Is Cron Itself Not Working or Is the Cron Job Not Running as Expected?

When a cron job doesn’t run as expected, it can be challenging to pinpoint the exact issue. Cron, the time-based job scheduler in Unix-like operating systems, offers flexibility and precision for scheduling tasks. However, this can sometimes lead to confusion when jobs fail to run. The most effective first step in troubleshooting is to check the cron log. This can immediately provide insights into what went wrong.

Start with the Cron Log

Cron logs are invaluable for troubleshooting because they record every cron attempt, whether successful or not. By examining these logs, you can quickly determine if Cron attempted to run your job at the scheduled time and whether there were any errors.

The location of cron logs varies by system but typically can be found in /var/log/cron or /var/log/syslog. Use the following command to tail the cron logs, which allows you to view the most recent entries:

tail -f /var/log/cron
# or
tail -f /var/log/syslog | grep CRON

If your cron job is listed in the logs but did not execute as expected, this means the issue is with the command being run in the cron rather than an issue with Cron. You will want to skip to the Verify Cron Job Syntax and Permissions section. Though if it’s not listed at all there is a chance the issue could be with cron.

Is Cron Running?

After checking the logs, ensure that the cron service itself is running. Use the following command to check the cron service’s status:

systemctl status cron  # For systemd systems
# or
service cron status    # For non-systemd systems

If the service isn’t running, start it with:

systemctl start cron  # For systemd systems
# or
service cron start    # For non-systemd systems

Verify Cron Job Syntax and Permissions

If the cron service is running and your job appears in the logs but isn’t executing correctly, the next steps are to verify the job’s syntax and permissions:

• Cron Syntax: Ensure your cron job’s syntax is correct. Even a misplaced asterisk can change the schedule you intended.
• Permissions: Check that the script or command your cron job is attempting to run is executable and that the user under which cron is running has the necessary permissions.

Check the Execution Environment

Cron jobs run in a minimal environment, which means they might not have access to all the environment variables your script expects when running in a standard terminal session. This often causes scripts to fail silently when run as cron jobs. To diagnose such issues, you can compare the environment Cron provides to your script with your usual environment.

Redirect the output of the env command to a file from within a cron job to inspect the environment it provides:

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

Conclusion

When a cron job doesn’t run as expected, starting your investigation with the cron logs can provide immediate and valuable insights into the issue. Whether it’s a misconfiguration, a permissions issue, or an environment variable mismatch, the cron logs are an essential first step in diagnosing the problem. From there, verifying the cron service status, checking the job syntax, ensuring correct permissions, and understanding the execution environment are systematic steps that can help resolve most cron job issues. Remember, detailed attention to cron’s operation and job configuration is key to leveraging its full potential for automating tasks on Unix and Linux systems.