Unix Fundamentals: CRON

CRON

CRON is a unix utility which allows processes to be executed on a scheduled basis. It’s power is in it’s simplicity. Each user on the server has their own cron table; so user x can’t see the cron table of user y. More concretely, gswadmin doesn’t have access to oracle cron table and vice versa.

Jobs and commands can be scheduled in the cron table using the crontab command. Entries into the cron table have three parts; scheduling, command, output redirection. The scheduling piece has allows jobs to be run up to the at any time increment between a minute and a year.

An example of a cron table entry is as follows:

* * * * * /oracle/bin/example.sh > /tmp/example.log 2>&1

‘*’ is a wild card that indicates every increment. Here are some scheduling examples,

05 * * * * /oracle/bin/example.sh > output.log

Runs every 5 minutes after every hour, every day of the month, every month, every day of the week.

* 13 * * * /oracle/bin/example.sh > output.log

Runs every minute between 1pm and 2pm, every day, every month, every day of the week.

00 20 15,30 * * /oracle/bin/example.sh > output.log

Runs at 8pm on the 15^th and 30^th of every month

00 12 * * 5 /oracle/bin/example.sh > output.log

Runs every Friday at noon.

00 20 15,30 * 4 /oracle/bin/example.sh > output.log

Runs at 8pm on the 15^th and 30^th of every month AND every Thursday.

Cron has it’s own utility to build cron tables. This utility is appropriately called crontab.

COMMANDS

• Crontab –l  
• shows the contents of the cron table for user
• crontab –e
• edits the crontab in the VI editor