Script produces different result when executed by Bash than by cron

Question

Please consider following crontab (root):

SHELL=/bin/bash
...
...
0 */3 * * * /var/maintenance/raid.sh

And the bash script /var/maintenance/raid.sh:

#!/bin/bash

echo -n "Checking /dev/md0... "
if ! [ $(mdadm --detail /dev/md0 | grep -c "active sync") -eq 2 ]; then
    mdadm --detail /dev/md0 | mail -s "Raid problem /dev/md0" "[email protected]";
    echo "ERROR"
else 
    echo "ALL OK"
fi;

#-------------------------------------------------------

echo -n "Checking /dev/md1... "
...

And this is what happen when...

...executed from shell prompt (bash):

Mail with mdadm --detail /dev/md0 output is sent to my email (proper behaviour)

...executed by cron:

Blank mail is sent to my email (subject is there, but there is no message)

---

Why such difference and how to fix it?

Answer 1

As indicated in the comments, do use full paths on crontab scripts, because crontab does have different environment variables than the normal user (root in this case).

In your case, instead of mdadm, /sbin/mdadm makes it.

How to get the full path of a command? Using the command command -v:

$ command -v rm
/bin/rm

Answer 2

cron tasks run in a shell that is started without your login scripts being run, which set up paths, environment variables etc.

When building cron tasks, prefer things like absolute paths and explicit options etc