RCS stands for Revision Control System. It's not as sophisticated as some of the newer revision systems such as CVS or GNU Arch but for a single-developer project or maintaining a single-user Linux machine, it's more than adequate.
Putting a new file under version control.
Create directory 'RCS'.
$ mkdir RCS
This is not required but keeps the working directory from being cluttered with the version control files.
Initialize the file:
$ rcs -i new-file RCS new-file: RCS/new-file,v enter description, terminated with single '.' or end of file: NOTE: This is NOT the log message! >>
Enter file description at '>>' prompt, press RETURN, period, RETURN
This places a file named 'new-file,v' in the RCS directory.
Edit the file
$ co -l new-file
This creates 'new-file' with read-write permissions. Edit as usual.
Place the newly edited file under version control.
$ ci -l new-file RCS/new-file,v <-- new-file new revision: 1.2; previous revision: 1.1 enter log message, terminated with single '.' or end of file: >>
Enter log message at '>>' prompt, press RETURN, period, RETURN
The file is now under revision control and a copy of the file with read-write permissions is left in the working directory for further editing.
Detailed Command Descriptions
By default, files must be locked in order to be returned to version control. This is the '-l' option for the co command. When checking a file back under version control, normally the working file is deleted. By using the '-l' command line switch with the ci command, the file is versioned and then automatically checked out and locked once again. It's a bit confusing at first, but as long as you use the '-l' switch with both commands, a working copy of the versioned file will always be present.
File locking can be disabled with the command:
$ rcs -u file-name
See the rcs man page.