feels good to get it right...

README.md

add_update_cron_job

http://stackoverflow.com/questions/878600/how-to-create-cronjob-using-bash/8106460#8106460

Thanks everybody for your help. Piecing together what I found here and elsewhere I came up with this:

The Code

command="php $INSTALL/indefero/scripts/gitcron.php"
job="0 0 * * 0 $command"
cat <(fgrep -i -v "$command" <(crontab -l)) <(echo "$job") | crontab -

I couldn't figure out how to eliminate the need for the two variables without repeating myself.

command is obviously the command I want to schedule. job takes $command and adds the scheduling data. I needed both variables separately in the line of code that does the work.

Details

1. Credit to duckyflip, I use this little redirect thingy (<(*command*)) to turn the output of crontab -l into input for the fgrep command.
2. fgrep then filters out any matches of $command (-v option), case-insensitive (-i option).
3. Again, the little redirect thingy (<(*command*)) is used to turn the result back into input for the cat command.
4. The cat command also receives echo "$job" (self explanatory), again, through use of the redirect thingy (<(*command*)).
5. So the filtered output from crontab -l and the simple echo "$job", combined, are piped ('|') over to crontab - to finally be written.
6. And they all lived happily ever after!

In a nutshell:

This line of code filters out any cron jobs that match the command, then writes out the remaining cron jobs with the new one, effectively acting like an "add" or "update" function. To use this, all you have to do is swap out the values for the command and job variables.

add_update_cron_job

#!/bin/bash

command="php /some/script.php"
job="0 0 * * 0 $command"
cat <(fgrep -i -v "$command" <(crontab -l)) <(echo "$job") | crontab -