Categories
cron Linux Ubuntu

Execute Cron Job After System Reboot on Linux

Is there is an easy way to run script or command at boot time after fresh reboot command?

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. You or user can use crontab program to edit cron jobs.

Running job at statup (boot)

You need to use special string called @reboot. It will run once, at startup after reboot command.

@reboot  /path/to/job
@reboot /path/to/shell.script
@reboot /path/to/command

This is an easy way to give your users the ability to run a shell script or command at boot time without root access. First, run crontab command:
$ crontab -e

OR
# crontab -e -u UserName
# crontab -e -u vivek

Run a script called /home/vivek/bin/installnetkit.sh
@reboot /home/vivek/bin/installnetkit.sh

You also need to enable crond service via sys v / BSD init style system. Under RHEL / CentOS / Fedora, you need to use chkconfig (ntsysv) command to enable crond on boot:
# chekconfg crond on

Under Debian / Ubuntu Linux use update-rc.d as follows to turn on service on boot:
# update-rc.d cron defaults

Save and close the file.

For further information see this tutorial on cron jobs.

Categories
cron Linux

How to add jobs to cron under Linux or UNIX

How do I add cron job under Linux or UNIX like operating system?

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)

Example
If you wished to have a script named /root/backup.sh run every day at 3am, my crontab entry would look like as follows:

Install your cronjob:
# crontab -e


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 ..., every day:
23 0-23/2 * * * /path/to/command

Run at 5 after 4 every sunday:
5 4 * * sun /path/to/command

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

  1. 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.
  2. The comma (,) : This operator specifies a list of values, for example: “1,5,10,15,20, 25”.
  3. 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.

How do I disabling 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 particluer email account let us say sysadmin@powercram.com you need to define MAILTO variable to your cron job:
MAILTO="sysadmin@powercram.com"
0 3 * * * /root/backup.sh >/dev/null 2>&1

Task:To list your crontab jobs use the command

Type the following command:
# crontab -l

To remove or erase all crontab jobs use the command:
# crontab -r

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

Make a backup everyday:
@daily /path/to/backup/script.sh

Understanding /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.

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

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 cronjobs. You can directly drop your scripts here. run-parts command run scripts or programs in a directory via /etc/crontab

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