Ubuntu – Why crontab scripts are not working

cron

Often, crontab scripts are not executed on schedule or as expected. There are numerous reasons for that:

  1. wrong crontab notation
  2. permissions problem
  3. environment variables

This community wiki aims to aggregate the top reasons for crontab scripts not being executed as expected. Write each reason in a separate answer.

Please include one reason per answer – details about why it's not executed – and fix(es) for that one reason.

Please write only cron-specific issues, e.g. commands that execute as expected from the shell but execute erroneously by cron.

Best Answer

Different environment

Cron passes a minimal set of environment variables to your jobs. To see the difference, add a dummy job like this:

* * * * * env > /tmp/env.output

Wait for /tmp/env.output to be created, then remove the job again. Now compare the contents of /tmp/env.output with the output of env run in your regular terminal.

A common "gotcha" here is the PATH environment variable being different. Maybe your cron script uses the command somecommand found in /opt/someApp/bin, which you've added to PATH in /etc/environment? cron ignores PATH from that file, so runnning somecommand from your script will fail when run with cron, but work when run in a terminal. It's worth noting that variables from /etc/environment will be passed on to cron jobs, just not the variables cron specifically sets itself, such as PATH.

To get around that, just set your own PATH variable at the top of the script. E.g.

#!/bin/bash
PATH=/opt/someApp/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

# rest of script follows

Some prefer to just use absolute paths to all the commands instead. I recommend against that. Consider what happens if you want to run your script on a different system, and on that system, the command is in /opt/someAppv2.2/bin instead. You'd have to go through the whole script replacing /opt/someApp/bin with /opt/someAppv2.2/bin instead of just doing a small edit on the first line of the script.

You can also set the PATH variable in the crontab file, which will apply to all cron jobs. E.g.

PATH=/opt/someApp/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

15 1 * * * backupscript --incremental /home /root
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