Tasks management on Linux using crontab

Crontab

The crontab (cron derives fromchronos, Greek for time; tab stands fortable) command, found in Unix and Unix-like operating systems, is used to schedule commands to be executed periodically. To see what crontabs are currently running on your system, you can open a terminal and run:

crontab -l

To edit the list of cronjobs you can run:

crontab -e

This will open a the default editor ( vi or pico) to manipulate the crontab settings. If you save and exit the editor, all your cronjobs are saved into crontab. Cronjobs are written in the following format:

* * * * * /bin/execute/this/script.sh

Scheduling explained

As you can see there are 5 stars. The stars represent different date parts in the following order:

1. minute (from 0 to 59)
2. hour (from 0 to 23)
3. day of month (from 1 to 31)
4. month (from 1 to 12)
5. day of week (from 0 to 6) (0=Sunday)

Execute every minute

If you leave the star, or asterisk, it means every. Maybe that's a bit unclear. Let's use the the previous example again:

* * * * * /bin/execute/this/script.sh

They are all still asterisks! So this means execute /bin/execute/this/script.sh:

1. every minute
2. of every hour
3. of every day of the month
4. of every month
5. and every day in the week.

In short: This script is being executed every minute. Without exception.

Execute every Friday 1AM

So if we want to schedule the script to run at 1AM every Friday, we would need the following cronjob:

0 1 * * 5 /bin/execute/this/script.sh

Get it? The script is now being executed when the system clock hits:

1. minute: 0
2. of hour: 1
3. of day of month: * (every day of month)
4. of month: * (every month)
5. and weekday: 5 (=Friday)

Execute 10 past after every hour on the 1st of every month

Here's another one, just for practicing

10 * 1 * * /bin/execute/this/script.sh

Fair enough, it takes some getting used to, but it offers great flexibility.

If you want to run something every 10 minutes

0,10,20,30,40,50 * * * * /bin/execute/this/script.sh

Mailing the crontab output of just one cronjob

And change the cronjob like this:*/10 * * * * /bin/execute/this/script.sh 2>&1 | mail -s "Cronjob ouput" yourname@