Automation periodically tasks with CRON job scheduler

What is CRON? Unix-like operating systems provide utility for run periodical tasks. For example you need to run script every day at 08:00 AM. CRON read tasks from configuration file line by line and have special syntax.

Windows have their own tool named as "Windows Task Scheduler".

Open terminal and run crontab -e command. When you open crontab first time, you must choose editor, which is convenient for your use. Enter number of preferred editor and press "Enter":

user@localhost:~$ crontab -e
no crontab for user - using an empty one

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

Choose 1-2 [1]:

When you open cron system display generated configuration file with description:

# 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

Structure of executable command:

# ┌───────────── minute (0–59)
# │ ┌───────────── hour (0–23)
# │ │ ┌───────────── day of the month (1–31)
# │ │ │ ┌───────────── month (1–12)
# │ │ │ │ ┌───────────── day of the week (0–6) (Sunday to Saturday;
# │ │ │ │ │                                   7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * <command to execute>

Here cheat-sheet of some examples:

Run task every minute:

* * * * * cd /home/user && ./my_script.sh

Run task every 5 minutes:

*/5 * * * * cd /home/user && ./my_script.sh

Run task every 1 hour:

0 * * * * cd /home/user && ./my_script.sh

Or use @hourly definition.

Run task every day at 00:00:

0 0 * * * cd /home/user && ./my_script.sh

Or use @daily definition.

If your command not running by any reason use bash -c:

*/5 * * * * bash -c 'cd /home/user && ./my_script.sh'

Run task at system startup:

@reboot cd /home/user && ./my_script.sh

Save changes and exit from editor. Now we're need to reload CRON service. Its need for read configuration file again. Just use service cron reload command:

user@localhost:~$ service cron reload
Reloading configuration files for periodic command scheduler: cron.