What is CVS?

CVS is a version control system. Using it, you can record the history of your source files.

For example, bugs sometimes creep in when software is modified, and you might not detect the bug until a long time after you make the modification. With CVS, you can easily retrieve old versions to see exactly which change caused the bug. This can sometimes be a big help.

You could of course save every version of every file you have ever created. This would however waste an enormous amount of disk space. CVS stores all the versions of a file in a single file in a clever way that only stores the differences between versions.

CVS also helps you if you are part of a group of people working on the same project. It is all too easy to overwrite each others' changes unless you are extremely careful. Some editors, like GNU Emacs, try to make sure that the same file is never modified by two people at the same time. Unfortunately, if someone is using another editor, that safeguard will not work. CVS solves this problem by insulating the different developers from each other. Every developer works in his own directory, and CVS merges the work when each developer is done.

CVS started out as a bunch of shell scripts written by Dick Grune, posted to comp.sources.unix in the volume 6 release of December, 1986. While no actual code from these shell scripts is present in the current version of CVS much of the CVS conflict resolution algorithms come from them.

In April, 1989, Brian Berliner designed and coded CVS. Jeff Polk later helped Brian with the design of the CVS module and vendor branch support.

You can get CVS via anonymous ftp from a number of sites, for instance prep.ai.mit.edu in `pub/gnu'.

There is a mailing list, known as info-cvs, devoted to CVS. To subscribe or unsubscribe send a message to info-cvs-request@prep.ai.mit.edu. Please be specific about your email address. As of May 1996, subscription requests are handled by a busy human being, so you cannot expect to be added or removed immediately. The usenet group comp.software.config-mgmt is also a suitable place for CVS discussions (along with other configuration management systems).

CVS is not...

CVS can do a lot of things for you, but it does not try to be everything for everyone.

CVS is not a build system.

Though the structure of your repository and modules file interact with your build system (e.g. `Makefile's), they are essentially independent. CVS does not dictate how you build anything. It merely stores files for retrieval in a tree structure you devise. CVS does not dictate how to use disk space in the checked out working directories. If you write your `Makefile's or scripts in every directory so they have to know the relative positions of everything else, you wind up requiring the entire repository to be checked out. If you modularize your work, and construct a build system that will share files (via links, mounts, VPATH in `Makefile's, etc.), you can arrange your disk usage however you like. But you have to remember that any such system is a lot of work to construct and maintain. CVS does not address the issues involved. Of course, you should place the tools created to support such a build system (scripts, `Makefile's, etc) under CVS. Figuring out what files need to be rebuilt when something changes is, again, something to be handled outside the scope of CVS. One traditional approach is to use make for building, and use some automated tool for generating the depencies which make uses.

CVS is not a substitute for management.

Your managers and project leaders are expected to talk to you frequently enough to make certain you are aware of schedules, merge points, branch names and release dates. If they don't, CVS can't help. CVS is an instrument for making sources dance to your tune. But you are the piper and the composer. No instrument plays itself or writes its own music.

CVS is not a substitute for developer communication.

When faced with conflicts within a single file, most developers manage to resolve them without too much effort. But a more general definition of "conflict" includes problems too difficult to solve without communication between developers. CVS cannot determine when simultaneous changes within a single file, or across a whole collection of files, will logically conflict with one another. Its concept of a conflict is purely textual, arising when two changes to the same base file are near enough to spook the merge (i.e. diff3) command. CVS does not claim to help at all in figuring out non-textual or distributed conflicts in program logic. For example: Say you change the arguments to function X defined in file `A'. At the same time, someone edits file `B', adding new calls to function X using the old arguments. You are outside the realm of CVS's competence. Acquire the habit of reading specs and talking to your peers.

CVS does not have change control

Change control refers to a number of things. First of all it can mean bug-tracking, that is being able to keep a database of reported bugs and the status of each one (is it fixed? in what release? has the bug submitter agreed that it is fixed?). For interfacing CVS to an external bug-tracking system, see the `rcsinfo' and `editinfo' files (see section Reference manual for the Administrative files). Another aspect of change control is keeping track of the fact that changes to several files were in fact changed together as one logical change. If you check in several files in a single cvs commit operation, CVS then forgets that those files were checked in together, and the fact that they have the same log message is the only thing tying them together. Keeping a GNU style `ChangeLog' can help somewhat. Another aspect of change control, in some systems, is the ability to keep track of the status of each change. Some changes have been written by a developer, others have been reviewed by a second developer, and so on. Generally, the way to do this with CVS is to generate a diff (using cvs diff or diff) and email it to someone who can then apply it using the patch utility. This is very flexible, but depends on mechanisms outside CVS to make sure nothing falls through the cracks.

CVS is not an automated testing program

It should be possible to enforce mandatory use of a testsuite using the commitinfo file. I haven't heard a lot about projects trying to do that or whether there are subtle gotchas, however.

CVS does not have a builtin process model

Some systems provide ways to ensure that changes or releases go through various steps, with various approvals as needed. Generally, one can accomplish this with CVS but it might be a little more work. In some cases you'll want to use the `commitinfo', `loginfo', `rcsinfo', or `editinfo' files, to require that certain steps be performed before cvs will allow a checkin. Also consider whether features such as branches and tags can be used to perform tasks such as doing work in a development tree and then merging certain changes over to a stable tree only once they have been proven.

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