Running Crond from Daemontools

Crond is a background daemon which reads crontab files and executes the commands within. It is used to ensure the running of scheduled tasks, such as nightly backups.

Daemontools is a collection of tools for managing Unix services. It provides a means of monitoring a service, starting and stopping it and logging any debug and/or error messages. Daemontools provides easy service installation and removal, easy first time service startup, reliable restarts, easy, reliable signalling, clean process state and OS portability.

Running crond from daemontools helps to ensure that scheduled jobs do not fail to run simply because the crond daemon crashed. It also allows you to maintain a standard build across the Unix operating systems. Other alternatives to crond from daemontools are runwhen and uschedule.

Setting up the Service Directories

First verify the daemontools "svscan" process is running. Choose a location where you want the physical service directories. I usually use "/var/service", however any directory may be used as long as it is not "/service".

Create the service directories with the following commands.

# mkdir -m 1755 /var/service/crond
# mkdir -m 755 /var/service/crond/log

Download the "run" scripts for crond and its log.

# cd /var/service/crond
# wget -c http://www.antagonism.org/scripts/crond-run
# mv crond-run run
# chmod 755 run
# cd log
# wget -c http://www.antagonism.org/scripts/log-run
# mv log-run run
# chmod 755 run

Warning, before using either of the crond "run" scripts, make sure you understand what the commands do.

• "-n" forces crond to run in the foregound. This is required for operation with daemontools.

Activating the Service

(The below section is taken almost verbatim from the following page created by John Simpson. I felt that his description on what happens when you activate a service was the most clear and easy to understand, so why change a thing?)

Once the directories are set up, you need to make them start running. This is done by creating a symbolic link from /service/(whatever) to the physical directory where the service lives. The "svscan" program checks /service every five seconds, and when it sees a new directory (or symbolic link) there, it starts a "supervise" process for that directory. In addition, if the directory has the sticky bit set and a child directory called "log", it starts a "supervise" process for the "log" child directory and sets up a pipe between the two processes (so that the main process's logs end up being sent to the log process).

The "supervise" program works by running the "run" script inside of whatever directory it's watching. If that child process (either the "run" script itself, or whatever process it runs using "exec") stops, it starts it back up by running the "run" script again.

The following command will create the symbolic link needed to start the crond service.

# ln -s /var/service/crond /service/

After running this command, wait ten seconds (to give it time to start) and then run the "svstat" command to see what's running:

# svstat /service/crond /service/crond/log
/service/crond: up (pid 2508) 7 seconds
/service/crond/log: up (pid 2510) 7 seconds

As long as the new services show "up" with a timer of more than one second, the services are running correctly. If the timer on a service is 0 or 1 second, then wait about five seconds and run the same command - it should now be higher than 1 second. If it's still 0 or 1, then the service is having a problem and you need to fix it. This page provides some steps to troubleshoot daemontools service installations.

Downloads

Copyright 2006-2008 Patrick R. McDonald <marlowe@antagonism.org> (GPG key)
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