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I'm trying to create an interface that will show a message on the screen every set time. Cron is an ideal tool for this case only it doesn't read data from a file during it's run (as far as I could tell). I could create a bunch of file to read but this is redundant.

Is there a way to add a line into the user crontab file or remove a line from it after I have read it from a file?

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I can't really see how your first paragraph connects to your second. What exactly are you trying to accomplish? "show a message on the screen every set time" is trivial; is the message changing each time? Does the contents of the message come from a file? – Michael Mrozek Sep 11 '11 at 20:45
I've difficulties to understand what you really want, but I wonder if at wouldn't be a better solution than modifying the crontab. – AProgrammer Sep 23 '11 at 19:46
up vote 1 down vote accepted

You can always write a cronjob that calls a script that conditionally calls crontab -l > oldcrontab and then executes crontab file. Where file would be the new crontab that should be installed, constructed from oldcrontab modulo an appended/removed line.

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