Crontab
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