Crontab

Setting up cronjobs in Unix and Solaris

cron is a unix, solaris utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon. These tasks are often termed as cron jobs in unix , solaris.
Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times.

Crontab Restrictions

You can execute crontab if your name appears in the file /usr/lib/cron/cron.allow. If that file does not exist, you can use crontab if your name does not appear in the file /usr/lib/cron/cron.deny. 
If only cron.deny exists and is empty, all users can use crontab. If neither file exists, only root user can use crontab.The allow/deny files consist of one user name per line.

Crontab Commands
crontab -e     Edit your crontab file, or create one if it doesn’t already exist.
crontab -l      Display your crontab file.
crontab -r      Remove your crontab file.
crontab -v      Display the last time you edited your crontab file. (This option is only available on a few systems.)

Crontab file
A crontab file has five fields for specifying day , date and time  followed by the command to be run at that interval.

min hr dom month dow cmd

*     *   *   *    *  command to be executed

-     -    -    -    –

|     |     |     |     |

|     |     |     |     +—– day of week (0 – 6) (Sunday=0)

|     |     |     +——- month (1 – 12)

|     |     +——— day of month (1 – 31)

|     +———– hour (0 – 23)

+————- min (0 – 59)

* in the value field above means all legal values as in braces for that column.

The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).

     * 12 16 * Mon root cmd

Will run cmd at midday every Monday and every 16th, and will produce the

same result as both of these entries put together would:

     59 11 * * 1,2,3,4,5 root backup.sh

Will run backup.sh at 11:59 Monday, Tuesday, Wednesday, Thursday

and Friday,as will:

59 11 * * 1-5 root backup.sh

Cron also supports ‘step’ values.A value of */2 in the dom field would mean the command runs every two daysand likewise, */5 in the hours field would mean the command runs every 5 hours.e.g.

* 12 10-16/2 * * root backup.sh

is the same as:

* 12 10,12,14,16 * * root backup.sh

*/15 9-17 * * * root connection.test

Will run connection.test every 15 mins between the hours or 9am and 5pm

To collect the cron execution execution log in a file :

30 18  *    *   *    rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log

LAST DAY of EVERY MONTH

cron job executing ‘myJob.sh’ to be ran on the LAST day of every month at 23:58 :

58 23 * * * [ `date +%d` -eq `echo \`cal\` | awk ‘{print $NF}’` ] && myJob.sh

 

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