How to Configure Cron Jobs on RedHat Enterprise Linux (RHEL) 6

Table of Contents:

1. Cron Directories
2. Crontab

1. Cron Directories

• There are four cron directories :
  • /etc/cron.hourly/
  • /etc/cron.daily/
  • /etc/cron.weekly/
  • /etc/cron.monthly/
     
• Scripts in these directories will be executed on recurring basis (every hour, day, week, or month)
• Perfect for scheduled tasks that do not need to be run on a specific day, specific time, etc.
• Copy your script into the corresponding folder
• Ensure scripts have the file permissions of 755 or -rwxr-xr-x
• # chmod 755 scriptfile.sh
• Default :
  • /etc/cron.daily starts at 0305 (3:05 AM)
  • /etc/cron.weekly starts at 0325 (3:25 AM)
  • /etc/cron.monthly starts at 0345 (3:45 AM)
     
• View your current configuration :
• # cat /etc/anacrontab
• Use your favorite editor to edit '/etc/anacrontab'
• Change your configuration :
• What time the scripts will be executed in these directories :

2. Crontab

• System administrators can use crontab to create cron jobs to schedule tasks to execute at a specific date and time
• Using crontab is the same as you would use the 'vi' editor
• Usage :
• Edit cron jobs :
  • # crontab -e
     
• List cron jobs for current logged in user :
• # crontab -l
• # crontab -l -u username

• # crontab -r
• # crontab -r -u username

• List cron jobs for a different user :
• Remove cron jobs for current logged in user :
• Remove cron jobs for a different user :
• M H D Mo W /path/to/script
  • M = Minute (00 to 59)
  • H = Hour (00 to 23)
  • D = Day of month (01 to 31)
  • Mo = Month (01 to 12)
  • W = Day of week (0 to 6 .. 0 = Sunday)
     
• Use * to imply every hour, day, etc.
• Example :
• 30 02 15 6 * /root/scriptfile.sh
• Executes '/root/scriptfile.sh' on June 15th at 2:30AM
• 30 02 * * 0 /root/scriptfile.sh
• Executes '/root/scriptfile.sh' every Sunday at 2:30AM
• Example :
  • 30 02,14 15 6 * /root/scriptfile.sh
  • Executes '/root/scriptfile.sh' on June 15th at 2:30AM and 2:30PM
     
• Example :
  • 30 02-05 * * 0 /root/scriptfile.sh
  • Executes '/root/scriptfile.sh' every Sunday at 2:30AM, 3:30AM, 4:30AM, and 5:30AM
     
• @yearly - execute script every year
• @monthly - execute script every month
• @weekly - execute script every week
• @daily - execute script every day
• @hourly - execute script every hour
• @reboot - execute script every reboot
• Use these variables instead of the M H D Mo W format
  • Example :
    • @monthly /root/scriptfile.sh
    • Executes '/root/scriptfile.sh' on the first day of every month
       
• Example :
• Use , for multiple hours, days, etc.
• Use - to span hours, days, etc.
• Additional variables :
• Formatting entries :

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