incron seems to be a pretty neat tool. It’s cron-for-file-activities. Cron is a common unix tool that allows you to run any command at a specified time. It’s immensely useful for running anything that you want to occur on a regular basis: backups, daily reminder emails, downloading podcasts, etc. If you’ve used unix to any significant extent, you’ve used cron.

incron takes that cron metaphor and applies it to file activity. You specify which files you want to watch, and then you specify which activities you are interested in. If incron notices any of those activities occurring on your files, it launches the command that you specified. Activities could include creating a new file, modifying a file, accessing a file, among multiple other possibilities. incron takes advantage of the inotify system built into recent linux kernels, which supposedly is more efficient than previous ways of doing this same thing.

incron seemed to be just what I wanted. I wanted to watch a specific directory and launch a tool to rebuild my website if any of those files changed. The problem is that incron doesn’t work recursively on directories. You have to specify each directory, the activities on that directory, and the command to launch. If you create new directories, you have to tell incron about them specifically. This ruins the benefit of the tool for me. It seems the developer has been planning to implement recursive watching for the past 4 years ago, but hasn’t done it yet. If it hasn’t happened in 4 years, it’s probably not going to happen soon.

But, I found a perfectly usable way to get what I want. inotify-tools includes a program called inotifywait. It takes a file or directory name and listens for specific activities that you are interested in. It basically waits until it sees one of those activities and then returns control back to you. So, you can something like (pseudocode):

while (inotifywait /home/vinod/web/kurup.org):
    # triggered
    rebuild my server

inotifywait waits for my activities. If it finds one, it returns TRUE and my server gets rebuilt, and then the while loop restarts. Best of all, inotifywait can work recursively on a directory. Which brings me to my watch-rebuild.sh script which watches my web source files for changes, and rebuilds the web generated files on demand. It also pops up a little GNOME notification using notify-send:

#!/bin/bash
    
BLOGOFILE='/usr/local/bin/blogofile'
BLOGDIR='/home/vinod/web/kurup.org'
    
# requires libnotify-bin for notify-osd notification
# requires inotify-tools
while LINE=$(inotifywait -rq --timefmt '%F %R' --format '%T %e %f' \
      -e close_write -e move -e delete "$BLOGDIR");
do
        echo -n "Rebuilding $LINE ..."
        $BLOGOFILE -s $BLOGDIR build
        echo "done"
        notify-send -u low "Rebuild Done" "Watching again"
done

-r = recursive
-q = be quiet
--timefmt & --format = what we want to display on the command line
-e = each of the individual events that we are interested in