CRON TAB SETUP IN THE UBUNTU 15.10 LTS

Cron is a daemon that executes scheduled commands. 

If you want to do some task repetitively, you can add your task 

in the crontab, it will handle all for you.

Crontab is the program 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.

Each user root or other has their own crontab, and commands in 

any given crontab will be executed as the user who is the owner of 

the crontab.

Crontab Commands

crontab -e For creating new cron file

crontab -l list the crontab files

crontab -r remove crontab files

cron files options:

m     -    minute 0-59

h      -    hour 0-11

dom -    day of month 1-31

mon -    month 1-12 OR Jan-Dec

dow -    day of week 0-6 OR Sun-Sat

command - you command (ls -la or and script name ./foo)

* means irrespective of(For example I do not want specify the cron 

to run without taking consideration “dow” I will write as * in that 

place like this 41 19 23 7 * ls -la)

Example for crontab.

I want to run the command

ls -la

and output should be store in the file cronLog file which is located 

in the /var/log directory.I will write in the cron file as

ls -la >>/var/log/cronLog

It will list all your file and store the output in cronLog file

Cron searches its spool(jobs to execute) area in 

/var/spool/cron/crontabs directory for crontab files.

We will execute the above example with step by step.

Step1: Adding Command in the Cron

cd ~

crontab -e

a file will be opened with content as:

# Edit this file to introduce tasks to be run by cron.

#

# Each task to run has to be defined through a single line

# indicating with different fields when the task will be run

# and what command to run for the task

#

# To define the time you can provide concrete values for

# minute (m), hour (h), day of month (dom), month (mon),

# and day of week (dow) or use '*' in these fields (for 'any').#

# Notice that tasks will be started based on the cron's system

# daemon's notion of time and timezones.

#

# Output of the crontab jobs (including errors) is sent through

# email to the user the crontab file belongs to (unless redirected).

#

# For example, you can run a backup of all your user accounts

# at 5 a.m every week with:

# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/

#

# For more information see the manual pages of crontab(5) and cron(8)

#

# m h dom mon dow command

Here you need to only focus only on the last line that' it

# m h dom mon dow command

m – minute 0-59

h – hour 0-11

dom- day of month 1-31

mon- month 1-12 OR Jan-Dec

dow- day of week 0-6 OR Sun-Sat

command – you command ls -la or and script name ./foo

When you will save the file, you can see that a file will be created 

as in /var/spool/cron/crontabs 

-rw------- 1 root crontab 1128 Jul 23 19:39 root

in the /var/spool/cron/crontabs directory, where cron daemon

will read it and execute it.

Step2: Add the following line at the end of file, like this

# m h dom mon dow command

41 19 23 7 * ls -la >>/var/log/cronLog

Here * means irrespective of that thing.

What it will do, it list all the file in the current directory at 

Jul 23^rd,19:41, irrespective of the day of week and store that in 

/var/log/cronLog

That's it you have made your first cron.