Run cron job at random time

crontab

# Run /your/script.sh at random time
04  */3 *   *   *   sleep $((RANDOM\%90))m; /your/script.sh

# RANDOM versions:
#  ${RANDOM:0:2}m
#  $[RANDOM\%90]m
#
# No bash version:
#  $(( $(od -N1 -tuC -An /dev/urandom) \% 90 ))m 
#
# Note: You need to render modulus as '\%' above because cron (well, at least Linux 'vixie-cron') terminates the line when it encounters an unescaped '%'.

# Debugging cron
#  * You should have `mailutils` installed
#  * Use `mail` to check mails in your box from CRON
#  * Output of the crontab jobs (including errors) is sent through email to the user the crontab file belongs to (unless redirected).
#  * To see when it was ran: 
#    grep CRON /var/log/syslog
#
# `crontab -e` to edit crontab for current user
# `crontab -l` list jobs