In this tutorial, we will explain ten useful cron commands in Linux with examples.
The cron command is used for scheduling tasks to be executed in a specific period, like a month, day, hour, minute, or even seconds. The cron uses a table of tabs called crontabs which have a list of commands that need to be executed with their times respectively. These cron commands are used by system administrators mostly, to make their work easier and be sure not to forget to execute something during a specific period of their work.
In this tutorial, we are going to use Ubuntu 20.04, but you can choose any Linux distro you want. Let’s get started!
Prerequisites
- Fresh install of Ubuntu 20.04 OS
- User privileges: root or non-root user with sudo privileges
Update the System
Before we start with the ten useful cron commands, we need to update the system packages to the latest versions available.
sudo apt update -y && sudo apt upgrade -y
Once the system is updated, we are ready to show you the basic cron commands in Linux.
1. Manage crond service
To check the status of the crond service, execute the following command:
systemctl status cron.service
You should receive the following output:
root@host:~# systemctl status cron.service
● cron.service - Regular background program processing daemon
Loaded: loaded (/lib/systemd/system/cron.service; enabled; vendor preset: enabled)
Active: active (running) since Mon 2022-08-08 23:57:40 CEST; 1 weeks 3 days ago
Docs: man:cron(8)
Main PID: 583 (cron)
Tasks: 1 (limit: 4611)
Memory: 11.7M
CGroup: /system.slice/cron.service
└─583 /usr/sbin/cron -f
For starting, stopping, and restarting the cron service, you can use the following commands respectively:
systemctl start cron.service systemctl stop cron.service systemctl restart cron.service
2. List the jobs
To list the jobs for the current user, execute the following command:
crontab -l
The output should look similar to this because we do not have cron jobs set yet:
root@host:/var/www/html# crontab -l no crontab for root
3. Create your first cron job
To create your first cron job execute the following command:
crontab -e
After executing this command, the file will be opened, and you can paste the following lines of code:
1 0 * * * root cd /var/www/html && /usr/bin/sh script.sh
Save the file and close it. If you execute the command crontab -l, you will get this:
root@host:/var/www/html# crontab -l # Edit this file to introduce tasks to be run by cron. # # Each task to run has to be defined through a single line # indicating with different fields when the task will be run # and what command to run for the task # # To define the time you can provide concrete values for # minute (m), hour (h), day of month (dom), month (mon), # and day of week (dow) or use '*' in these fields (for 'any'). # # Notice that tasks will be started based on the cron's system # daemon's notion of time and timezones. # # Output of the crontab jobs (including errors) is sent through # email to the user the crontab file belongs to (unless redirected). # # For example, you can run a backup of all your user accounts # at 5 a.m every week with: # 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/ # # For more information see the manual pages of crontab(5) and cron(8) # # m h dom mon dow command 1 0 * * * cd /var/www/html && /usr/bin/sh script.sh
4. Schedule a job every 10 minutes
To schedule a job to run every 10 minutes, add the following lines with the crontab -e command:
*/10 * * * * root cd /var/www/html && /usr/bin/sh script.sh
This cron will call the script every 10 minutes.
5. Schedule a job every hour
To schedule a cron job to run every hour, add the following lines with the crontab -e command:
0 * * * * root cd /var/www/html && /usr/bin/sh script.sh
6. Schedule a job every day at 10:00 AM
Now, we will show you a different way to schedule a cron job. Open the file /etc/crontab. You should see the following lines of code:
# /etc/crontab: system-wide crontab # Unlike any other crontab you don't have to run the `crontab' # command to install the new version when you edit this file # and files in /etc/cron.d. These files also have username fields, # that none of the other crontabs do. SHELL=/bin/sh PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin # Example of job definition: # .---------------- minute (0 - 59) # | .------------- hour (0 - 23) # | | .---------- day of month (1 - 31) # | | | .------- month (1 - 12) OR jan,feb,mar,apr ... # | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat # | | | | | # * * * * * user-name command to be executed 17 * * * * root cd / && run-parts --report /etc/cron.hourly 25 6 * * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily ) 47 6 * * 7 root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly ) 52 6 1 * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly ) #
Now to add the cron job to execute every day at 10:00 AM, just add the following line at the bottom of the file, save it and close it.
0 0 * * * root cd /var/www/html && /usr/bin/sh script.sh
7. Schedule a job At 12:00 AM, only on Friday
To schedule this job, open the /etc/crontab file and paste the following command:
0 0 * * FRI root cd /var/www/html && /usr/bin/sh script.sh
8. Schedule a job for a daily backup
To schedule a daily backup, for example, with some script that will execute commands to generate a daily backup, you do not have to specify intervals. You just need to place the script to the /etc/cron.daily directory on your server:
9. Understanding the cron directories
In the previous example, we explained how to enable daily backup by putting some script into it. What if we want our script to be executed hourly, weekly, or monthly? Do not worry about that. Just execute ls -al /etc/cron and hit tab in your terminal server:
root@host:/var/www/html# ll /etc/cron cron.d/ cron.daily/ cron.hourly/ cron.monthly/ crontab cron.weekly/
As you can see, there are directories for different periods where you can place your scripts, which will be executed automatically.
10. Man command for cron
If you want to learn more about the cron, you must execute man cron in your terminal. You should receive complete documentation about the cron command:
CRON(8) System Manager's Manual CRON(8)
NAME
cron - daemon to execute scheduled commands (Vixie Cron)
SYNOPSIS
cron [-f] [-l] [-L loglevel]
DESCRIPTION
cron is started automatically from /etc/init.d on entering multi-user runlevels.
OPTIONS
-f Stay in foreground mode, don't daemonize.
-l Enable LSB compliant names for /etc/cron.d files. This setting, however, does not affect the parsing of files under /etc/cron.hourly,
/etc/cron.daily, /etc/cron.weekly or /etc/cron.monthly.
-n Include the FQDN in the subject when sending mails. By default, cron will abbreviate the hostname.
-L loglevel
Tell cron what to log about jobs (errors are logged regardless of this value) as the sum of the following values:
That’s it. You just learned how to set up cron jobs on your own in different ways. If you find this difficult and you want some specific cron at a specific time, just contact our technical support, and they will do the rest. We are available 24/7.
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