Crontab basic commands

In Linux, Cron is a daemon/service that executes shell commands periodically on a given schedule. Cron is driven by a crontab, a configuration file that holds details of what commands are to be run along with a timetable of when to run them.

1. Create a cron file

$ crontab -e

It will open crontab file. A new blank file if you do not have an existing crontab file.

2. List active crontab

$ crontab -l

3. crontab syntax
A crontab file has six fields for specifying minute, hour, day of month, month, day of week and the command to be run at that interval.

4. crontab examples

* * * * * #Runs every minute
30 * * * * #Runs at 30 minutes past the hour
45 7 * * * #Runs at 7:45 am every day
45 17 * * * #Runs at 5:45 pm every day
00 2 * * 0 #Runs at 2:00 am every Sunday
00 2 * * 7 #Runs at 2:00 am every Sunday
00 2 * * Sun #Runs at 2:00 am every Sunday
30 9 1 * * #Runs at 9:30 am on the first day of every month
00 0–23/2 02 08 * #Runs every other hour on the 2nd of August

5. Special strings that can be used in crontab

@reboot #Runs at boot
@yearly #Runs once a year [0 0 1 1 *]
@annually #Runs once a year [0 0 1 1 *]
@monthly #Runs once a month [0 0 1 * *]
@weekly #Runs once a week [0 0 * * 0]
@daily #Runs once a day [0 0 * * *]
@midnight #Runs once a day [0 0 * * *]
@hourly #Runs once an hour [0 * * * *]

6. Multiple commands
&& can be used to run multiple commands consecutively.

@daily command_01 && command_02

7. Specifying a user’s crontab file to use

$crontab -u username file

8. Removing a crontab file

$crontab -r