Collaborating Using Git

Share your code the kernel developer way.

At the heart of collaboration using git is the ability to use public repositories as the basis for your own development, and in turn to publish your repository so others may use it. This article introduces the means to base your development on public git repositories and details the installation and configuration procedures for publishing your own repositories.

Two prior Linux Magazine articles, How To Git It and Embrace the Git Index, cover basic concepts about git and explain some of the details about the git index. This article introduces many new git concepts and techniques that can be used for collaborative development, including the use and manipulation of topic and tracking branches, cloning a remote repository, details on installing a server for git repositories using HTTP and git- native protocols, and publishing a public repository via the Web.

Topic And Tracking Branches

Normally, branches are used to identify conceptual lines of development within a repository. Since branches are very inexpensive in git and easy to create, it’s often desirable to have several small branches that each contain very well-defined concepts or small incremental, yet related changes. Such branches are termed topic branches. Often these branches are short-lived, and might be merged into either a temporary test branch or a long-lived “master” branch.

Another typical use for branches is to track the development from another repository and bring those changes into your repository. Branches used in this way are called tracking branches. These branches are typically much longer lived, and again, are typically merged into a “master” branch within your local repository.

Since tracking branches are used exclusively to follow the changes from another repository, you shouldn’t make any merges or perform any direct commits onto a tracking branch. Doing so will cause your tracking branch to become out of sync with the so-called upstream version of that branch, and future updates from that repository will continually require merging.

Cloning a Remote Repository

The first step in collaborating with another git project is to locate the remote, public repository on which your development will be based. Typically, the remote repository will either be on a file-system you can directly access or at a URL where the repository may be found on the Internet. You can also locate a repository URL using a Web-based interface such as gitweb.

Once you’ve found the repository, use git clone to make a local copy of it. For example, the “primes.git” repository can be cloned using one of these two commands:

$ git clone http://www.example.com/pub/software/primes.git

$ git clone git://www.example.com/pub/software/primes.git

You can make a clone using either the HTTP or git native protocols. The former is more universally available yet slower, while the latter is less common but faster.

By default, cloning a repository establishes two branches for you, master and origin. The master branch is, by convention only, for your development. git clone also establishes a remote reference called origin that points upstream to the original source of the repository, namely the URL you supplied on the command line:

$ cat .git/remotes/origin
URL: git://www.example.org/pub/software/primes.git
Pull: master:origin

$ git show-branch master origin
* [master] some commit message
! [origin] some commit message
++ [master] some commit message

The additional Pull: master:origin line contains a reference specification or refspec that establishes a mapping that the remote master branch should be fetched into the local origin branch. The branch named origin, because it appears on the right-hand-side of the refspec, is a tracking branch. As mentioned above, it should be used exclusively for following the changes from the remote repository. Any local changes should occur on either the master branch or some other topic branch.

Fetching Changes

Two basic commands obtain the changes made in a remote repository: git fetch and git pull. git fetch is used to obtain the updates from a remote repository. Building on top of that, git pull does one more step: it executes a git fetch followed by a git merge operation.

If the remote repository is really a URL referencing some other repository, git fetch, using the master:origin refspec, obtains all of the necessary objects from the remote repository’s master branch, places them in your object database, and updates the local origin tracking branch.

git pull goes one step further. It merges the newly fetched changes into the current branch as well. Typically, the current branch for a git pull operation is the master branch. In the default git clone setup, while on the master branch, the command git pull origin causes the remote master branch to be fetched into the local origin tracking branch, then merged onto the local master development branch. If you had additional changes in your local master branch, the merge results in both your changes and the updates from the remote master branch being unified and present in your master branch.

In the article “Embrace the Git Index,” git pull was used to cause some named branch in a repository to be merged into the current branch of the same repository. In that case, the “remote” repository was a reference, ., to the same repository, and the fetch step was trivially satisfied by having all the necessary objects already present in the object database within the same repository. However, the subsequent merge operation was then performed into the current branch.

Often your development requires you to incorporate changes from multiple, different but related repositories so that you may obtain changes from different developers. It’s easy to create new remote references and pull in those changes as well. Simply create a new remote reference, add the URL and the branch name you want to use, and fetch it! In the following example, both Anne’s and Bob’s changes to the primes project are obtained from their master branches and placed in local branches named anne and bob, respectively:

$ cat .git/remotes/anne
URL: git://www.bobs-place.org/pub/scm/primes.git
Pull: master:anne

$ git fetch anne

$ cat .git/remotes/bob
URL: git://www.annes-place.org/pub/scm/primes.git
Pull: master:bob

$ git fetch bob

Now that Anne’s and Bob’s changes have been brought into the local repository, you can easily manipulate and compare the various branches. You may inspect them using git show-branch, merge them using git checkout master; git pull.bob, or compare them using git diff bob master.

