Productive Monday: Incron - Execute commands based on filesystem activity

Cron is a tool that executes commands when a certain time has been reached. Incron is a new tool modeled after cron which executes commands when a filesystem event occurs. Without polling, Incron can watch a specific file or an entire directory for new files, file writes, closures, deletions, or other activity.

There are dozens of uses for incron, including:

• playing a sound when a file changes
• reloading a server config when the config file changes
• notifying a user of new mail
• watching for changes in critical files
• processing uploaded files

This package contains a daemon named incrond. Use the server and chkconfig commands to start the server and configure it to be started at boot:

# service incrond start
Starting incrond: [ OK ]
# chkconfig incrond on

Incron uses per-user incrontab files, similar to crontab files. Each line in an incrontab contains a filename, a comma-separated list of events to watch, and a command to be executed. To display a list of available events, use the incrontab command with the -t (type) flag:

$ incrontab -t
IN_ACCESS,IN_MODIFY,IN_ATTRIB,IN_CLOSE_WRITE,
IN_CLOSE_NOWRITE,IN_OPEN,IN_MOVED_FROM,
IN_MOVED_TO,IN_CREATE,IN_DELETE,IN_DELETE_SELF,
IN_CLOSE,IN_MOVE,IN_ONESHOT,IN_ALL_EVENTS,
IN_DONT_FOLLOW,IN_ONLYDIR,IN_MOVE_SELF

Most of these event names will be self-explanatory; you can find a brief description of each in /usr/include/linux/inotify.h (from the kernel-headers package -- this is the main header file for the inotify system, which incron is based upon).

To create an incrontab, use the incrontab command with the -e (edit) option. By default, vi will be used as the editor; you can change this with the EDITOR environment variable or in the /etc/incron.conf file.

Here are some examples of incrontab entries:

Play a sound whenever a writable file in /tmp is closed (use play instead of paplay if you're not using PulseAudio)	/tmp IN_WRITE_CLOSE paplay /usr/share/sounds/KDE_Beep_Bottles.wav
Reload the DNS configuration whenever /etc/named.conf is modified	/etc/named.conf IN_MODIFY /sbin/service named reload
Run a script whenever a file in the inbound mailbox directory changes	/var/spool/mail IN_WRITE_CLOSE /usr/local/bin/mail-notify-script $#

The third example shows that you can can insert information about the filesystem event into the arguments of the executed command. The documentation file /usr/share/doc/incron-0.5.5/README notes that these substitutions are available:

$$ - a dollar sign
$@ - the watched filesystem path - this is the directory name if watching a directory
$# - the event-related file name - this is the relative filename if watching a directory; the absolute name is $@/$#
$% - the event flags (textually)
$& - the event flags (numerically)

Incron promises to be an useful tool for system administrators and end-users alike.

Thanks to Mattew Miller for suggesting today's package. If you have packages you would like to see covered, please use the Suggest a Package form on the right-hand side of the main page.

Installing from the command line: yum install incron
Installing using the graphical installer: Not available in browse mode; use the search or list tabs to install incron.
Menu location after installation: (Not applicable)
Command: /usr/bin/incrontab
Upstream website: http://inotify.aiken.cz/