RCS (Revision Control System) is a good tool for controlling revisions of programs and documents. It allows a user to maintain a catalog of revisions to a program and to "roll back" to a previous revision. It is well integrated with other UNIX tools such as emacs and make.

Consider the following scenario: You have two files (a.c and b.c) in a directory named stuff, and you want to control those files using RCS. Here are the steps:

1. In the stuff directory, create a directory named RCS. This directory will store the database of changes to your files.
2. Set the permission to allow rw priviledges to the correct users. For example, if you are working with users in the UNIX group grad, the permissions for a.c and b.c should be read and write for both user and group (rw-rw----) and their group should be "grad". The RCS and stuff directory should have read, write, and execute permissions for both user and group (rwxrwx---) and their group should be "grad".
3. Now we must check the files into source control. In the stuff directory, type "ci a.c" and "ci b.c". Each time you check in a file, RCS asks you for a comment describing the changes you made. If you look in the RCS directory, you will see a.c,v and b.c,v. These are the files that RCS uses to record changes to a.c and b.c, respectively. Only in extreme circumstances would you edit or delete these files.
4. If you just want to use a file (e.g., compile a.c), you can check it out without a lock by typing "co a.c" in the stuff directory. This gives you a read-only copy of a.c that you can compile but not edit. WARNING: By default when you check in a file in RCS, there is no longer a copy of it in the stuff directory so to get a copy of it back in the stuff directory you must check it out without a lock to use it, even if you don't want to edit it.
5. If you want to edit a file, you check it out with a lock by typing "co -l a.c" in the stuff directory. Now you are the only person who can edit that file. When you are done, check it back in with "ci a.c". WARNING: While you may be the only person who can edit a locked file, other people may still use (e.g., compile) the file in the stuff directory. If your edits temporarily "break" (e.g., no longer compiles) the file, it will interfer with others who are using it; consequently, you may want to check it out with a lock, copy and make changes to it in another location, and copy it back to the stuff directory when edits are complete.