Getting started with crontab

Remarks#

Crontab stands for cron table. It is a utility used to add, edit and delete crons. A cron is a command or program that needs to be run on a regular schedule. Eg: A backup script that runs every day or every hour.

Install crontab on Linux

Debian/Ubuntu

# apt-get update & apt-get -y upgrade
# apt-get install cron

Fedora/CentOS

# yum -y update
# yum install vixie-cron

Arch

# pacman --noconfirm -Syu
# pacman -S cronie

Creating a new cron on ubuntu (or most other linux os)

You can create a new cron entry by simply typing

crontab -e

on the command line. If it’s the first time you want to edit your crontab(le), you will be prompted with an editor selection:

no crontab for <user> - using an empty one

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

Choose 1-4 [2]:

Simply choose your editor by following the prompt, and your crontab will open with an empty file (only containing some commented lines of explanation):

# 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

As you can see, there’s already an example entry in the text:

0 5 * * 1 tar -zcf /var/backups/home.tgz /home/

This would create a backup file called home.tgz inside /var/backups/. The timing for this crontab would be

every monday (first day of week) at 5:00 A.M.

If you had entered that line as your crontab, all you had to do now would be to save the crontabfile. For example with the nano editor, this is done with <Ctrl> + <X> - then confirm to save with Y.

To check your crontab, simply type

crontab -l

in the console.

Some more information on crontimings you can choose:

 # * * * * *  command to execute
 # │ │ │ │ │
 # │ │ │ │ │
 # │ │ │ │ └───── day of week (0 - 6) (0 to 6 are Sunday to Saturday, or use names; 7 is Sunday, the same as 0)
 # │ │ │ └────────── month (1 - 12)
 # │ │ └─────────────── day of month (1 - 31)
 # │ └──────────────────── hour (0 - 23)
 # └───────────────────────── min (0 - 59)

Special characters in cronjobs are:

Asterisk ( * )

The asterisk indicates that the cron expression matches for all values of the field. E.g., using an asterisk in the 4th field (month) indicates every month.

Slash ( / )

Slashes describe increments of ranges. For example 3-59/15 in the 1st field (minutes) indicate the third minute of the hour and every 15 minutes thereafter. The form ”*/…” is equivalent to the form “first-last/…”, that is, an increment over the largest possible range of the field.

Comma ( , )

Commas are used to separate items of a list. For example, using “MON,WED,FRI” in the 5th field (day of week) means Mondays, Wednesdays and Fridays.

Hyphen ( - )

Hyphens define ranges. For example, 2000-2010 indicates every year between 2000 and 2010 AD, inclusive.

Percent ( % )

Percent-signs (%) in the command, unless escaped with backslash (), are changed into newline characters, and all data after the first % are sent to the command as standard input.