Add a one-off scheduled task through a shell script?

Question

I'd like to execute a task at a given time, once.

I know I can just use crontab -e to add it there, but I'd like to do that through a shellscript. crontab -e gives me some temporary file to edit which is unpredictable.

Also, once the task executes I'd like to remove it from the crontab again to make sure it'd not leaving behind mess.

So is there a standardized way to add/remove entries from my personal crontab through a script?

I know I could roll my own system: Have a script that executes every minute that executes and deletes .sh files from a folder and have my "addtask" script create .sh files in that folder, but before rolling my own I wonder if there is something already.

If it matters, I use Mac OS X and zsh but I wanted to use something that works on other *nixes as well.

Answer 1

I think the at command is what you are after.

E.g.:

echo "mail -s Test mstumm < /etc/group" | at 16:30

This will e-mail you a copy of /etc/group at 4:30 PM.

You can read more about at here: http://www.softpanorama.org/Utilities/at.shtml

Answer 2

Adding a new cron job:

newJob="/path/to/new/script.sh"
newJobSchedule="0 0 * * 0 $newJob"
cat <(crontab -l) <(echo "$newJobSchedule") | crontab -

Deleting an old cron job:

oldJob="/path/to/old/script.sh"
cat <(fgrep -i -v "$oldJob" <(crontab -l)) | crontab -

Replacing an outdated cron job:

oldJob="/path/to/old/script.sh"
newJob="/path/to/new/script.sh"
newJobSchedule="0 0 * * 0 $newJob"
cat <(fgrep -i -v "$oldJob" <(crontab -l)) <(echo "$newJobSchedule") | crontab -

Temp files are avoided by using pipes and redirects.