Serving Repositories

Setting up a machine to serve git repositories is straightforward, but you need root access and the usual bit of care. The setup described here presents a unified view of a repository directory in which all repositories present in the directory are published and visible via both the git- native protocol and HTTP. Furthermore, the repositories are directly browsable through HTTP requests. You don’t have to do all of these steps, but it provides a good, unified view.

This setup is on a Debian Linux system, and some directories are either standard or semi-arbitrarily chosen. You may have to adjust for your distribution, file-system, and other requirements.

Start by selecting a location on your local filesystem to store all of your repositories. In this article, /pub/software is used. This directory will be directly browsable through HTTP requests and served through the Apache HTTP Server. Create the directory so that your web server has access rights to it. You may want to allow yourself or a software development group rights as well. For example:

$ ls -lad /pub /pub/software
drwxr-xr-x 3 root root 4096 Jul 23 2005 /pub
drwxrwsr-x 9 jdl www-data 4096 Mar 24 22:40 /pub/software

Modify your Apache /etc/apache/httpd.conf file to allow direct browsing on this same directory.

Alias /pub/software /pub/software
<Directory "/pub/software">
Options Indexes

While you don’t have to allow the Indexes option, it can be convenient especially if you want to make other files related to the repositories available as well. For example, you may want to publish release snapshot tar files or related patches; those could be published in this same /pub/software directory, too.

At this point, you’ve enabled the HTTP protocol access to the git repositories and have made them available for someone else to clone using:

$ git clone http://www.example.com/pub/software/primes.git

In addition to serving repositories via HTTP, you should enable the git native protocol, too. You can enable the git-daemon via the inetd mechanism by adding the following entry to your /etc/inetd.conf file:

git  stream  tcp   nowait  nobody  \
/usr/bin/git–daemon git–daemon ––inetd \
––syslog ––export–all /pub/software

(Naturally, that is all on one physical line within the file, and assumes that you have installed git in /usr/bin.) Make any adjustments for your /pub/software location if needed.

The option –export-all states that all of the repositories within /pub/software should be visible and published. If you only want to publish a subset of the git repositories that are actually present in this directory, you should instead use the git-daemon-export-ok file mechanism described in the git-daemon man page.

Additionally, enable the port on which git-daemon listens by adding the following entry to /etc/services.

git  9418/tcp    # git daemon

Port 9418 is the default git-daemon port. If that’s not the one you’re using, substitute your own.

After restarting the inetd daemon, you should be serving repositories requested like this:

$ git clone git://www.example.com/pub/software/primes.git

Installing gitweb

Unless you have some other way of publishing your repositories, you should consider installing gitweb, a web browser written by Kay Sievers. It is the standard tool in use today for publishing git repositories on the web. You may clone it using:

$ git clone git://git.kernel.org/pub/scm/git/gitweb.git

Installing gitweb is a simple matter of placing a copy of gitweb.cgi somewhere within your web server’s DocumentRoot hierarchy. This example places it within the directory git_repos at the top of the web server’s DocumentRoot, though others have used the directory git as well. Make the git_repos directory and copy the gitweb.cgi and indextext.html files from a gitweb.git repository to it:

$ mkdir /var/www/www.example.com/git_repos
$ cd /var/www/www.example.com/git_repos

… copy gitweb.cgi and indextext.html files here …

$ ls –ls .
-rwxr-xr-x 1 jdl www-data 82195 Mar 21 15:27 gitweb.cgi
-rw-r--r-- 1 jdl www-data 757 Mar 23 08:29 indextext.html

Next, modify the gitweb.cgi script to reflect the location of the directory where your repositories are stored. Furthermore, gitweb must have a means to determine which repositories are valid to display and browse. By pointing projects_list at the same directory, the information is determined dynamically from the repositories present. Here are some example settings:

gitbin => "/usr/bin"
git_temp => "/tmp/gitweb"
projectroot => "/pub/software"
projects_list => "/pub/software"
home_text => "indextext.html"

Finally, customize the base page by modifying the indextext.html file.

It’s likely that you will have to extend your Apache configuration to recognize the git_repos directory and allow access to it, and to allow gitweb.cgi to be executed and treated as the default “index” file for the directory. For example, modify your /etc/apache/httpd.conf as follows:

<Directory /var/www/www.example.com/git_repos/>
AddHandler cgi-script .cgi
Options +ExecCGI -Indexes
<IfModule mod_dir.c>
DirectoryIndex gitweb.cgi

With that, accessing www.example.com/git_repos should present a list of published repositories found in the /pub/software directory!

Creating a Published Repository

Now that the infrastructure is installed and set up, you can publish your own repositories. Once published, other people can them using gitweb.

