Crontab - The Linux Schedulers

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 in Vixie Cron. 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)

crond* -----> Binary or App server daemon
/etc/rc.d/init.d/crond -----> Initscript to start crond server
/etc/crontab -----> System crontab file
mins    hrs DOM  MOY  DOW
00-59 00-23 1-31 1-12 0-7 (0=Sun 1=Mon, 2=Tue, 3=Wed,4=Thu, 5=Fri, 6=Sat and 7=Sun)

Each of the time-related fields may contain:

• A '*', which matches everything, or matches any value
• A single integer, which matches exactly
• Two integers seperated by a dash, matching a range of values ie 8-10 in the hr field would match 8am,9am and 10am.8-10,13 would match 8am,9am,10am and 1pm
• A comma-seperated series of ints or ranges, matching any listed value 
• */2 in the hr field refers to midnote, 2am, 4am and so forth ie the cmd is executed every 2 hrs 
• 0-10/2 in the hr field refers to midnite, 2am, 4am, 6am, 8am and 10am

Note:

• A crontab entry is considered to match the current time when the min and hr fields match the curr time and the mth field matches the current month
• An entry is considered to match the current date when the day of month field [3rd] matches the current day of the mth OR the day of week [5th] field matches the current day of the week:
• IT IS NOT NECESSARY THAT BOTH THE DAY OF THE MTH AND DAY OF THEWEEK MATCH!
• If both the time and date match the current time and date the cmd is executed!
• Never put a '*' in the first field unless u want the cmd to run every minute
• You MAY hand-edit this file but it is never necessary since run-parts does everything. Simply put a shell script in the appropriate /etc/cron.*/ dirs

Also the crond* daemon need not be restart. It will do just that every minute anyway

Example: 
Users often forget to shutdown their machines and go home. Hence,
machine should auto shutdown at 11 pm

/etc/crontab

Install your cronjob:# crontab –e

00 23 * * * root /sbin/shutdown -h now

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:

0 22 * * 1-5 /path/to/command

Run 23 minutes after midnigbt, 2am, 4am ..., everyday:

23 0-23/2 * * * /path/to/command

Run at 5 after 4 every sunday:

5 4 * * sun /path/to/command

If you run many sites, you can use this tip to make managing your cron jobs easier. To minimize the clutter, create a /etc/cron.
5min directory and have crontab read this directory every five minutes.

*/5 * * * * root run-parts /etc/cron.5min

45 * * * * /usr/bin/lynx -source http://example.com/cron.php

45 * * * * /usr/bin/wget -O - -q -t 1 http://www.example.com/cron.php

45 * * * * curl --silent --compressed http://example.com/cron.php

00 09-18 * * * /home/alex/bin/check-db-status
* – Every day of the week
00 – 0th Minute (Top of the hour)
09-18 – 9 am, 10 am,11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm
* – Every day
* – Every month
* – Every day of the week
*/10 * * * * /home/alex/check-disk-space

Cron jobs saved in to /var/spool/cron/$username

$ crontab –l --> To list your crontab jobs
$ crontab –r --> To remove or erase all crontab jobs

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.
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 * * * *".

Run ntpdate every hour:
@hourly /path/to/ntpdate

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

Directory Description
/etc/cron.d/ Put all scripts here and call them from /etc/crontab
file.
/etc/cron.daily/ Run all scripts once a day
/etc/cron.hourly/ Run all scripts once an hour
/etc/cron.monthly/ Run all scripts once a month
/etc/cron.weekly/ Run all scripts once a week

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

Cron Access Perms

/etc/cron.allow and /etc/cron.deny
If a user is only in /etc/cron.allow, then all others are denied
If a user is only in /etc/cron.deny then all others are allowed/not affected
If cron.deny is touched, then no users is allowed to create a crontab
If cron.allow is touched, then no users is allowed to create a crontab