How do I add jobs to cron under Linux or UNIX oses?

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Q. How do I add cron job under Linux or UNIX like operating system?

A. Cron job are used to schedule commands to be executed periodically i.e. to setup commands which will repeatedly run at a set time, you can use the cron jobs.

crontab is the command used to install, deinstall or list the tables used to drive the cron daemon. Each user can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly. You need to use crontab command for editing or setting up your own cron jobs.

To edit your crontab file, type the following command:
$ crontab -e

Syntax of crontab

Your cron job looks like as follows:
1 2 3 4 5 /path/to/command arg1 arg2

Where,

• 1: Minute (0-59)
• 2: Hours (0-23)
• 3: Day (0-31)
• 4: Month (0-12 [12 == December])
• 5: Day of the week(0-7 [7 or 0 == sunday])
• /path/to/command - Script or command name to schedule

Same above five fields structure can be easily remembered with following diagram:

* * * * * command to be executed
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

Example(s)
If you wished to have a script named /root/backup.sh run every day at 3am, my crontab entry would look like as follows:
(a) Install your cronjob:# crontab -e (b)Append following entry:0 3 * * * /root/backup.sh Run five minutes after midnight, every day:5 0 * * * /path/to/command Run at 2:15pm on the first of every month:15 14 1 * * /path/to/command Run at 10 pm on weekdays (Mon - Fri): 0 22 * * 1-5 /path/to/command Run 23 minutes after midnigbt, 2am, 4am ..., (every 2 hours) everyday:23 0-23/2 * * * /path/to/command Run at 5 after 4 every sunday:5 4 * * sun /path/to/command

Use of operators

An operator allows you to specifying multiple values in a field. There are three operators:

1. The asterisk (*) : This operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to every hour or an asterisk in the month field would be equivalent to every month.
2. The comma (,) : This operator specifies a list of values, for example: "1,5,10,15,20, 25".
3. The dash (-) : This operator specifies a range of values, for example: "5-15" days , which is equivalent to typing "5,6,7,8,9,....,13,14,15" using the comma operator.
4. The forward slash (/) : This specifies a repeat pattern, for example: "*/10" minutes , which is equivalent to typing "0,10,20,30,40,50" using the comma operator. (Not on all systems).

How do I disabling Email output?

By default the output of a command or a script (if any produced), will be email to your local email account. To stop receiving email output from crontab you need to append ">/dev/null 2>&1". For example:0 3 * * * /root/backup.sh >/dev/null 2>&1To mail output to particluer email account let us say vivek@nixcraft.in you need to define MAILTO variable to your cron job:MAILTO="vivek@nixcraft.in"
0 3 * * * /root/backup.sh >/dev/null 2>&1

Task:To list your crontab jobs use the command

Type the following command:# crontab -l To remove or erase all crontab jobs use the command:# crontab -r Display the last time you edited your crontab file. (Not on all systems.):# crontab -v

Use special string to save time

Instead of the first five fields, you can use any one of eight special strings. It will not just save your time but it will improve readability.

Run ntpdate every hour:
@hourly /path/to/ntpdate
Make a backup everyday:
@daily /path/to/backup/script.sh

Understanding /etc/crontab file and /etc/cron.d/* directories

/etc/crontab is system crontabs file. Usually only used by root user or daemons to configure system wide jobs. All individual user must must use crontab command to install and edit their jobs as described above. /var/spool/cron/ or /var/cron/tabs/ is directory for personal user crontab files. It must be backup with users home directory.

Typical /etc/crontab file entries:

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

Additionally, cron reads the files in /etc/cron.d/ directory. Usually system daemon such as sa-update or sysstat places their cronjob here. As a root user or superuser you can use following directories to configure cronjobs. You can directly drop your scripts here. run-parts command run scripts or programs in a directory via /etc/crontab

How do I use above directories to put scripts?

Here is a sample shell script (clean.cache) to clean up cached files every 10 days. This script is directly created at /etc/cron.daliy/ directory i.e. create a file called /etc/cron.daily/clean.cache:

 #!/bin/bash

CROOT="/tmp/cachelighttpd/"
DAYS=10
LUSER="lighttpd"
LGROUP="lighttpd"
 
# start cleaning
/usr/bin/find ${CROOT} -type f -mtime +${DAYS} | xargs -r /bin/rm

 
# if directory deleted by some other script just get it back
if [ ! -d $CROOT ]
then

        /bin/mkdir -p $CROOT
        /bin/chown ${LUSER}:${LGROUP} ${CROOT}

fi

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 the root user can use crontab. The allow/deny files consist of one user name per line.

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