To publish a repository, a bare copy of the repository needs to be made inside the /pub/software directory. A bare repository is normally an appropriately named directory with a .git suffix that doesn’t have a locally checked-out copy of any of the files under revision control. That is, all of the git administrative and control files that would normally be present in the hidden .git sub-directory are directly present in the project.git directory instead, and no other files are present and checked out. There are two ways to establish a bare git repository.

First, you can explicitly construct a new repository. Use this approach if you have no existing repository and want to start a brand new project.

$ cd /pub/software
$ mkdir primes.git
$ cd primes.git
$ setenv GIT_DIR .
$ git init-db

(Depending on your shell, you may need an alternate syntax for setting the GIT_DIR environment variable to be ..) The steps listed above create an empty, bare repository. However, since such a repository has no files, and no commit history, it still won’t show up using gitweb. But as soon as a commit it present in such a repository, it will appear.

On the other hand, if you already have an existing repository that you’d like to publish, you can use the ––bare option to git clone and directly clone the repository into the /pub/software directory.

$ ls -a primes
. .. .git .gitignore Makefile primes.c

$ git clone --bare primes /pub/software/primes.git

$ ls -a /pub/software/primes.git
. .. HEAD branches config description
hooks info objects refs remotes

Here, the entire project in the primes repository has been copied as a bare clone into the /pub/software/primes.git directory. The files that were in primes/.git have been promoted to the /pub/software/primes.git directory, and all of the files under revision control, such as primes.c and Makefile, have been eliminated. Those files are now only present in the object database.

As a final step in publishing these repositories, edit the file named description to contain a meaningful yet short description of your project. It’s used by gitweb as the one-line description presented for each project.

Publishing Changes

Once a git repository has been established, you can use git push to update it. git push uses the same refspec used by git fetch and git pull to map local source reference names to remote references in a target repository, and brings the remote references up-to-date with respect to the local source. Any local repository objects needed on the remote end to bring it up-to-date are transferred to the remote repository first. Usually, branch names on the local and remote repositories are consistent, and the local branches are clearly direct commit-descendents of the remote branch HEAD, and thus a very fast and simple fast forward update can be performed on the remote HEAD once the new commit objects have been transfered to the repository being updated.

To set up a clear publishing path from your working repository to your public, published repository, create a new file within the .git/remotes directory that contains the destination URL and the mapping of branch names.

For example, here is the content of a file named publish that can be used to push the changes in your working primes repository master branch to the public repository’s master branch. Since an absolute path is used in this URL, clearly both the working repository and the public repository are on the same machine.

$ cd primes
$ cat .git/remotes/publish
URL: /pub/software/primes.git
Push: master:master

To perform an update across the network, you may use a network URL with a hostname specification such as this:

$ cat .git/remotes/publish
URL: www.example.com:/pub/software/primes.git
Push: master:master

In either case, the invocation to perform the transfer is simply:

$ git push publish

You may specify multiple references and branches to be pushed by using multiple Push: refspec lines.

In the following example, all the objects needed by the master, test, and dev branches are transferred to the remote primes repository on www.example.com, and their respective HEADS are updated as well.

$ cat .git/remotes/publish
URL: www.example.com:/pub/software/primes.git
Push: master:master
Push: test:test
Push: dev:dev

There is no hard requirement that the same name be used on both sides of the refspec colon such as test:test. However, care should be taken to track the flow of objects and the expected use patterns carefully. If you’re expecting to have other users clone and pull from this repository, it is quite likely, though not required, that you want to ensure that your push updates somehow land in the master branch. Furthermore, since refspecs can potentially match branch names, tags and heads, and so on, you can be more explicit if needed. For example:

Push: refs/heads/test:ref/heads/test
Push: refs/tags/today:ref/tags/today

If you’re providing either HTTP protocol support for clones of your repositories, or a gitweb interface to them as described here, you should perform one more important change to your repositories to support them. Both of these tools rely on a few files of pre-built information located in the .git/info and .git/objects/info directories.

After a git push operation has updated a repository, the git update-server-info command must also be executed. While you may run this by hand from within the repository with the GIT_DIR environment variable set to ., it is easier and more reliable to enable the post-update hook. It’s located in the hooks subdirectory, and can be enabled by simply making it executable. The post-update hook is automatically invoked on the remote repository after it has finished updating all of the references initiated by a local git push operation. The default action for the post-update hook is to perform the git update-server-info command!

Go Forth and Collaborate!

You now have all of the necessary tools to collaborate with git. You know how to create new git repositories, base new development off of a clone of some pre-existing public repository, set up a git server using HTTP and git protocols through Apache and basic Linux services, provide a web-based front-end for your repository, and finally, know how to create and update it. With that, other developers will be able to use and leverage your work.

There is still plenty of room to explore how best to provide development branches, publish works in progress, leverage distributed development efforts, and coordinate between different remote sites!

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