100 Useful Command-Line Utilities

by Oliver; 2014

82. crontab

crontab is a tool to automatically run a command or script at periodic intervals. The first hit on Google has a nice how-to about crontab: To quote directly from this source, you need to make a file of the form:

Field	Range of Values
minute	0-59
hour	0-23
day	1-31
month	1-12
day-of-week	0-7 (where both 0 and 7 mean Sun, 1 = Mon, 2 = Tue, etc)
command-line-to-execute	the command to run along with the parameters to that command if any

Once you make a file of this form, you can run it. For example, if:

$ cat mycron.txt
45 10 * * * rm -r /full/path/trash/* > /full/path/out.o 2> /full/path/out.e

Then you run this as:

$ crontab mycron.txt

And it will empty the trash folder every day at 10:45 a.m.

Display your cron jobs:

$ crontab -l

One peculiar thing about cron is that, if you're on a shared computer system, your system administrator might be the one running cron in his name and, when you invoke cron, you won't start a process running under your name. For this reason, you have to use absolute paths in the commands you put in your cron job: cron doesn't know about the local folder from which you run the command.

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