Scheduling tasks in linux cron
Windows has scheduled tasks which most people are only halfway aware of. Linux has very powerful scheduling capabilities coming from it’s unix heritage. cron is the daemon that deals with scheduled tasks under most linux distributions. There are a couple ways that you can schedule cron tasks. The first is from the command line.
Open a console and become the user that you want to schedule tasks for.
Now type crontab -e
This brings up the editor for your crontab or your cron table, which is a list of commands to run and times to run them.
If your new to vi this can be frustrating, vi is the default text editor for most distributions. *(I’ve seen pico in some places.) If it’s pico you can start typing with no problems, if it’s vi, you’ll need to press the insert key first (or the letter i)…
Anyway, if you don’t have any scheduled jobs you’ll start out with a blank slate…
The format for entries is something like this…
00 14 * * * /home/userdir/scripts/starttunnel
The first 5 columns control WHEN a job runs. the first column is minutes, the second hours, the third is the day of the month, the fourth is for the month of the year and the 5th column is for the day of the week. After that you can enter the absolute path to the command or script that you want to run. Asterisks mean to match anything, so the above runs EVERY day at 14:00 hours *(2PM)
Let’s say, I want the above script to run at 12:36 PM on July 3rd each year…
36 12 3 7 * /home/userdir/scripts/starttunnel
As for the days of the week, 0 and 7 both refer to Sunday, 1 is Monday, 2 Tuesday, 3 Wednesday, 4 Thursday, 5 Friday and 6 Saturday… So we could make the above run only for those years where July 3rd is a Friday by doing this…
36 12 3 7 5 /home/userdir/scripts/starttunnel
Multiple run times are fairly easy too… Let’s say we want the script to run every thirty minutes…
0,30 * * * * /home/userdir/scripts/starttunnel
This one runs at 0 and 30 of each hour every day. If you’re using vi, you’ll need to press escape, then :wq (colon followed by wq) and enter to write and quit. Under pico ctrl-o to write the file and ctrl-x quits.