11 Cron Scheduling Task Examples in Linux

In this article, we are going to review and see how we can schedule and run tasks in the background automatically at regular intervals using the Crontab command.

Dealing with a frequent job manually is a daunting task for system administrators and such tasks can be scheduled and run automatically in the background without human intervene using cron daemon in Linux or Unix-like operating system.

[ You might also like: How to Create and Manage Cron Jobs on Linux ]

For instance, you can automate Linux system backup, schedule updates, and synchronization of files, and many more using Cron daemon, which is used to run scheduled tasks from commandline or use online tools to generate cron jobs.

Cron wakes up every minute and checks schedule tasks in countable – Crontab (CRON TABle) is a table where we can schedule such kinds of repeated tasks.

Tips: Each user can have their own crontab to create, modify and delete tasks. By default cron is enabled to users, however, we can restrict users by adding an entry in /etc/cron.deny file.

Crontab file consists of command per line and has six fields actually and separated either of space or tab. The beginning five fields represent time to run tasks and the last field is for command.

  • Minute (hold values between 0-59)
  • Hour (hold values between 0-23)
  • Day of Month (hold values between 1-31)
  • The month of the year (hold values between 1-12 or Jan-Dec, you can use the first three letters of each month’s name i.e Jan or Jun.)
  • Day of week (hold values between 0-6 or Sun-Sat, Here also you can use the first three letters of each day’s name i.e Sun or Wed. )
  • Command – The /path/to/command or script you want to schedule.

1. List Crontab Entries

List or manage the task with crontab command with -l option for the current user.

# crontab -l

00 10 * * * /bin/ls >/ls.txt

2. Edit Crontab Entries

To edit the crontab entry, use -e the option as shown below. In the below example will open schedule jobs in VI editor. Make necessary changes and quit pressing :wq keys that save the setting automatically.

# crontab -e

3. List Scheduled Cron Jobs

To list scheduled jobs of a particular user called tecmint using option as -u (User) and -l (List).

# crontab -u tecmint -l

no crontab for tecmint

Note: Only root user have complete privileges to see other users’ crontab entries. Normal users can’t view others.

4. Remove Crontab Entry

Caution: Crontab with -r the parameter will remove complete scheduled jobs without confirmation from crontab. Use -i option before deleting user’s crontab.

# crontab -r

5. Prompt Before Deleting Crontab

crontab with -i the option will prompt you confirmation from the user before deleting the user’s crontab.

# crontab -i -r

crontab: really delete root's crontab?

6. Allowed Special Characters (*, -, /, ?, #)

  • Asterisk(*) – Match all values in the field or any possible value.
  • Hyphen(-) – To define range.
  • Slash (/) – 1st field /10 meaning every ten minutes or increment of range.
  • The Comma (,) – To separate items.

7. System-Wide Cron Schedule

System administrator can use predefine cron directory as shown below.

  • /etc/cron.d
  • /etc/cron.daily
  • /etc/cron.hourly
  • /etc/cron.monthly
  • /etc/cron.weekly

8. Schedule a Jobs for Specific Time

The below jobs delete empty files and directory from /tmp at 12:30 am daily. You need to mention the user name to perform the crontab command. In the below example root user is performing a cron job.

# crontab -e

30 0 * * *   root   find /tmp -type f -empty -delete

9. Special Strings for Common Schedule

Strings Meanings
@reboot Command will run when the system reboot.
@daily Once per day or may use @midnight.
@weekly Once per week.
@yearly Once per year. we can use the @annually keyword also.

Need to replace five fields of the cron command with keywords if you want to use the same.

10. Multiple Commands with Double ampersand(&&)

In the below example, command1 and command2 run daily.

# crontab -e

@daily <command1> && <command2>

11. Disable Email Notification.

By default, cron sends mail to the user account executing cronjob. If you want to disable it add your cron job similar to the below example. Using >/dev/null 2>&1 option at the end of the file will redirect all the output of the cron results under /dev/null.

[[email protected] ~]# crontab -e
* * * * * >/dev/null 2>&1

conclusion: Automation of tasks may help us to perform our tasks in better ways, error-free, and efficiently. You may refer to a manual page of crontab for more information by typing the ‘man crontab‘ command in your terminal.

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50 thoughts on “11 Cron Scheduling Task Examples in Linux”

  1. Hey, Ravi,

    Can you provide some examples of the common things that Linux System Administrators automate via Crontab?

    I have two jobs set, one to output the first two lines of df -H, and to output the uptime to a log. Both for historical record-keeping, but I’m struggling to find things that *should* be automated.

    01 1 1 * * df -H | head -n 2 >> /var/log/df.log 2>&1
    01 3 1 * * uptime >> /var/log/uptime.log 2>&1

    I get that it will always be contextually related to the things that need to happen in your environment, and my environment is just that of a person learning Linux administration.

    I just learn by example and am drawing a blank.

    -Dalton

    Reply
  2. Hello users here is a very nice website for creating a crontab command within few second with your schedule,

    crontabgenerator.org

    Please review it and let me know if you have any query.

    thanks

    Reply
    • Hello users here is a very nice website for creating a crontab command within few second with your schedule,

      crontabgenerator.org

      Please review it and let me know if you have any query.

      thanks

      Reply
        • Daniel is right. to run at every 45 days, you need to enter 2 lines.

          1. to run of every 30th day for month#1,3,5,7,9,11
          2. to run of every 15th day for month#2,4,6,8,10,12
          Reply
  3. Thanks for this share, I need your help on an real urgent basis, I need my server to automatically update its dynamic ip with the DDNS provider every time my system reboots or boots, kindly help me with the command, i am using ddclient for dynamic ip updating to my DDNS provider with the following command:

    "/usr/sbin/ddclient -daemon 300 -syslog -verbose -noquiet -debug"
    

    kindly tell me the exact command for running the above command as root as soon as my system boots up.

    Secondly i want my server to execute this command on every reboot even if no one has logged in yet, is it possible to run the command using crontab before even anyone logs in the server?

    Really Appreciate any help from any of you who knows how to do it..

    Regards

    Reply
    • @Junaid,

      Which Linux distribution you’re using? if it’s CentOS, you can add the below command to /etc/rc.local file, this file executes commands when system boots up.

      /usr/sbin/ddclient -daemon 300 -syslog -verbose -noquiet -debug
      
      Reply

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