Crontab

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You might also see: cron

Description

Crontab is an interface that enables to edit cron tables, located in /var/spool/cron/crontabs. These tables don't have to be manually edited. They enable to plan jobs.

Usage

Basic usage is as follows:

crontab [-u user] file
crontab [-u user] { -e | -l | -r }

Options

-e
Edit <user>'s crontab
-l
List <user>'s crontab
-r
Purge crontab of <user>

Environment variables

To change default editor (nano), you can define $EDITOR environment variable as follows:

export EDITOR=/usr/bin/vim

To make this variable known each time you boot your machine, insert this line in your ~/.bashrc.

To avoid to accidentally remove crontab with the -r option, you can also define an alias to replace the command:

alias crontab="crontab -i"

The -i parameter enables to display a confirmation before deleting a crontab.

crontab syntax

Each line of a crontab must be formatted as follows, with a space or tab between each item:

24 3 * * 1 root /foo/bar/script.sh
0-59 0-23 1-31 1-12 0-7    ╰➙ Script to execute
 ╰➙ Command will be launched as root
 ╰➙ Day of week (1=Monday, 2=Tuesday, ..., 7=0=Sunday)
 ╰➙ Month (1=January, 2=February, ..., 12=December)
 ╰➙ Day of month
 ╰➙ Hour
 ╰➙ Minute

Examples

To launch a task every minute:

* * * * * root /every/minute/task.sh

To launch a task every 5 minutes:

*/5 * * * * root /every/fiveminutes/task.sh

To launch a task every Tuesday, at 04:30 AM :

30 4 * * 2 root /every/tuesday/task.sh