How to Schedule the Crotab Utility in UNIX Operating Systems

crontab: crontab is a Unix utility to schedule periodic  jobs  and it works in background.

crontab Options:

crontab  -l   The current crontab will be displayed on standard output

crontab  -r   The current crontab will be be removed.

crontab -e   This option is used to edit the current crontab using the editor specified by  the  visual  or  editor  environment  variables

Crontab Fields:  Crontab fields are separated by spaces and structure of the stars follows the day, month, hours and mins

*     *     *   *    *        write the execution command here.
-     -     -   -    -

50    23    *   *    *        sh /home/shutm.sh   (example)

|     |     |   |    +----- day of week (0 - 6) (Sunday=0)

|     |     |   +------- month (1 - 12)

|     |     +--------- day of month (1 - 31)

|     +----------- hour (0 - 23)

+------------- min (0 - 59)

Here I m giving the small example for scheduling the crontab job

The following example shows the request is scheduled at 11:50 p.m at every day. Crontab starts the process in background and it executes the script when scheduled time comes.

In this script I ve included the init 0 command for shut downing the machine.

Vi shutm.sh

/sbin/init 0

:wq

Here

/sbin is a standard subdirectory of the root directory in Linux and other Unix-like operating systems that contains executable (i.e., ready to run) programs

init (short for initialization) is the program on Unix and Unix-like systems that spawns all other processes and 0 represents for to shutdown the Unix machine.