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Ben K.
Asked: 2009-07-22 11:44:59 +0800 CST

Is there a way to validate /etc/crontab’s format?

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I prefer to stick scheduled tasks in /etc/crontab so I can see at a glance what's scheduled to run, regardless of which user the task runs as.

The only gotcha is that the format isn't validated on save, unlike crontab -e -- so a stray character can quietly break the entire cron.

Is there a way to validate the /etc/crontab format before/after save?

cron

6 Answers

  1. The only reliable way I found is to check the log.

    cron checks /etc/crontab every minute, and logs a message indicating that it has reloaded it, or that it found an error.

    So after editing, run this:

    sleep 60; grep crontab /var/log/syslog | tail

    Or, to not wait a full minute, but only until the next minute + 5 seconds:

    sleep $(( 60 - $(date +%S) + 5 )) && grep cron /var/log/syslog | tail

    Example output with an error:

    Jan  9 19:10:57 r530a cron[107258]: Error: bad minute; while reading /etc/crontab
Jan  9 19:10:57 r530a cron[107258]: (*system*) ERROR (Syntax error, this crontab file will be ignored)

    Good output:

    Jan  9 19:19:01 r530a cron[107258]: (*system*) RELOAD (/etc/crontab)

    That's on Debian 8. On other systems, cron might log to a different file.

    (I thought I could avoid hunting for the right log file by using systemd's journalctl -u cron, but that didn't show me these log entries, and actually seems to have stopped logging cron events 2 days ago for some reason)

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  2. Another more recent solution is the python script chkcrontab

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  3. Wicked cool shell scripts has a shell script that validates crontab files.

    You can get the zip archive containing the script here

    The script is called verifycron

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  4. I found this cool solution here: https://crontab.guru

    It doesn't just validate the crontab, it tells you explicitly what and when the crontab will run, and highlights where errors are.

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  5. On Ubuntu, it seems like I can just run:

    crontab path/to/crontab/file

    NOTE: this has the side effect of starting this cronjob (thanks @NZD)

    If the file is invalid, I will an error, like:

    "crontab":11: bad minute
errors in crontab file, can't install.
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  6. run crontab -T path/to/crontab

    If you want to automatically do it before/after, you can write your own visudo style wrapper, such as

    $EDITOR /etc/crontab
crontab -T /etc/crontab

    Personally, since breaking the crontab is not as bad as breaking sudoers, I think it's fine to just print the message, but you could also everything to a tempfile if you wanted.

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