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Cronjobs

Cronjobs are a time-based scheduler utility in Unix-like operating systems, including Linux and macOS. They allow users to schedule tasks to run automatically at specified intervals, whether it's hourly, daily, weekly, or even minutely. With cronjobs, you can automate repetitive tasks such as backups, system maintenance, or running scripts, freeing up your time and ensuring timely execution of important processes.


You can create a crontab / cronjob by executing following command: 


  # crontab -e

It will then ask you what text editor you want to use. I recommend nano, is is the simplest of them.

Here is a quick breakdown on how cronjobs are constructed: 

  # /etc/crontab: system-wide crontab
  # Unlike any other crontab you don't have to run the 'crontab'
  # command to install the new version when you edit this file
  # and files in /etc/cron.d. These files also have username fields,
  # that none of the other crontabs do.

  # Set the default shell for the cron jobs
  SHELL=/bin/bash

  # Set the default path for the cron jobs
  PATH=/sbin:/bin:/usr/sbin:/usr/bin

  # Specify the email address to which cron job output should be sent
  MAILTO=root

  # For details see man 4 crontabs

  # Example of job definition:
  # .---------------- minute (0 - 59)
  # |  .------------- hour (0 - 23)
  # |  |  .---------- day of month (1 - 31)
  # |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
  # |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
  # |  |  |  |  |
  # *  *  *  *  * command to be executed

  # Run the cron job every hour
  0 * * * * root /bin/bash /usr/local/bin/hourly_backup.sh

  # Run the cron job every day at 2:30 AM
  30 2 * * * root /bin/bash /usr/local/bin/daily_cleanup.sh

  # Run the cron job every Sunday at 3:00 AM
  0 3 * * 0 root /bin/bash /usr/local/bin/weekly_report.sh

This is what a cronjob example could look like: It will execute the script "/var/www/merge.sh" every day at 12 AM, with the bash shell. 


  0 0 * * * /bin/bash /var/www/merge.sh

Cronjobs can also trigger commands on certain events, for example on system startup: 


  @reboot /bin/bash /var/www/merge.sh

Or scheduled but without specifying the time: 

  @hourly /bin/bash /var/www/merge.sh
  
  @daily /bin/bash /var/www/merge.sh
  
  @weekly /bin/bash /var/www/merge.sh
  
  @monthly /bin/bash /var/www/merge.sh
  
  @yearly /bin/bash /var/www/merge.sh

Note for troubleshooting: If your cronjob does not work as intended, try putting an empty space at the end of it!

Cronjobs often don't work if they don't end with a space because this is part of the cron daemon's syntax requirements. The cron daemon expects each line in the crontab file to end with a newline. If a line does not end with a newline character, the cron daemon may not be able to interpret or execute the line correctly.



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