Scheduling Tasks with cron

Description

This document outlines how to use cron for scheduling events.

Prerequisites

Install cron if it is not already setup on your system.

sudo apt update && sudo apt install cron

Basic Usage

Open the terminal and type,

crontab -e

This will open the crontab (cron configuration file) for your user account.

The first time you run this command it may give you the following prompt:

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no crontab for anthony - using an empty one

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

Choose 1-4 [1]: 

I recommend selecting option 1.

Then you should see a crontab file open on your screen that looks like this,

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# 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

Each line you add can define one command to run and its schedule.

The format structure is listed in the file as m h dom mon dow command or spelled out thats,

minute (0-59) hour (0-23) day-of-month (1-31) month (1-12) day-of-week (0-7) followed by the command or /path/to/script

Examples

The following entry would execute a shell script (incremental-backup) at 11:00 and 16:00 (twice) each day.

00 11, 16 * * * /home/user/bin/incremental-backup

The comma separated value in a field allows us to specify that the command needs to be executed at multiple times.

Administrator

Commands that normally run with administrative privileges should be added to the root crontab file.

To edit the root crontab type,

sudo crontab -e

Sources

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