Configuring event-triggered commands with Incron

Trigger Happy

While cron doggedly keeps to a fixed schedule, Incron monitors directories and runs commands when changes occur.

By Charly Kühnast

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Cron is a constant companion for admins like myself. My crontab and I have both grown over the years - Crontab has gotten longer, and I've gotten wider. Maybe I'm sentimental, but I decided it was time to let my old friend venture out into unexplored territory with a little help from Incron [1]. This cron extension uses an event triggered approach, rather than traditional time-based scheduling, monitoring directories, and running commands when specific changes occur.

Before you can start working with Incron, you need kernel 2.6.13 with built-in Inotify support and the matching header file inotify.h. The file is typically located in /usr/include/sys/; some distributions add a file called inotify- syscalls.h.

If you like, you can change the installation paths in the Makefile and then go on to make && make install. After completing the build and installing, you should have incrond, incrontab, and the manpages.

Path, Event, Command

Incrond is a daemon, and it disappears into the background after launching. Of course, Incrontab is the central element. Calls to Incrontab follow the Crontab syntax for the main: icrontab -e opens the table for editing, icrontab -l displays the content, and icrontab -r deletes the table. The incrontab format is very simple. Each line contains three entries:

path event command+parameter

Whenever an event occurs in a monitored path, Incron runs the matching command. Table 1 shows you the events Incron can monitor. When an event occurs, Incron sets the parameters listed in Table 2, and the command can then use them for its own purposes. If you modify the Incrontab, there is no need to let Incrond know, as the daemon parses the control file periodically.

Let's look at an example. Whenever a user deletes a file in the /var/run/daemon/ directory, I want Incrond to delete the /var/log/daemon.log file, too. The incrontab line looks like this:

/var/run/daemon IN_DELETE rm /var/log/daemon.log

This simple entry illustrates the power of Incron. At last - cron has a friend it can play with.

[1] Incron:

Charly Kühnast is a Unix System Manager at the data center in Moers, near Germany's famous River Rhine. His tasks include ensuring firewall security and availability and taking care of the DMZ (demilitarized zone).