12.7. Which Version Control System?

We said earlier that VC uses any of a number of version control systems (more may be added in the future). It chooses which to use for any given file by looking for a corresponding master file—that is, a file containing a change history.

If you're using RCS, each of your project directories usually has a subdirectory in which RCS masters live. If you're using SCCS, there are SCCS subdirectories. CVS is a little trickier; your project directory has a CVS subdirectory with control information in it, but CVS masters are typically kept in one central repository directory, the location of which is typically given by the CVSROOT environment variable, and will likely be on another machine completely, using the pserver network protocol. Subversion, too, uses a separate server machine to store the revision repository; it generally uses WebDAV over HTTP for its transactions. Your local Subversion master files are kept in a subdirectory named .svn.

If VC can't find a master in any of these special directories, it looks for a master in the same directory as your work file (so you don't have to create SCCS or RCS directories if you don't mind your work directories being cluttered with masters). VC checks each of these possibilities (so you can actually use more than one system in the same directory, although we don't recommend it).

If VC can't find a master anywhere, it looks for an RCS, SCCS, CVS, or .svn directory. The order in which these are attempted is controlled by the variable vc-handled-backends, described in "Customizing VC" later in this chapter. The first one it finds tells it which version control system to register new files with. If it can't find any of these directories, and you tell it to register a file, it assumes you want to use RCS and creates the master right alongside your work file.

To find out which of SCCS, RCS, CVS, or Subversion is available on your system, simply execute the commands comb, rcs, cvs, and svn respectively, with no arguments. If you see an error or usage message, the corresponding system is ready to use; if you see command not found, it's not.