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Crontab : Scheduling Tasks

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Crontab permits to schedule tasks on your computer. For example you can program a safeguard every month on the 13th, or administrative tasks such as booting tasks (checking emails, log off network stations , etc...).

We first notice that crontab stands out the at utility.
Indeed crontab permits to schedule tasks, that is to say repetitive tasks,
whereas at only executes one task.

Allow a user to use crontab

My username is nadir. First you have to allow nadir to use the crontab command. We log in as root and we check if the
/etc/cron.allow file exists. If it exists, add the user nadir in the file, if it does not exist create it and add the user nadir.

root@ipower:~# nano -w /etc/cron.allow

or write (vi,gedit, nedit, kwrite, etc...)
and fill out the file accordingly.

From now on, the nadir user is allowed to use crontab. So it is possible to specify who are the
(users) allowed to use crontab and those who are not allowed. To do that, we use the /etc/cron.allow and
/etc/cron.deny files.

If the /etc/cron.allow file exists, only the users mentioned will have the right to use the cron command.

If the /etc/cron.allow file does not exist, it is the /etc/cron.deny file which is taken into account: the mentioned users
will not have the right to use the cron command.

If neither of the two files exists, only the super user (root) will have the right to use the cron command.

Note: an empty /etc/cron.deny file means that all the users can use the cron commande.

Use of crontab

Once the nadir user is allowed, this one is able to use crontab.
Using the crontab -l option, list the current tasks:

nadir@ipower:~$ crontab -l
no crontab for nadir

We can clearly see that no task is defined. Well, it is now or never!

First of all, create a task file

nadir@ipower:~$ crontab -e

Now you had to fill out it. The syntax is:
m h dom mon dow command
 m forminute between 0 and 59
 h for hour between 0 and 23
 dom for day of month between 1 and 31
 mon for month between 1 and 12
 dow for day of week between 0 and 7, sunday is represented by 0 or 7, monday by 1, etc ...
 command to execute the command.

Now, take an interest in some special characters (metacharacters) :
 *, if one of the m h dom mon dow fields
owns the * character, then it indicates evey minute or evey hour or every day or every day of the month or every month or every day of the week, it depends on which field is placed *.
 / permits to specify a repetition.
 - permits to define a range.
 , permits to specify several values.

Some examples:

*/5 * * * * command to execute a command every 5 minutes.

0 22 * * 1-5 command to execute a command every day, monday to friday, at 10 p.m.

17 19 1,15 * * command means the first and the fifteenth day of the month at 19h17 (7.17 p.m.)

23 0-16/2 * * * command means every 2 hours at the twenty-third minute, between midnight and 16h00 (4.00 p.m.)

There are also special strings of characters :

String Action
@reboot execution at boot
@yearly execution once a year, "0 0 1 1 *"
@annually execution once a year, "0 0 1 1 *"
@monthly execution onnce a month, "0 0 1 * *"
@weekly execution once a week, "0 0 * * 0"
@daily execution once a day, "0 0 * * *"
@midnight execution once a day, "0 0 * * *"
@hourly execution once an hour, "0 * * * *"

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