove all later commits from this branch. If this branch is the only branch containing those commits, they will be lost. Also, don’t use git reset on a publicly-visible branch that other developers pull from, as it will force needless merges on other developers to clean up the history. If you need to undo changes that you have pushed, use git revert instead.

The git grep command can search for strings in any version of your project, so

searches for all occurrences of "hello" in v2.5.

If you leave out the commit name, git grep will search any of the files it manages in your current directory. So

is a quick way to search just the files that are tracked by Git.

Many Git commands also take sets of commits, which can be specified in a number of ways. Here are some examples with git log:

You can also give git log a "range" of commits where the first is not necessarily an ancestor of the second; for example, if the tips of the branches "stable" and "master" diverged from a common commit some time ago, then

will list commits made in the master branch but not in the stable branch, while

will show the list of commits made on the stable branch but not the master branch.

The git log command has a weakness: it must present commits in a list. When the history has lines of development that diverged and then merged back together, the order in which git log presents those commits is meaningless.

Most projects with multiple contributors (such as the Linux kernel, or Git itself) have frequent merges, and gitk does a better job of visualizing their history. For example,

allows you to browse any commits from the last 2 weeks of commits that modified files under the "drivers" directory. (Note: you can adjust gitk’s fonts by holding down the control key while pressing "-" or "+".)

Finally, most commands that take filenames will optionally allow you to precede any filename by a commit, to specify a particular version of the file:

You can also use git show to see any such file:

This tutorial should be enough to perform basic distributed revision control for your projects. However, to fully understand the depth and power of Git you need to understand two simple ideas on which it is based:

Part two of this tutorial explains the object database, the index file, and a few other odds and ends that you’ll need to make the most of Git. You can find it at gittutorial-2[7].

If you don’t want to continue with that right away, a few other digressions that may be interesting at this point are:

gittutorial-2[7], gitcvs-migration[7], gitcore-tutorial[7], gitglossary[7], git-help[1], gitworkflows[7], giteveryday[7], The Git User’s Manual

Part of the git[1] suite.