Last Updated: February 08, 2022
· janosgyerik

Scheduling jobs in UNIX without cron

If you happen to be unfortunate enough to be in a system where you're not allowed to use cron, you might be able to do something similar with at instead. Here's a script that reschedules itself to run the next day at 8:45am:

the_main_thing &>/dev/null
[[ $0 = /* ]] && script=$0 || script=$PWD/$0
at -f "$script" 0845 &>/dev/null

the_main_thing is the command or script you want to run periodically.

The purpose of [[ $0 = /* ]] && script=$0 || script=$PWD/$0 is to get the absolute path of the current script itself.

In at -f "$script" 0845 &>/dev/null, the -f is to specify the script to run: the current script itself with absolute path, otherwise it might not work.

To run something every 5 hours, you can use relative times, for example:

at -f "$script" 'now + 5 hours' &>/dev/null

Similar to cron, the output of scheduled at jobs is emailed to the UNIX account. To prevent polluting the mailbox, it's probably a good idea to redirect both with &> /dev/null, and you might want to do the same for the_main_thing too.

To view your scheduled tasks, use atq. To remove scheduled tasks, use atrm.

2 Responses
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Thanks! It's harder than it should be to find fully explained UNIX tips. Keep it up!

over 1 year ago ·

Thanks for sharing. I adapted your technique to separate the scheduling from the task. For example, save the following as scheduled

# run script at scheduled time
# (derived from script by Janos Gyerik
# usage:  scheduled <when> <script> [args...]
# e.g. scheduled '7:15' script arg1 arg2
#      scheduled 'now + 5 minutes' script arg1 arg2
# also note script will execute in $PWD
[[ $0 = /* ]] && script=$0 || script=$PWD/$0
echo "$script '$when' $*" | at "$when" &>/dev/null
$* &>/dev/null

Then use with: scheduled 'now + 10 minutes' another_script arg1 arg2 arg3

over 1 year ago ·