Job scheduling with cron

Cron is the task scheduling daemon in linux. The daemon is called 'crond' and the program that manipulate cron jobs is 'crontab'. Once the daemon is started, it goes into sleep and wakes up every minute to examine all the files containing crontab entries. If a job is scheduled to run at the current minute, it will be executed and the daemon will go back to sleep.

A user can manipulate the cronjobs for his account using the command 'crontab'. Some security restrictions are imposed on use of this command with two files /etc/cron.allow & /etc/cron.deny.

If the file /etc/cron.allow exists, the user should be listed in that inorder to run crontab. If this file doesnt exist the user should not be listed in /etc/cron.deny to run crontab. If both the files does not exist, no users other than root is allowed to use crontab.

Cron jobs for individual users are stored in a seperate text file /var/spool/cron/username which is edited by corresponding user with the command 'crontab -e'

The system wide crontab configuration file is /etc/crontab. What this does is run the cronjob files under a set of directories.

/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


As the name of the directories indicate, the files inside the directories are inspected by cron once every hour, day, week and month. The files insied this directories will be in the standard cronjob format as indicated below.

For per user crontab files (/var/spool/cron/username) the format is:

minute hour day month dayofweek command-to-be-scheduled

For all other crontab files the format is

minute hour day month dayofweek USER command-to-be-scheduled

Values for each field:

minute: 0-59
hour: 0-23
day: 1-31
month: 1-12 (or short names as jan,feb,jun etc can be used)
dayofweek: 0-7 ( or short names as sun, mon,fri etc; 0 & 7 represent Sunday )

For all these fields following rules apply:

Asteric (*) in any field indicate all applicable values for that field.
A hiphen (-) between two digits specify all the values in between inlcuding the two.
A comma seperated list specify all the values listed.

Eg:

10 9 * * 0 = 10:10 AM every sunday
0 10-17 * * * = Everyday at 10AM, 11AM..5PM
15,30,45 9 * * * = Everyday 9:15 AM, 9:30AM and 9:45 AM
*/5 * * * * = Every 5 minutes

USER: the user account under whose privilege the cron job should be run.

The command to execute can be a standatd linux command or a script. Whatever thing that can be executed in shell can be substituted here.

At the starting of the file, we can set the environemnt variables under which the cron job should run. Comments can be inserted with '#'. You can see both these in the /etc/crontab example above.

The script `run-parts` that you see in the crontab file is a standard script that takes a directory as its argument and run all the binaries under that directory.

Individual users can manipulate only their personal crontab file. All other files are managed by root.

In addition to the directories mentioned above, there is one more directory /etc/cron.d. If you want to run task at a custom time, ie not every hour/day/week/month the files should be put in this directory.

crontab command

For individual users to manipulate their cronjobs, use the command crontab.

To add a new job or edit an existing:
[safeer@LinuxBox1 ~]$crontab -e

This will open /var/spool/cron/safeer in the default editor. Now the user can edit this file just like any text file.

To add a job to run a script under the home directory to run at 10 minutes after 10 AM every day

10 10 * * * ~/bin/backup.sh >> backup.log 2>&1

To view safeer's current crontab entries:
[safeer@LinuxBox1 ~]$crontab -l
10 10 * * * ~/bin/backup.sh >> backup.log 2>&1

To remove all the cuurent cron jobs for safeer:
[safeer@LinuxBox1 ~]$crontab -r


The root user can edit anu users cronjob with the -u switch. With this command root can edit the croontab for user safeer:
[root@LinuxBox1 ~]#crontab -[e/l/r] -u safeer

The service name for cron is crond. Use this to start or stop the service. You will hardly need to do this ever.
[root@LinuxBox1 ~]#service crond restart