When not given a file, cat will just read from stdin, so you can use "cat > file.c", write some text, and send EOF with ^D when you're done.

Obviously, there's no way to go back and edit anything mid-stream, you have to write the whole thing out in one shot.

You can join files together.

    $ cat foo bar > baz

will join the files foo and bar together into a single file called baz