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Automating tasks on Ubuntu 20.04 using Cronjobs

Reviewed on 09 November 2021Published on 09 November 2019
  • cron
  • cronjob
  • crontab
  • cronfile
  • automate
  • automation
  • ubuntu

Cron is daemon to execute scheduled commands on Unix-like operating systems, including Ubuntu Linux 20.04 LTS (Focal Fossa). The commands to be executed are stored in a table. This table is called Crontab, while the tasks executed by the Cron daemon are called Cronjobs. Crojobs are executed automatically, making cron useful for automating maintenance-related tasks.

Requirements:

Installing Cron

Cron is part of the Ubuntu operating system and is preinstalled on most Ubuntu distributions. However, you can manually install Cron using the apt package manager if necessary.

  1. Make sure your apt package index is updated:

    apt update
  2. Install Cron using the apt package manager:

    apt install cron
  3. Make sure the service is enabled and running in the background to be able to schedule tasks:

    systemctl enable cron

    The following output displays:

    Synchronizing state of cron.service with SysV service script with /lib/systemd/systemd-sysv-install.
    Executing: /lib/systemd/systemd-sysv-install enable cron

Understanding Cron

Cronjobs are managed from a cronfile, specific for each user of the system. This allows each user to schedule their own cronjobs. These cronfiles are located in the /var/spool/cron/crontabs/ directory. Cron allows you to run almost any command you would normally run from the command line.

Each crontab uses the following structure:

MINUTE (0 - 59)HOUR (0 - 23, 0 = MIDNIGHT)DAY OF THE MONTH (1 - 31)MONTH (1 - 12)DAY OF THE WEEK (0 - 6, 0 = SUNDAY)COMMAND OR DIRECTORY PATH AND SCRIPT NAME
*****e.g. /var/www/websites/backup.sh
Tip:

You can also use the following notation for:

  • Months: JAN-DEC
  • Day of the week: SUN-SAT

Below is an example that runs a backup every night at 4:30:

30 04 * * * /var/www/websites/backup.sh

Special strings

Cron provides a range of special strings, which can be used in place of the five time-and-date fields:

StringMeaning
*Wildcard variable that represents “all”, ` * * *` run every minute of every hour of every day of every month.
,Break up scheduling values to form a list, 0,30 * * * * run at the beginning and the middle of every hour.
-Range of values in the schedule field, 0-29 * * * * run very minute of the first 30 min of every hour.
*/Express a step value,*/10 * * * * run on every minute divisible by 10 (10, 20, 30 etc.).
@rebootRun once, at startup.
@yearlyRun once a year, 0 0 1 1 *.
@annually(same as @yearly)
@monthlyRun once a month, 0 0 1 * *.
@weeklyRun once a week, 0 0 * * 0.
@dailyRun once a day, 0 0 * * *.
@midnight(same as @daily)
@hourlyRun once an hour, 0 * * * *.

Managing crontabs

Once you have decided when you want to run your cronjobs, provide the info in your crontab file so that the daemon can read it.

Important:

The crontab file is a regular text file, however it is not recommended to edit it directly. Use the crontab command instead, which will also check your edits for syntax errors.

  1. Open the crontab file:

    crontab -e 

    If you are running the command for the first time, and no crontab exists for your user, a new one will be created. The following prompt displays:

    Select an editor.  To change later, run 'select-editor'.
    1. /bin/nano <---- easiest
    2. /usr/bin/vim.basic
    3. /usr/bin/vim.tiny
    4. /bin/ed

    Choose 1-4 [1]:

    Enter the number corresponding to the editor of your choice, or alternatively press Enter to use the default choice, nano - which is the most user-friendly option.

  2. Edit your crontab by adding your tasks at the end of the file.

    Note:

    By default, the file contains some commented-out information on how to edit the crontab file. Go to the end of the file to add your tasks.

    # 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

    Once done, press CTRL and O to save your modifications, then CTRL and X to quit the text editor.

Tip:

crontab will remember your choice of editor. When you run crontab -e in the future, it will open the file automatically in the same text editor.

Displaying your cronjobs

If you want to display the content of your crontab, but not edit it, use the following command:

crontab -l 

Deleting your crontab

If you want to delete your crontab, run the following command:

crontab -r -i

When prompted, press y to confirm the deletion, or n to cancel the process.