Crontab examples on system wide file /etc/crontab
# /etc/crontab: system-wide crontab
#
#
# Cron Syntax
#
# .---------------------------- Minute
# | .----------------------- Hour
# | | .------------------ Day of month
# | | | .--------------- Month
# | | | | .------------ Day of week
# | | | | | .-------- User
# | | | | | | .-- Command
# | | | | | | |
# * * * * * * *
# do it once at startup, after 120s
@reboot root sleep 120 && myFullPathCommand
# Every 4h, start program
0 */4 * * * root myFullPathCommand