cron: Job Scheduler

cron: Job Scheduler

The crontab utility is mostly used to run jobs at the same time every day, every month, or every year. Every minute, the system checks a system file, called crontab, to see which (if any) processes/jobs need to be started.

There are four ways to manipulate the crontab file:     

crontab [filename] 

crontab -l [username] 

crontab -r [username]  

NOTE: Be careful in using the first method: if a crontab file already exists, it will be overwritten!! (To avoid the file being overwritten, escape using CTL-c, NOT CTL-d).

Every line in the crontab file contains six fields, each separated by a space. A field may contain more than one value, each separated by a comma. An means ``all values".

The fields are:

1. minute: values are 0 to 59.
2. hour: legal values are 0 to 23.
3. date of the month: 1 to 31.
4. month: 1 to 12 (Jan - Dec).
5. day of the week: 0 to 6 (Sun - Sat).
6. Bourne shell command.

Here is an example of what a crontab file would look like:

0 0 * * * /usr/bin/new.messages 
0,15,30,45 * * * * /usr2/people/cantin/check.mail 
0 0 1 1 * /usr2/people/cantin/new.year.message

The first line on the above example runs the command new.messages at 0 minutes, 0 hours (midnight), every day, every month (i.e., every day at midnight).

The second line runs /usr2/people/cantin/check.mail every fifteen minutes.

Finally, the third entry runs new.year.message the first of January, at midnight.