Job Scheduling Using Crontab

Cron is available with each Linux distribution. It has two major programs, crond and crontab. crond is the daemon which runs in the background and ensures in which programs run as scheduled. The crontab is the program which permits the programs that are scheduled to be run to be modified. The /etc/crontab file stores system scheduling information; every user has their own crontab file in /var/spool/cron.

Note : Within Solaris environment the crontab file is stored in the /var/spool/cron/crontabs.

Cron has data about the tasks to be executed and their schedule through setting up the crontab files. The Cron has to be informed about the task to be run and the exact time at that is has to be executed in months, minutes, days, hours, and years. Asterisks can be used to denote all possible values. The 'crontab -e'command could be used to edit the crontab file. Cron command will bring up an editor window and permit the crontab file to be edited.

A crontab file could be created without using crontab. Other than crontab, has to be used to set up a particular file to be the crontab file. A command 'crontab file', has to be used. At this point 'file' refers to the file which is to be used. This will exchange the old crontab file and not append to it.

The'crontab -e'command uses vi through default. The EDITOR environment variable has to be set if the user prefers to use a different editor. For instance, to use emacs, in bash, use the 'export EDITOR=emacs'.