Niklas Tech Blog

Every now and then I need to create jobs that run on intervalls or at special times. In *nix there is a handy command called crontab that takes care of this. However the crontab command is not always self-explanatory so I made this quick intro to the command so I don’t have to keep it in my head

Command line options:

crontab -l   # Show my current cron jobs
crontab -e   # Edit my cron jobs. Opens your default editor (e.g. VIM)
crontab -r   # Removes all cron jobs

Crontab syntax (user):

#Example
#min hour day month weekday command
  5   *    *    *      *    echo Hi #Runs 5 min past every hour
 */5  *    *    *      *    echo Hi #Runs in 5 min intervall
  *  8,20  *    *      *    echo Hi #Runs at 08:00 and 20:00  
  *  12   */2   *      *    echo Hi #Runs every other day at 12:00
 30  23    *    *      0    echo Hi #Runs every Sunday at 23:30   

Crontab syntax (root):

#Example
#min hour day month weekday user command
  0   *    *    1      *    root   echo Hi #Runs every hour in January
 */10 *    *    *      *    niklas echo Hi #Runs in 10 min intervall  

Only differens between root crontab and a user crontab is that root needs to tell cron who will be running the job

Syntax details:
min = minutes (0-59)
hour = hours (0-23)
day = day of month (1-31)
month = month (1-12)
weekday = day of week (0-6) (0 is Sunday)
user = the user that runs the job
command = command to run. Same syntax as if you would have run it on the command line. More then one command can be run, just separate the commands with a semicolon (;)

Run a cron job in “silent mode”:
Cron likes to mail that everything has gone well after running the command. If you do not want these mails you can put a cryptic pice of code at the end of your command like this:

#min hour day month weekday command
  0   0    *    *      *    rm -rf /home/niklas/tmp/* >/dev/null 2>&1

This will delete everything in my /home/niklas/tmp directory every night at 00:00. No mail will be sent

Tested on Red Hat Enterprise Linux AS release 3 (Taroon Update 9)