Setting Up a Cron Job

Setting up a Cron Job

Using cron

 Use the personal crontab to setup your jobs

type

crontab -e  ** use the -e switch, do not edit the file directly 

Everthing is the same as below except the 6th space in the personal crontab is the field to execute as oposed to the user to run the job

example

* * * * * /backup/script.sh

 0 7 */15 * * /var/www/scripts/htmlbackup.sh 2>> /var/www/cronhtml.txt
 0 4 */15 * * /var/www/scripts/sitesavailablebackup.sh 2>> /var/www/cronsites.txt

as opposed to

* * * * * root /backup/script.sh

 

crontab -l - shows scheduled jobs

crontab -r  remove the current crontab file

 

In the /etc directory you will probably find some sub directories called 

'cron.hourly', 'cron.daily', 'cron.weekly' and 'cron.monthly'. If you place 

a script into one of those directories it will be run either hourly, daily, 

weekly or monthly, depending on the name of the directory. 


If you want more flexibility than this, you can edit a crontab (the name 

for cron's config files). The main config file is normally /etc/crontab.

On a default RedHat install, the crontab will look something like this:

 
root@pingu # cat /etc/crontab

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

 
The first part is almost self explanatory; it sets the variables for cron.

SHELL is the 'shell' cron runs under. If unspecified, it will default to 

the entry in the /etc/passwd file.


PATH contains the directories which will be in the search path for cron 

e.g if you've got a program 'foo' in the directory /usr/cog/bin, it might 

be worth adding /usr/cog/bin to the path, as it will stop you having to use

the full path to 'foo' every time you want to call it.

 
MAILTO is who gets mailed the output of each command. If a command cron is 

running has output (e.g. status reports, or errors), cron will email the output 

to whoever is specified in this variable. If no one if specified, then the 

output will be mailed to the owner of the process that produced the output.


HOME is the home directory that is used for cron. If unspecified, it will 

default to the entry in the /etc/passwd file.

 

Now for the more complicated second part of a crontab file.

An entry in cron is made up of a series of fields, much like the /etc/passwd

file is, but in the crontab they are separated by a space. There are normally

seven fields in one entry. The fields are:

 
minute hour dom month dow user cmd

 
minuteThis controls what minute of the hour the command will run on,

and is between '0' and '59'

hourThis controls what hour the command will run on, and is specified in

         the 24 hour clock, values must be between 0 and 23 (0 is midnight)

domThis is the Day of Month, that you want the command run on, e.g. to

run a command on the 19th of each month, the dom would be 19.

monthThis is the month a specified command will run on, it may be specified

numerically (0-12), or as the name of the month (e.g. May)

dowThis is the Day of Week that you want a command to be run on, it can

also be numeric (0-7) or as the name of the day (e.g. sun).

userThis is the user who runs the command.

cmdThis is the command that you want run. This field may contain 

multiple words or spaces.

 

Fields

 .---------------- minute (0 - 59) 

 |  .------------- hour (0 - 23)

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

 |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ... 

 |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7)  OR sun,mon,tue,wed,thu,fri,sat 

 |  |  |  |  |

 *  *  *  *  *  

 

If you don't wish to specify a value for a field, just place a * in the 

field.

 

e.g.

01 * * * * root echo "This command is run at one min past every hour"

17 8 * * * root echo "This command is run daily at 8:17 am"

17 20 * * * root echo "This command is run daily at 8:17 pm"

00 4 * * 0 root echo "This command is run at 4 am every Sunday"

* 4 * * Sun root echo "So is this"

42 4 1 * * root echo "This command is run 4:42 am every 1st of the month"

01 * 19 07 * root echo "This command is run hourly on the 19th of July"

Notes:

Under dow 0 and 7 are both Sunday.

If both the dom and dow are specified, the command will be executed when

either of the events happen. 

e.g.

* 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:

* 12 16 * * root cmd

* 12 * * Mon root cmd

Vixie Cron also accepts lists in the fields. Lists can be in the form, 1,2,3 

(meaning 1 and 2 and 3) or 1-3 (also meaning 1 and 2 and 3).

e.g.

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 days

and 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

Lists can also be combined with each other, or with steps:

* 12 1-15,17,20-25 * * root cmd

Will run cmd every midday between the 1st and the 15th as well as the 20th 

and 25th (inclusive) and also on the 17th of every month.

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

is the same as:

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

When using the names of weekdays or months, it isn't case sensitive, but only

the first three letters should be used, e.g. Mon, sun or Mar, jul.

Comments are allowed in crontabs, but they must be preceded with a '#', and

must be on a line by them self. 

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