How to check cron syntax by script like "crontab -e" does?

Question

Let´s say i write cron files via script and modify them direct under /var/spool/cron/crontabs/. When using the command crontab -e crontab checks the syntax when i exit the editor. Is there any way, to do the same check via script?

Answer 1

Crontab with -e option does open your default editor with the current cron file and installs it after exiting.

First of all, save your actual cron file to be sure of not losing or breaking anything.

crontab -l > backup.cron

You can directly install a file that you have cooked:

crontab yourFile.text

Or use a pipe in a script:

#/bin/bash
Variable="your scheduled tasks"
echo $Variable | crontab

You will get the error messages in the case of bad formatting.

More info: man crontab

Answer 2

Use chkcrontab:

pip3 install chkcrontab

chkcrontab /etc/cron.d/power-schedule
Checking correctness of /etc/cron.d/power-schedule
E: 15: 0 12 * foo * * root echo hi
e:     FIELD_VALUE_ERROR: foo is not valid for field "month" (foo)
e:     INVALID_USER: Invalid username "*"
E: There were 2 errors and 0 warnings.

Or try a shell script 48-verifycron from Wicked Cool Shell Scripts, 2nd Edition, but it's not as good.