Simple version control with RCS

I had to write quite a bit of code for one of my projects this term. To keep the project manageable, I wanted a simple way to take snapshots of my files at particular milestones, so I could back up and look at previous snapshots if I ever screwed up my code. I ended up doing this with RCS.

RCS is best for situations where a single user is modifying files locally. For remote repositories or when you need to coordinate actions between multiple users, you should use another version control system.

To set up RCS in a directory:

  • Create a subdirectory named RCS.
  • To manage FILE using RCS, run $ ci -u FILE. Type a description of the file when prompted.

Emacs can deal with RCS-managed files out of the box. Here's a typical workflow:

  • Open an RCS-managed file in Emacs. Emacs automatically recognizes that the file is under RCS and displays the version number in the mode line.
  • Lock the file for editing with C-x v v.
  • Make your changes.
  • When you're at a milestone, commit your changes with C-x v v. Emacs prompts you for a commit message. Enter it in the provided buffer and press C-c C-c.
  • To continue editing the file, lock it again wih C-x v v and proceed as above.

Now you get to use the other cool features provided by the VC facility in Emacs:

  • C-x v = displays a diff between your current version and the previous committed version. C-u C-x v = lets you diff any two previous versions of the file. I like to use unified diffs whenever possible, so I have (setf vc-diff-switches "-u").
  • C-x v ~ prompts you for a version number and displays that version of the file in a new buffer.
  • C-x v g displays an annotated version of the file showing, for each line, when that line was last modified, and a heat-map displaying older and newer code in different colors.

Bridging the gap between Emacs and Gnome

I use Emacs whenever I can (which is, nowadays, for almost all the work I do) but I still need to switch away occasionally for some apps. Usually this involves starting up Nautilus, navigating to a directory, finding some files (e.g. PDFs or web pages), and double-clicking them. I wrote some Emacs functions to do the equivalent from Dired, avoiding the need to use Nautilus at all. This makes Dired a lot more useful as a file manager/browser for all kinds of files, not just text files.

gnome-open opens a file using the same application which would have been used to open it had you double-clicked it in Nautilus (Evince for PDFs, OpenOffice for OO.o docs, etc.). The following function takes a file and calls gnome-open on it:

(defun gnome-open-file (filename)
  "gnome-opens the specified file."
  (interactive "fFile to open: ")
  (let ((process-connection-type nil))
    (start-process "" nil "/usr/bin/gnome-open" filename)))

In a Dired buffer, the following function gnome-opens the file on which your cursor is sitting:

(defun dired-gnome-open-file ()
  "Opens the current file in a Dired buffer."
  (gnome-open-file (dired-get-file-for-visit)))

I bound it to 'E':

(add-hook 'dired-mode-hook (lambda () (local-set-key "E" 'dired-gnome-open-file)))


I discovered this real key-saver a few weeks ago. It makes Emacs appear to be positively psychic.

M-/ or dabbrev-expand completes the word you are in the middle of typing by looking at other words in the buffer. If no match is found it looks in other buffers. So dabbrev can complete the word or name you are typing on the basis of anything else you are reading in Emacs: the text you've previously typed, code or text in other files you have open, even documentation or web pages you have open.

If dabbrev completes your text incorrectly, you can press M-/ repeatedly to get other possible completions.