Pragmatic Automation

Letting CVS Pull The Trigger
13 Aug 04 -
A version control system is one safety blanky I carry around. When I’m working on a project, I’ll commit files frequently throughout the day: after I write a test and make it pass, before I go to lunch (provided all the tests are still passing), and after updating a section of documentation. Just knowing that those short durations of work are safely tucked away gives me a warm, fuzzy feeling. A successful commit means I can always go back in time and rewrite computing history if I don’t like where I’ve ended up in the present.

The commit process also offers an opportunity to leverage triggered automation—automation that is triggered by an event. CVS makes this relatively easy by exposing two "hooks" into the commit process: a pre-commit hook and a post-commit hook. (Subversion has these hooks too, but I’ll save the details for a future post.) These internal CVS hooks are convenient because I can continue to type cvs commit and an arbitrary amount of work happens automatically behind the scenes. I don’t have to remember to do extra steps every time I commit changes. And all that good automation also happens for everyone that shares the CVS repository.

The CVS commit hooks are well documented, but often overlooked. So let’s take a walk through a quick example to learn (or revisit) a powerful automation trick.

Write A Pre-Commit Filter

The pre-commit hook is triggered after you type cvs commit, but before the changes are saved into the repository. You get notified of a pre-commit event by registering a program to be run when the commit is attempted. The program is handed the name of the CVS directory where the commit is occurring and the name of each of the files in the commit.

Here’s a trivial Unix shell script called that simply logs the commit directory and each file in the commit attempt:


  DATE=`date "+%m/%d/%y [%H:%M:%S]"`

  echo "$DATE: A commit is occurring in $1" >> $LOG_FILE

  for file in $*
    echo "$DATE: Pre-commit check for $file" >> $LOG_FILE

Notice that the value of the $1 variable is the name of the commit directory and the $* variable contains the files in the commit.

You can grow this script to take on more complex automation steps, but you have to be careful about how much it does. The cvs commit command will block until all registered programs complete. So it’s best to think of this program as a quick filter. If the program returns successfully, then the files will be committed. If the program returns with a nonzero exit status, then the files won’t be committed.

In other words, the pre-commit hook can be used to make sure only good stuff ends up in your CVS repository. Example uses might include verifying that changed files adhere to coding standards, examining the contents of changed files for CVS conflict markers, or changing tabs to spaces.

Write A Post-Commit Notifier

The post-commit hook is triggered after changes have been committed to the CVS repository. As such, the post-commit hook is useful for providing notification that the repository has been changed. Similar to the pre-commit hook, you get notified of a post-commit event by registering a program that gets handed the name of each file that was committed.

Here’s a trivial Unix shell script called that logs each file that was committed:


  DATE=`date "+%m/%d/%y [%H:%M:%S]"`

  for file in $*
    echo "$DATE: Post-commit notification for $file" >> $LOG_FILE

  # Eat extra input
  cat > /dev/null

Notice that the last line uses cat to eat any extra input. This is important because the post-commit event can hand the program gratuitous status (you can redirect cat to $LOG_FILE to see it), and if the program fails to read all of its input, then you’ll get the error

  cvs [commit aborted]: received broken pipe signal

Again, be sure to limit how much work this script does so that the cvs commit command doesn’t hang for too long. For example, the post-commit program could play a rewarding sound when someone successfully commits code. If it’s a relatively short sound, the code to play the sound can be inlined right in the program.

But the post-commit hook is where you’ll likely want to do a bit more automation. Say, for example, you want the post-commit trigger to include playing the world’s longest standing ovation over the company intercom system followed by sending an SMS message to each member of your team? If your cvs commit command hangs while all this is happening, you probably won’t commit changes very often. One easy solution is to have the post-commit program "touch" another file that’s being monitored by another process. The monitoring process can then do mountains of work without tying up CVS.

Register The Programs

To register the pre-commit and post-commit programs, you need to update two internal CVS files. These files live inside the CVSROOT directory of your CVS repository. You could edit the files in that directory, but it’s safer to use version control. Start by checking out the CVSROOT module into a temporary directory:

  [~] $ mkdir temp
  [~] $ cd temp/
  [~/temp] $ cvs checkout CVSROOT
  cvs checkout: Updating CVSROOT
  U CVSROOT/checkoutlist
  U CVSROOT/commitinfo
  U CVSROOT/config
  U CVSROOT/cvswrappers
  U CVSROOT/editinfo
  U CVSROOT/loginfo
  U CVSROOT/modules
  U CVSROOT/notify
  U CVSROOT/rcsinfo
  U CVSROOT/taginfo
  U CVSROOT/verifymsg

All of those files are different kinds of triggers, but we’re only interested in two files: the commitinfo file and the loginfo file. The commitinfo file contains a list of pre-commit programs and the loginfo file contains a list of post-commit programs. Both files use the same format. Each line is of the form:

  regular_expression program_to_run

The regular_expression is used to match the directory (relative to $CVSROOT) that includes the changed file(s). The program_to_run is the name of the program to run when the matched directory’s contents change.

For example, to run the program when a post-commit event occurs in a sub-directory of the dms module, edit the commitinfo file and add the line

  dms/* /path/to/

To run the program when a post-commit event occurs in a sub-directory of the dms module, edit the loginfo file and add the line

  dms/* /path/to/ %s

(CVS expands the %s argument to include the name of each file committed.)

After editing the commitinfo and loginfo files, check them in:

  [~/temp/CVSROOT] $ cvs commit -m "Added hooks" commitinfo loginfo

That’s all there is to configuring CVS.

Commit Changes

Here’s where the rubber meets the road. Change directory to your local workspace that contains a checked out version of the dms project. Then change and commit a couple of files in the dms project. For example:

  [~/work/dms] $ emacs src/com/pragprog/dms/
  [~/work/dms] $ emacs bin/
  [~/work/dms] $ cvs commit -m "Testing the hooks"
  Checking in bin/;
  /CVSRepo/dms/bin/,v  <--
  new revision: 1.4; previous revision: 1.3
  Checking in src/com/pragprog/dms/;
  /CVSRepo/dms/src/com/pragprog/dms/,v  <--
  new revision: 1.15; previous revision: 1.14

The output looks as it did before registering the hook programs. To test that the pre-commit and post-commit programs were run, peek inside of the $CVSROOT/CVSROOT/commitlog log file:

  [~] $ tail -f $CVSROOT/CVSROOT/commitlog

  08/13/04 [14:59:38]: A commit is occurring in /CVSRepo/dms/bin
  08/13/04 [14:59:38]: Pre-commit check for
  08/13/04 [14:59:38]: A commit is occurring in /CVSRepo/dms/src/com/pragprog/dms
  08/13/04 [14:59:38]: Pre-commit check for
  08/13/04 [14:59:38]: Post-commit notification for dms/bin
  08/13/04 [14:59:38]: Post-commit notification for
  08/13/04 [14:59:38]: Post-commit notification for dms/src/com/pragprog/dms
  08/13/04 [14:59:38]: Post-commit notification for

Notice that the pre-commit program ran first to filter all the changed files. Then, because both files passed the filter, the files were logged by the post-commit program.


By writing callback programs and registering them with CVS commit hooks, triggered automation happens transparently. Indeed, these hooks are great jumping-off points for automation big and small. If you’re already automating chores in your project with version control hooks, or this trick inspires you to do so, please share your story.