Practical CVS Tutorial

The purpose of this tutorial is to provide practical examples of how to use CVS during a typical source development cycle. The complexity of the CVS interface and underlying structure tends to prevent casual use of the system and prevents us from taking full advantage of the multi-user, parallel development features. With the examples provided in this tutorial, it will be possible to use CVS as just another tool, simplifying back porting bug fixes, maintainence of released packages, and safely going about large scale code changes (such as a platform port, architectural changes, or adding experimental code). Those using CVS after this tutorial will be able to enjoy some of its more mysterious benefits with as little trouble as possible.

Being a tutorial, or 'how to', only necessary procedures are provided. Those wishing to understand the nuance, history, and detail of CVS are invited to read the manual.

The Environment

 

In the beginning there is nothing. No package, no directories, no source, nothing.

First, CVS needs a directory in which to create a repository. The repository is a special directory managed by CVS that will hold all files you want CVS to manage, as well as all changes made to these files. The directory may be reached through your local filesystem or remotely via ssh. In both cases, the environment variable CVSROOT must be set to specify the repository location.

If the repository is accessable through the filesystem, CVSROOT maybe set to the absolute path to the CVS directory. For instance, a workstation in the UT Computer Science lab using the LoCI CVS repository could set CVSROOT as: export CVSROOT=/cvs/homes/
However, for remote access from laptops or non-NFS mounted workstations, a different variation makes this possible via SSH.

export CVSROOT=:ext:username@enterprise.cs.utk.edu:/cvs/homes
export CVS_RSH=ssh

Be sure to replace username with your username.

To experiment on your own and become more comfortable with the CVS commands, you may set CVSROOT to a directory in your $HOME area without any danger of breaking a shared repository. Once you have chosen and set CVSROOT, run cvs init to initialize the repository. This needs to be done only once, but running the command multiple times will not hurt anything.


Adding a package to your CVS repository.

 

If you have write permission to your CVSROOT, you may add a module to the CVS repository using cvs import. (A cvs module is another word for a source package name. For instance, "ibp", "lors", "libexnode" are all examples of modules currently in the LoCI CVS repository. "Module name" or "package name" may be used interchangably.) 

UNIX> cvs import <package name> <vendor tag> <symbolic tagname>
The vendor tag is not important for now. But you must have something in its place.

CVS does not allow tag names to contain any spaces, dashes, or periods. This is unfortunate, because we are used to naming packages with dashes and periods: "lors-0.80", for instance. However, this name would become lors_0_80, if it were to be used as a tag name.

From the PWD of the package you wish to import into CVS (not below the directory; in the directory), run the import command. 
UNIX> cat source.c
#include <stdio.h>
int main()
{
    printf("Hello broccoli.\n");
}
UNIX> cvs import lilprogram loci lilprogram_0_0
N lilprogram/source.c

No conflicts created by this import
In this example, I have a single C file named "source.c". CVS prepends the package name to each file or directory imported. The 'N' signifies that this file is new. And, assuming you've not chosen a package name that is already in the CVS repository, the command will exit without an error.

 

Retrieving a module from CVS

 



To begin working from CVS you must retrieve the code you just imported by running cvs checkout. Move to a directory where you wish to place the working directory of your CVS package. 
UNIX> cvs checkout lilprogram
cvs checkout: Updating lilprogram
U lilprogram/source.c
In this example, CVS creates a directory named lilprogram, and populates it with source.c. The 'U' means CVS has updated this file. You may now change to the new directory and begin editing and saving changes.

 

Updating your working directory and Committing changes

 


Once you modify and save your source files, you will wish to commit them back to the CVS repository eventually. Before doing this, it is important to run cvs update.

When you are working with more than one developer, they may have committed changes since the last time you checked out or updated your working directory. Update will first look in the CVS repository and compare your files to those in CVS. If there have been additions committed by others, then CVS brings the files in your working directory up to date with the content of the repository. If you try to commit your changes before updating, CVS will warn you if your code is not up to date with the code in the repository. But, updating first saves you a step and allows you to verify that your code still works with the updates of others.


In my lilprogram, I realize that broccoli is not a friendly vegetable, and I would rather say "Go away" than "Hello". I make this change, and run cvs update. 
UNIX> cvs update
cvs update: Updating .
M source.c
The 'M' tells me that source.c was modified by me and no other changes were made by anyone else. It is now safe commit the changes. 
UNIX> cvs commit source.c
Checking in source.c;
/Users/soltesz/cvsroot/example/source.c,v  <--  source.c
new revision: 1.2; previous revision: 1.1
done
This commit introduces the first revision in the history of source.c in the main branch.

 

Branching and Tagging

 


So far, we have assumed that there is only one branch in CVS, and every checkout, update or commit operates on the most current revision in that branch (also known as the "head" of the branch.) One of the strengths of CVS is its ability to manage independent, parallel development branches within a single module (or package).

Examples when branching the versions of a package may be beneficial are at the time of a package release, during code porting, or when experimental features are added that may be unstable or otherwise unsuitable for the main branch. For releases, development can continue on the main branch, but if several weeks later bugs are discovered in the release, it may be difficult to recreate a new release and add the bug fixes in the CVS repository. As for porting and experimental features, once the new code is stable, it can be brought back into the main branch.

The important point to recognize is that release maintenance and experimental development can be done inside and with the assistance of CVS, rather than outside of CVS.

