Crontab Examples

Crontab is a great way to schedule and run background tasks in Linux.

What is Cron?

In Linux, Cron is the daemon where a user can have their scheduled task listed which they would want to run at specific time periods.

What is Crontab?

Crontab is the program where all the cron tables are installed, uninstalled and listed for a user.

A user can check his crontab, by using the below command

crontab -l

To list any other users’ crontab

crontab -u username -l 

How to use crontab in Linux?

The crontab command in Linux is used to schedule tasks.

How to schedule cron job in Linux?

In order to add or schedule a cron job in Linux, a user will edit his crontab file, with the below command

crontab -e

And to edit other users’ crontab

crontab -u username -e

Below is the syntax of the Crontab file as image

Below is the format of a Crontab file

Minute    Hour    DayOfMonth  Month   DayOfWeek   Command
  *         *         *         *        *        /user/bin/ls

There are 6 fields in the Crontab file, those are:

The First field states the Frequency in Minutes
* means Do this every Minute

The Second field states the Frequency in Hours
* means Do this every Hour

The Third field states the Frequency in Days of Month
* means Do this on All Days of Month

The Fourth field states which Month(s)
* means Do this on All Months

The Fifth field states the Frequency Days of Week
* means Do this on All Days of the Week

The Sixth field states the Script/Command to execute
command_or_path_of_script_to_run

The values in the first 5 fields being:

* = Any/All values # e.g. *
- = Range of values # e.g. 1-5 
, = Multiple/List of values # e.g. 1,2,3
/ = Step values # e.g. 1/3

Note: If the day-of-month or day-of-week part starts with a *, they form an intersection, else, they form a union. 

* * 1 * 1 runs on the 1st day of the month and on Monday (union) (These can be 2 different days), whereas,

* * */2 * 1 runs on every second day of the month only if it's also a Monday (intersection) (Both the condition have to fall on the same day).

Let’s see a few cron job examples

1. Cron expression every minute

Run a Cron job every minute

* * * * * /path/to/script

2. Cron at every 0th Minute (Will run Every Hour on the strike of 0th minute)

0 * * * * /path/to/script

3. Cron expression every hour

Every Strike of Hour 1:00 (Will run Every day exactly at 1 am)

0 1 * * * /path/to/script

4. Every Minute after Hour 1:00 (Will run Every minute after 1 am)

* 1 * * * /path/to/script

5. Every minute on the 1st day of the Month (Will run Every minute on Date 1 of a New Month)

* * 1 * * /path/to/script

6. Every Minute in 1st Month i.e. January (Will run Every minute in January)

* * * 1 * /path/to/script

7. Every Minute on 0th Day of Week i.e. Sunday (Will run Every minute on Sunday)

* * * * 0 /path/to/script

8. Crontab example every 5 minutes

To run a crontab every 5 minute, we will add the below in out crontab file

0/5 * * * * /path/to/script

9. Cron expression every 5 seconds

So, We have 12 scripts, with each starting their run at every minute, and each script after the first progressively sleeps for 5 extra seconds, so we can emulate a 5 second time period.

* * * * * command_or_path_to_script_to_execute
* * * * * sleep 5; command_or_path_to_script_to_execute
* * * * * sleep 10; command_or_path_to_script_to_execute
* * * * * sleep 15; command_or_path_to_script_to_execute
* * * * * sleep 20; command_or_path_to_script_to_execute
* * * * * sleep 25; command_or_path_to_script_to_execute
* * * * * sleep 30; command_or_path_to_script_to_execute
* * * * * sleep 35; command_or_path_to_script_to_execute
* * * * * sleep 40; command_or_path_to_script_to_execute
* * * * * sleep 45; command_or_path_to_script_to_execute
* * * * * sleep 50; command_or_path_to_script_to_execute
* * * * * sleep 55; command_or_path_to_script_to_execute

10. At minute 30 past every 3rd hour from 1 through 23.

 30 1/3 * * * /path/to/script

11. Run cron on a few specific days.

Run every minute on Monday, Tuesday and Friday

 * * * * mon,tue,fri /path/to/script

12. Run cron on specific months.

Will run every minute in January and June

 * * * jan,jun * /path/to/script

Please let us know in the comments below if you have any inputs for us! Thanks!