[SOLVED] crontab random delay not working

aristosv · 02-16-2015, 12:00 AM

I am trying to delay the daily reboot of multiple Linux machines by a random time, within one hour.

This is to avoid a simultaneous reboot of all the Linux machines at the same time.

I also want to avoid specifying a time in cron. I want it to be completely random between 00:00 and 01:00.

So far I tried the commands below, but no luck. The machines still reboot at midnight.

Code:

@daily /bin/sleep $((RANDOM\%3600)) && /sbin/reboot
@daily /bin/sleep $(/usr/bin/expr $RANDOM \% 3600); /bin/reboot

pan64 · 02-16-2015, 01:28 AM

see man crontab, you will see crontab uses /bin/sh, not /bin/bash, therefore $RANDOM is an empty string.

aristosv · 02-16-2015, 01:45 AM

So what should I do? If I add a /bin/sh at the front of the line, will it fix the problem? For example:

Quote:

@daily /bin/sh /bin/sleep $((RANDOM\%3600)) && /sbin/reboot
@daily /bin/sh /bin/sleep $(/usr/bin/expr $RANDOM \% 3600); /bin/reboot

pan64 · 02-16-2015, 01:49 AM

no, I suggest you to write a small shell script and crontab will invoke that only

Code:

script:
#!/bin/bash
/bin/sleep $((RANDOM\%3600))
/sbin/reboot
or whatever you want


crontab:
@daily /full/path/to/the/script