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How do I add cron job under Linux or UNIX like operating system?
Cron job are used to schedule commands to be executed periodically. You can setup commands or scripts, which will repeatedly run at a set time. Cron is one of the most useful tool in Linux or UNIX like operating systems. The cron service (daemon) runs in the background and constantly checks the /etc/crontab file, and /etc/cron.*/ directories. It also checks the /var/spool/cron/ directory.
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crontab command

Tutorial details
Difficulty	Intermediate (rss)
Root privileges	Yes
Requirements	crond
Estimated completion time	20m

crontab is the command used to install, deinstall or list the tables (cron configuration file) used to drive the cron(8) daemon in Vixie Cron. Each user can have their own crontab file, 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.

Types of cron configuration files

There are different types of configuration files:

The UNIX / Linux system crontab : Usually, used by system services and critical jobs that requires root like privileges. The sixth field (see below for field description) is the name of a user for the command to run as. This gives the system crontab the ability to run commands as any user.
The user crontabs: User can install their own cron jobs using the crontab command. The sixth field is the command to run, and all commands run as the user who created the crontab

Note: This faq features cron implementations written by Paul Vixie and included in many Linux distributions and Unix like systems such as in the popular 4th BSD edition. The syntax is compatible with various implementations of crond.

How Do I install or create or edit my own cron jobs?

To edit your crontab file, type the following command at the UNIX / Linux shell prompt:
$ crontab -e

Syntax of crontab (field description)

OR

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

Easy to remember format:

OR

Example: Run backup cron job script

If you wished to have a script named /root/backup.sh run every day at 3am, your crontab entry would look like as follows. First, install your cronjob by running the following command:
# crontab -e
Append the following entry:
0 3 * * * /root/backup.sh
Save and close the file.

More examples

To run /path/to/command five minutes after midnight, every day, enter:
5 0 * * * /path/to/command
Run /path/to/script.sh at 2:15pm on the first of every month, enter:
15 14 1 * * /path/to/script.sh
Run /scripts/phpscript.php at 10 pm on weekdays, enter:
0 22 * * 1-5 /scripts/phpscript.php
Run /root/scripts/perl/perlscript.pl at 23 minutes after midnight, 2am, 4am .., everyday, enter:
23 0-23/2 * * * /root/scripts/perl/perlscript.pl
Run /path/to/unixcommand at 5 after 4 every Sunday, enter:
5 4 * * sun /path/to/unixcommand

How do I use operators?

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

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.
The comma (,) : This operator specifies a list of values, for example: '1,5,10,15,20, 25'.
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.
The separator (/) : This operator specifies a step value, for example: '0-23/' can be used in the hours field to specify command execution every other hour. Steps are also permitted after an asterisk, so if you want to say every two hours, just use */2.

How do I disable 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>&1
To mail output to particular email account let us say vivek@nixcraft.in you need to define MAILTO variable as follows:

MAILTO='vivek@nixcraft.in'
0 3 * * * /root/backup.sh >/dev/null 2>&1

See 'Disable The Mail Alert By Crontab Command' for more information.

Task: List all your cron jobs

Type the following command:

# crontab -l
# crontab -u username -l

To remove or erase all crontab jobs use the following command:

# Delete the current cron jobs #
crontab -r

## Delete job for specific user. Must be run as root user ##
crontab -r -u username

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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.

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Special string	Meaning
@reboot	Run once, at startup.
@yearly	Run once a year, '0 0 1 1 *'.
@annually	(same as @yearly)
@monthly	Run once a month, '0 0 1 * *'.
@weekly	Run once a week, '0 0 * * 0'.
@daily	Run once a day, '0 0 * * *'.
@midnight	(same as @daily)
@hourly	Run once an hour, '0 * * * *'.

Examples

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

More about /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.

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Understanding Default /etc/crontab

First, the environment must be defined. If the shell line is omitted, cron will use the default, which is sh. If the PATH variable is omitted, no default will be used and file locations will need to be absolute. If HOME is omitted, cron will use the invoking users home directory.

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 cron jobs. You can directly drop your scripts here. The run-parts command run scripts or programs in a directory via /etc/crontab file:

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