Debugging crontab jobs

Question

I have added a crontab entry on a Linux server that will run a Java executable. The Java code uses its own class for logging errors and messages into a log file.

But when I checked the log file after the scheduled time, no messages were logged. There should have been at least one log message saying the execution had started.

So there are two possible causes:

1. The code executed but didn't log;
2. Or, the code didn't execute at all.

The log file specified has chmod 777 permissions so I'm guessing it's the second cause here.

Why wouldn't a crontab job execute at its scheduled time? And how do I debug this without any kind of logging happening?

I have read that if there is an error cron sends an email to the user. How do I find out which email address is associated with the user?

Answer 1

You can enable logging for cron jobs in order to track problems. You need to edit the /etc/rsyslog.conf or /etc/rsyslog.d/50-default.conf (on Ubuntu) file and make sure you have the following line uncommented or add it if it is missing:

cron.*                         /var/log/cron.log 

Then restart rsyslog and cron:

sudo service rsyslog restart sudo service cron restart 

Cron jobs will log to /var/log/cron.log.

Answer 2

Append 2>&1 to the end of your Crontab command. This will redirect the stderr output to the stdout. Then ensure you're logging the crontab's Unix command.

0 0,12 1 */2 * ( /sbin/ping -c 1 192.168.0.1; ls -la ) >>/var/log/cronrun 2>&1 

This will capture anything from the Unix command.

A couple of additional hints (after helping a colleague the other day ...). Write out the environment variables by issuing the command set with no parameters. And get the shell to echo each command with the set -x command. At the top of your script issue;

set set -x