Using cron basics

At one time cron was easy to describe: It involved only one or two files. All you had to do was edit the files and -- voilà! -- cron did the rest. Now cron has become several files and several programs, and at first glance it seems quite complex. Fortunately, someone was clever enough to create a simplified interface along with the new complexity.

Cron is really two separate programs. The cron daemon, usually called cron or crond, is a continually running program that is typically part of the booting-up process.

To check that it's running on your system, use ps and grep to locate the process.

ps -ef|grep cron 
root    387      1   0   Jun 29 ?     00:00:00 crond 
root  32304  20607   0   00:18 pts/0  00:00:00 grep cron 

In the example above, crond is running as process 387. Process 32304 is the grep cron command used to locate crond.

If cron does not appear to be running on your system, check with your system administrator, because a system without cron is unusual.

The crond process wakes up each minute to check a set of cron table files that list tasks and the times when those tasks are to be performed. If any programs need to be run, it runs them and then goes back to sleep. You don't need to concern yourself with the mechanics of the cron daemon other than to know that it exists and that it is constantly polling the cron table files.

The cron table files vary from system to system but usually consist of the following:

• Any files in /var/spool/cron or /var/spool/cron/crontabs. Those are individual files created by any user using the cron facility. Each file is given the name of the user. You will almost always find a root file in /var spool/cron/root. If the user account named jinx is using cron, you will also find a jinx file as /var/spool/cron/jinx.
  

  
  ls -l /var/spool/cron 
  -rw-------   1  root    root          3768 Jul 14  23:54  root 
  -rw-------   1  root    group          207 Jul 15  22:18  jinx 
  

• A cron file that may be named /etc/crontab. That is the traditional name of the original cron table file.
• Any files in the /etc/cron.d directory.

Each cron table file has different functions in the system.

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