54

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?

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    If, like me, you came here looking for a simple (non-scriptable) way to tell you whether you'd got your cron entry right, you might be interested in cronchecker.net — it's an online validator for cron entries with human-friendly output. Dec 20, 2014 at 11:18

6 Answers 6

39

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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    Tested on FreeBSD 10.3. Works like a charm, just replace log path with /var/log/cron
    – Jette
    Aug 5, 2019 at 8:15
16

Another more recent solution is the python script chkcrontab

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    It doesn't check users' cronjobs yet
    – userlond
    Oct 19, 2015 at 7:22
  • It told me that cron wouldn't run the file because it contains dots in the file name. Thanks!
    – cweiske
    Aug 19, 2021 at 7:26
11

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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    Link doesn't work anymore as of 11 jan 2017.
    – Epskampie
    Jan 11, 2017 at 11:37
  • Someone fixed the link. In the zip-file it is located in "wicked_cool_shell_scripts_2e-master/6/48-verifycron". Use it with ./48-verifycron /var/spool/cron/crontabs/root to verify your user root crontab-file.
    – MadMike
    May 25, 2021 at 12:08
5

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.

2

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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    this command does check the crontab file, but at the same time installs it (if it contains no errors). This is probably an unwanted side-effect for the OP.
    – NZD
    Dec 19, 2016 at 1:18
  • Thanks @NZD, I've added this to my reply to make sure the OP is aware of that. Dec 19, 2016 at 11:08
  • @conradk the command doesn't start the cronjob, it overwrites the user's existing crontab file with the file provided.
    – MadHatter
    Dec 19, 2016 at 11:59
  • This doesn't actually work for all possible problems with the crontab; it will catch some glaring problems, but it doesn't catch this, for example: * 4/0 * * /bin/myscript.sh -- the 4/0 is invalid. but isn't caught by this method
    – JDS
    Jul 31, 2017 at 14:23
  • @JDS Isn't it a step value? Is a step value of 0 forbidden? unix.stackexchange.com/questions/32027/… Aug 1, 2017 at 12:18
0

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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