To have an understanding of what happens when a branch is created in CVS, imagine that a new directory is created in the repository and a copy is made of the main branch into this new directory. The resulting two directories are entirely independent of one another. Changes committed to one will not show up in the other unless you ask CVS to do this.

This procedure is idential to what you would end up doing if you were adding a fix to a bug in a release available only in the distributed TAR.GZ. You would unpack the archive and add your fixes, entirely independent of the CVS repository. When you do this, you must now return to the CVS repository and make the fixes or changes a second time to commit them. The difference is, of course, that CVS could manage the separate directories and releases for you, allowing you to have a single working directory for all of these tasks if you wish.

cvs rtag expects the module name and the name of the branch you are creating. Because the branch name is a tag, the same rules for apply to branches as apply to tag names.


If I wished to create a branch in the lilprogram to allow me to work on translating the message I would do this: 
UNIX> cvs rtag -b lilprogram_0_1 lilprogram
cvs rtag: Tagging lilprogram
After the branch command, there are two branch heads in the repository. At this moment, they contain the same information. However, each branch is now independent of the other.


Once a branch has been created I can make my working directory mirror either branch using cvs update. In the first example, the new branch is brought into the user's working directory. This step is necessary before you can commit changes or additions to this branch. Now you may update and commit and CVS will know that you are working on the lilprogram_0_1 branch. 
UNIX> cvs update -r lilprogram_0_1
cvs update: Updating .
UNIX> cvs status source.c
$ cvs status source.c
===================================================================
File: source.c          Status: Up-to-date

   Working revision:    1.2     Wed Jun  4 00:00:21 2003
   Repository revision: 1.2     /Users/soltesz/cvsroot/lilprogram/source.c,v
   Sticky Tag:          lilprogram_0_1 (branch: 1.2.2)
   Sticky Date:         (none)
   Sticky Options:      (none)
The 'Sticky Tag' is lilprogram_0_1, showing that it is in fact from the lilprogram_0_1 branch.

To revert to the main branch: 

UNIX> cvs update -A
cvs update: Updating .
UNIX> cvs status source.c
===================================================================
File: source.c          Status: Up-to-date

   Working revision:    1.2     Wed Jun  4 00:05:16 2003
   Repository revision: 1.2     /Users/soltesz/cvsroot/lilprogram/source.c,v
   Sticky Tag:          (none)
   Sticky Date:         (none)
   Sticky Options:      (none)
The 'Sticky Tag' is now 'none', which is appropriate for the head of the main branch.

Using cvs update -A, files in your working directory will be updated or removed when needed. As long as you have committed any changes you've made to the other branch, nothing will be lost.

A user may also assign symbolic Tags to any revision in the CVS repository. Tags are static pointers, while branches are functional development trees. Tags can be turned into branches.

My second committed change will be to replace broccoli with celery. 
UNIX> cat source.c
#include <stdio.h>
int main()
{
    printf("Go away celery.\n");
}
UNIX> cvs commit
$ cvs ci source.c
Checking in source.c;
/Users/soltesz/cvsroot/lilprogram/source.c,v  <--  source.c
new revision: 1.3; previous revision: 1.2
done
UNIX> cvs tag liltag_0_0
cvs tag: Tagging .
T source.c
The head of the main branch is now different from the head of the lilprogram_0_1 branch. I can verify this by updating my working directory to the lilprogram_0_1 branch and checking source.c 
UNIX> cvs update -r lilprogram_0_1
cvs update: Updating .
U source.c
UNIX> cat source.c
#include <stdio.h>
int main()
{
    printf("Go away broccoli.\n");
}

 

Merging Branches

 


In the lilprogram_0_1 branch, I will translate my message to German, and commit this change. 

UNIX> cat source.c
#include <stdio.h>
int main()
{
    printf("Gehen brokkoli weg.\n");
}
UNIX> cvs ci source.c
Checking in source.c;
/Users/soltesz/cvsroot/lilprogram/source.c,v  <--  source.c
new revision: 1.2.2.1; previous revision: 1.2
done
While there may be more than one branch, it may ultimately be desirable to bring these branches back together. For instance, if the branch held the code needed to run on windows, this branch could be merged with the main branch, and any conflicts resolved.

To do this, make sure your working directory is the head of the main branch (or which ever branch you're merging into). Using cvs update -j <branchname>, where branchname is the name used during the 'rtag' command. Where there are conflicts CVS will print a warning, and surround the differences in the file with identifying marks.


If I wish to merge the branches of the lilprogram, this is the message and content of source.c: 
UNIX> cvs update -A
cvs update: Updating .
U source.c
UNIX> cvs update -j lilprogram_0_1
cvs update: Updating .
RCS file: /Users/soltesz/cvsroot/lilprogram/source.c,v
retrieving revision 1.2
retrieving revision 1.2.2.1
Merging differences between 1.2 and 1.2.2.1 into source.c
rcsmerge: warning: conflicts during merge
UNIX> cat source.c
#include <stdio.h>
int main()
{
<<<<<<< source.c
    printf("Go away celery.\n");
=======
    printf("Gehen brokkoli weg.\n");
>>>>>>> 1.2.2.1
}
It is the user's responsibility to resolve these conflicts before committing the file back to the main branch. In this example, I might make the message printed conditional on a command line argument or simply combine the messages into a single print statement.

 

Extra Good Stuff

 


 

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