Linux.Desktop.Hacks [Electronic resources] نسخه متنی

Hack 86. Grab the Latest Source Code

Source code is the lifeblood behind any open source project, and its welfare is
critical in pushing the code forward and improving it. To the
nonhacker, this random collection of phrases and letters appears to
be gibberish, but to the hacker, this code makes perfect sense. To
remain productive, it is essential that hackers maintain their code
in a clean and useful state. Although many hackers worry about
keeping the source code usable, this is difficult when you consider
that multiple developers from around the world working on a project
hack a single set of source code files. These projects rarely have a
single lead developer who decides what goes in and out of the source
code tree, so how is this development managed?

The solution to this problem is a source control system. These
systems essentially provide a metaphorical railway signal box to
manage the different needs of the different hackers. The system
stores the code in a repository on the server
and allows access to only those users who have an authorized login
account. With an account, users can copy a snapshot of the code to
their computers (this is called making a working
copy), hack the code, and then submit the changes to the
main server. The source control system ensures that users cannot
commit changes at the same time, and it only commits the differences
made to the code back into the system. This ensures that bandwidth is
not bogged down by the same content flying back and forth between the
hackers and the source control server.

In this hack, you are going to get started with a popular source
control system called CVS. This tool is in use by most of the large
software projects, and you can use it to get read-only access to the
developers' source code servers. This code can be
useful if you want to explore the possibility of hacking on some code
yourself, but it is also handy for those of us who like to compile
unstable development code to see just what the hackers are putting in
the software. For example, using the methods described here, you can
compile the development branches of KDE or GNOME for a bleeding-edge
desktop [Section 5.11 and Section 6.5].

9.18.1. Connect to a CVS Repository

To use CVS first you need to
specify where
the CVS source code server is located. You do this by setting an
environment variable called $CVSROOT. This
variable is often set in your shell's startup file
(such as .bash_profile for the bash shell), but
if you are unsure of which shell you are running, type the following
command to see which one it is:

foo@bar:~$ echo $SHELL

You need to specify a $CVSROOT that indicates the
location of the CVS server, the username to connect with, and where
the code is stored on the server. The $CVSROOT is
set as an environment variable with three distinctive parts:

[authentication type]:[user@location.domain]:[location of the code on server]

If you want to connect to the KDE CVS server, for example, you can
get read-only access in which you can just download the code by using
the anonymous username. Many CVS services offer an
anonymous user in which onlookers can download the code but not make
changes to it.

If, for example, you are using the bash shell, add this line to your
.bash_profile to add a connection to the KDE CVS
server:

export CVSROOT=:pserver:anonymous@kde.org:/home/kde

If you want to set this value from the command line in a bash shell,
you can use this command (good only for your current session):

foo@bar:~$ export CVSROOT=:pserver:anonymous@kde.org:/home/kde

When you have set the variable with one of these methods, you can
check that it is working by typing:

foo@bar:~$ echo $CVSROOT
:pserver:anonymous@kde.org:/home/kde

With this complete, now you can log in to the CVS server. To do this,
type:

foo@bar:~$ cvs login

You are prompted for a password. Just press Enter to use a blank
password. Now you are returned to the prompt.

Considering the vast amount of different open source projects and the huge amount of code sitting on the Internet in CVS servers, you might want to use different CVS servers. To do this, you need to set your $CVSROOT to the new server and log in again.

On a CVS server, code is grouped into modules that contain code for
related projects and tools. For example, the KDevelop project is
contained within the kdevelop module on the KDE
CVS server. You can see a list of these modules by typing:

foo@bar:~$ cvs co -c

To get a module from the server, you need to check it out using the
CVS co option. For example, you can check out the
kdevelop module with:

foo@bar:~$ cvs co kdevelop

Then you'll see the code checked out to your
computer, one file at a time. To update your local copy with the
latest changes from the CVS server, you need to go into a checked-out
directory (such as the kdevelop directory in our
example), and type:

foo@bar:~$ cvs update -PAd

When you run this command, only the files that were changed since
your initial checkout will be copied to your computer. This ensures
that a minimal amount of bandwidth is used for keeping your code
current.

CVS is a powerful and competent source control system, but it is also
one of the most confounding pieces of software to use, particularly
when adding files to a server. For more information, I recommended
you read the free book Open Source Development with CVS
at http://cvsbook.red-bean.com/.

9.18.2. New Kid on the Block: Subversion

Although CVS has been around for a
long time and a lot of open source
projects use it, it has some limitations which some developers find
frustrating, including not being able to store metadata (properties)
associated with files and not being able to properly version
directories. An alternative system called Subversion, which overcomes
these limitations, is rapidly taking over as a replacement for CVS,
and many projects now provide access to their source code using
Subversion.

The command for accessing a Subversion repository is very simple:

foo@bar:~$ svn checkout http://repo.example.com/project project

This will check out the latest copy of the project tree into a local
working copy named project.

Then you can keep your local source tree up-to-date by entering it
and typing svn up, which will
fetch any files that have changed since your last update and will
merge the changes into your local tree.

The Subversion project is very active and has a very helpful mailing
list. You can find the main project page at http://subversion.tigris.org, and you can
also access the free book Version Control with Subversion
at http://svnbook.red-bean.com.