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cron and crontab

If you want a detailed presentation of the topic, refer to this post. Here I write just a short summary.

Cron is the daemon that executes scheduled commands. Crontab is the program to maintain crontab files for individual users. If we want to schedule a command, we need to use crontab.

List your sceduled tasks:

crontab -l

Edit the list of your sceduled tasks:

crontab -e

Example #1:

* * * * * date >>/tmp/date.log

It will append the date to /tmp/date.log every minute. It can be used for testing to see if you have the rights to schedule cron jobs.


{minute} {hour} {day of month} {month} {day of week} {command}

Example #2:

0 22 * * * date >>/tmp/date.log

Meaning: every day at 22h, execute the given task.

Example #3:

0 */2 * * * date >>/tmp/date.log

Execute the task every 2 hours. Notice that the minute must be specified (can be different from 0)! If you write “* */2 * * *“, it will still be executed every minute.

Calling a graphical application from crontab
If you want to launch a GUI application, you’ll get an error message: “Error: Can’t open display:”. To solve this problem, add an extra line to your crontab:

0 */2 * * * /path/to/gui_app.py >>/tmp/log.txt


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