crontab¶

set a shell

SHELL=/bin/bash

set a PATH

PATH=/usr/bin:/usr/sbin:/usr/local/bin

incorrect way of seeting PATH

PATH=$PATH:/do/not/do/this

crontab format

* * * * *  command_to_execute
- - - - -
| | | | |
| | | | +- day of week (0 - 7) (where sunday is 0 and 7)
| | | +--- month (1 - 12)
| | +----- day (1 - 31)
| +------- hour (0 - 23)
+--------- minute (0 - 59)

example entries

every 15 min

*/15 * * * * /home/user/command.sh

every midnight

0 0 * * * /home/user/command.sh

every Saturday at 8:05 AM

5 8 * * 6 /home/user/command.sh

compute your crontab periodicity format online

https://crontab.guru/

be careful with % sign (percent), it has special meaning, see https://crontab.guru/ for explanation

% signs must be escaped such as \%

view log

journalctl | grep CRON
 tldr:crontab 

crontab

Schedule cron jobs to run on a time interval for the current user. More information: https://crontab.guru/. Edit the crontab file for the current user:

crontab -e

Edit the crontab file for a specific user

sudo crontab -e -u user

Replace the current crontab with the contents of the given file

crontab path/to/file

View a list of existing cron jobs for current user

crontab -l

Remove all cron jobs for the current user

crontab -r

Sample job which runs at 10:00 every day (* means any value)

0 10 * * * command_to_execute

Sample crontab entry, which runs a command every 10 minutes

*/10 * * * * command_to_execute

Sample crontab entry, which runs a certain script at 02:30 every Friday

30 2 * * Fri /absolute/path/to/script.sh

Append a file to existing cron

(crontab -l 2>/dev/null; cat crontab.tmpl) | crontab -