Created Saturday 14 January 2017
By default, Arch doesn't come with a Cron implementation. We'll be using cronie which handles scheduled jobs, even if the computer was shut down when the job was supposed to run.
Installation
$ pacman -S cronie
# And enable its service file
$ systemctl enable cronie.service
# In order to use Cron, a user needs to be a member of the users group (not default)
$ gpasswd --add stick users
Usage
# If you installed SSMTP in the previous step, you will be notified when a job has been run
# Cron can be used in two ways - place executable Shell scripts in one of the job folders.
# This will execute the file in the appropriate time according to which folder the script is located
# Places for Shell executable
# /etc/cron.daily
# /etc/cron.weekly
# Or, you place a Cron command inside the crontab file, which is provided for each user.
# The basic format for Cron is; (separated with a space)
# <HOUR> : from 0 to 23
# <DAY_OF_MONTH> : from 1 to 31
# <MONTH> : from 1 to 12
# <DAY_OF_WEEK> : 0 to 6 where 0 is Sunday
# <COMMAND> : The command to run
# Cron also has some basic keywords;
@yearly : once a year
@annually : same as yearly
@monthly : once a month
@weekly : once a week
@daily : once a day
@midnight : same as daily
@hourly : once an hour
# Ex; @reboot ~/bin/i_love_cron.sh will execute the script at startup
Basic Commands
# To view Crontabs (tasks)
$ contab -l
# To edit Crontabs
$ crontab -e
# Or edit root jobs (system based) with sudo
$ sudo crontab -e
# To remove all Crontabs
$ crontab -r
Examples and further Syntax explained
# Using the crontab/cron syntax
# * (star) is used as wildcard - ex, every hour, every day, every month (15min. past)
15 * * * * someCommand
# , (comman) is used to specify multiple time - ex, at 12 and 16 o'clock
00 12,16 * * * someCommand
# / (slash) to specify a interval - ex to run something every 10min.
*/10 * * * * someCommand
# - (subtract) is to specify a range - ex, 1215 daily, except the summer months (June, July & August)
15 12 * 1-5,9-12 * someCommand