at, for one-off cronjobs

Scheduling a cronjob is great, but sometimes you only want it to run once. You could add, then remove the job, or write a script that executes once, but there is a better way.

Install at

sudo apt install at

Schedule a script with at

The below examples show two different ways to run a script with at.

echo /script/to/run | at 20:30

at 20:30 -f ./script/to/run

For this, the script needs to exist, and needs executable permissions for your user, sudo chmod +x /script/to/run, if you need a lil' hint.

The time/date passed 20:30 in this example, can then be passed in a few assorted ways.

• at now +5 minutes
• at 11:00 today
• at 11:00 tomorrow
• at 20:30
• at 20:30 093024
• at 08:30PM Sep 30
• at 08:30PM Sep 30 2024
• at 08:30PM Sep 30 24

Batch a script

Similar to the regular means of scheduling, except the batch will wait for the system load drops to a set point.

echo /script/to/run | at -b 20:30

You can also run batch, as follows.

echo /script/to/run | batch 20:30

Check Queue

atq

Running the command above, will show you something akin to...

10      Sun Sep 29 14:14:00 2024 a user

Which broken down shows the ID 10, the date/time the item will run Sun Sep 29 14:14:00 2024, the queue the item is in a, and the user executing the item user

Remove from Queue

You can remove an item from the queue, with the atrm command, passing just the ID.

atrm 3

Mail the user

Like cron, at also allows the ability to mail the user about the task once executed. By default, if the command produces an output, at will mail the user, you can override this with:

at -M

This will prevent mail being sent to the user. You can also force a mail sent even if there is no output with:

at -m