## 19.2. Basic Operation

Normally, you maintain RCS files in a subdirectory called `RCS`, so the first step in using RCS should be:

>     mkdir RCS

Next, you place an existing file (or files) under RCS control by running the check-in command:

>     ci file

This creates a file called file`,v` in the `RCS` directory. file`,v` is called an RCS file, and it stores all future revisions of file. When you run `ci` on a file for the first time, you are prompted to describe the contents. `ci` then deposits file into the RCS file as revision 1.1.

To edit a new revision, check out a copy:

>     co -l file

This causes RCS to extract a copy of file from the RCS file. You must lock the file with `-l` to make it writable by you. This copy is called a working file. When you're done editing, you can record the changes by checking the working file back in again:

>     ci file

This time, you are prompted to enter a log of the changes made, and the file is deposited as revision 1.2. Note that a check-in normally removes the working file. To retrieve a read-only copy, do a check-out without a lock:

>     co file

This is useful when you need to keep a copy on hand for compiling or searching. As a shortcut to the previous `ci`/`co`, you could type:

>     ci -u file

This checks in the file but immediately checks out a read-only (“unlocked”) copy. In practice, you would probably make a “checkpoint” of your working version and then keep going, like this:

>     ci -l file

This checks in the file, and then checks it back out again, locked, for continued work. To compare changes between a working file and its latest revision, you can type:

>     rcsdiff file

Another useful command is `rlog`, which shows a summary of log messages. System administrators can use the `rcs` command to set up default behavior of RCS.

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