Scheduling

CRON

crontab files are stored in /var/spool/cron.

Crontab command is used to install, remove, list or edit the crontab files.

 # crontab -u <username> <file_name>
 # crontab -e  # To edit the crontab files
 # crontab -l # to display the contents of crontab file
 # crontab -r # to remove the crontab file

Fields of Crontab file is:

 Minute Hour Day_of_Month Month Day_of_week Command

at

 # at 9.00am April 5
 at> rm /tmp/*

 # atq  # lists the pending jobs scheduled with at
 # at -l # same as above
 # atrm <job_number> # cancels a job that is pending 
 # at -d <job_number> # same as above



 atrm # 

System crontab files

• Different format than user crontab files
• Master crontab file /etc/crontab runs executables in the following directories
  • /etc/cron.hourly
  • /etc/cron.daily
  • /etc/cron.weekly
  • /etc/cron.monthly
• /etc/cron.d/ directory contains additional system crontab files.