Cron system

1. CRON SYSTEM
 ABH,IEM

2. CRON….
 An experienced Linux sys admin knows the importance
of running the routine maintenance jobs in the
background automatically.
Linux Cron utility is an effective way to schedule a
routine background job at a specific time and/or day on
an on-going basis.
 The Linux utilities cron and at are related commands.
The cron utility allows you to schedule a repetitive task to
take place at any regular interval desired, and the at
command lets you specify a one-time action to take place
at some desired time

3. Important cron components
Name Utility
/usr/sbin/crond The daemon which runs automatically
/etc/contab System crontab file in which default
jobs are placed by OS
/var/spool/cron/crontab Cron files created by users
/etc/cron.allow List of users who can access cron utility
/etc/cron.deny List of users who can not access cron
utility

4. Activities ….
 One user is allowed to access and perform these
operations :
1.List the entries in crontab
2.Revise the entries in crontab
3.Delete the crontab

5. Listing and deleting
 To view your current crontab file, use the -l (for ``list'') option:
$ crontab –l
 To view crontab entries of other Linux users, login to root and use -
u {username} -l as shown below.
# crontab -u Akash -l
 To delete your file,
$crontab -d
 If you are superuser, you can delete any user's crontab file with:
# crontab -d username
where username is the user's login name.
@yearly /home/ramesh/annual-maintenance
*/10 * * * * /home/ramesh/check-disk-space

6. Format of crontab
 It consists of three types of line-
 Command lines
 Environment settings
 commands

7. Environment settings
Those lines have the following form:name=value
 Common environment variables are
 SHELL=/bin/bash
 LOGNAME=abh
 HOME=/home/abh
 MAILTO=abhishek.bhattacharya@iemcal.com
 All the lines which are started with # are called
comment.

8. Specifying cron commands
 Each crontab entry is a single line composed of these six fields separated by whitespace.
MIN HOUR DOM MON DOW CMD
Field Description Allowed Value
MIN Minute field 0 to 59 or *
HOUR Hour field 0 to 23 or *
DOM Day of Month 1-31 or *
MON Month field 1-12 or *
DOW Day Of Week 0-6 or *
CMD Command
Any command to be
executed

9. Scheduling a Job For a Specific Time
 The basic usage of cron is to execute a job in a specific
time as shown below. This will execute the Full backup
shell script (full-backup) on 10th June 08:30 AM.
Please note that the time field uses 24 hours format. So,
for 8 AM use 8, and for 8 PM use 20.
 30 08 10 06 * /home/ramesh/full-backup
 30 – 30th Minute
 08 – 08 AM
 10 – 10th Day
 06 – 6th Month (June)
 * – Every day of the week

10. Schedule a Job For More Than One Instance (e.g. Twice a Day)
 The following script take a incremental backup twice a
day every day.
 00 11,16 * * * /home/ramesh/bin/backup
 00 – 0th Minute (Top of the hour)
 11,16 – 11 AM and 4 PM
 * – Every day
 * – Every month
 * – Every day of the week

11. Schedule a Job for Specific Range of Time (e.g. Only on Weekdays)
 This example checks the status of the database
everyday (including weekends) during the working
hours 9 a.m – 6 p.m
 00 09-18 * * * /home/ramesh/bin/backup
 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

12. Cron Job every weekday during working hours
 This example checks the status of the database every
weekday (i.e excluding Sat and Sun) during the
working hours 9 a.m – 6 p.m.
 00 09-18 * * 1-5 /home/ramesh/binbackup
 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
 1-5 -Mon, Tue, Wed, Thu and Fri (Every Weekday)

13. Schedule a Background Cron Job For Every 10 Minutes, every 2 hours.
 Use the following, if you want to check the disk space every
10 minutes.
 */10 * * * * /home/ramesh/check-disk-space
 It executes the specified command check-disk-space every
10 minutes through out the year.
 * 0-10/2 * * * /home/ramesh/check-disk-space
 It executes the specified command check-disk-space every 2
hours between midnight to 10 AM(Midnight,2 AM,4AM,6
AM,8AM ,10 AM) through out the year.

14. Another form
 There are special cases in which instead of the above
5 fields you can use @ followed by a keyword — such
as reboot, midnight, yearly, hourly.
Table: Cron special keywords and its meaning
Keyword Equivalent
@yearly 0 0 1 1 *
@daily 0 0 * * *
@hourly 0 * * * *
@reboot Run at startup.

15. Example
 If you want a job to be executed on the first minute
of every year, then you can use the@yearly cron
keyword as shown below.
This will execute the system annual maintenance
using annual-maintenance shell script at 00:00 on
Jan 1st for every year.
 @yearly /home/ramesh/red-hat/bin/annual-
maintenance

16. Anacron:Desktop job schedular
 Cron assumes that system is continuously powered
on.So normally cron jobs are seldom executed. Who
will keep the machine on 2 AM in night?
 But if anacon is installed in the system then it will
execute any jobs skipped during the time system was
powered off.
 It is not like normal cron because it does not run
continuously as adaemon.It must be executed as a
part of start up script.It should be placed in
/etc/rc.d/init.d directory.

17. Contd
 Anacron does not gurantee that time at which a job is
run.
 To start anacron mannually
 $anacron –s
 The configuration file of anacron is /etc/anacrontab

18. The at command
 Use at when you want to execute a command or multiple commands
once at some future time.In Linux, the at command requires that
the atrun command be started in root's crontab file. Many Linux
distributions ship with at enabled, but some do not. To enable the at
utility on your system, become superuser and edit root's crontab
file:
 $ su root
 Password:
 # crontab -e
 and add the following line:* * * * * directory/atrun
where directory is the location where the atrun executable is stored.
 On my system that's /usr/lib, so the entry is:* * * * * /usr/lib/atrun
 This causes atrun to be executed every minute

19. Example
 To demonstrate the at command, let's have it print ``hello'' on your current
terminal window after a few minutes. First, get the time and your current
terminal device:
 $ date
 Tue Oct 3 15:33:37 PDT 2012
 $ tty
 /dev/ttyp2
 Now run the at command. Specify a time in the command line, press
Return, and then enter the command, followed by another Return and a
Control-D:
 $ at 15:35
 echo "hello" > /dev/ttyp2
 ^D
 Job c00ceb20b.00 will be executed using /bin/sh
 N.B :The at command takes input up to the end-of-file character (press
Control-D while at the beginning of a line). It reports the job number and
informs you that it will use /bin/sh to execute the command. In two
minutes, hello should appear on the display of /dev/ttyp2. Note that you
can enter a series of commands, one per line.

20. Example
 One way to tell at to do something is to use the
relative form of timing, specifying a time relative
to now. If you want your computer to beep at you in
25 minutes, enter:
 $ at now + 25 minutes
 echo ^G > /dev/ttyp4
 ^D
 Job c00ceb7fb.00 will be executed using /bin/sh

21. Example
 You must tell at your tty location or it won't send
output to your terminal window. If you prefer, you
can receive mail:
 $ at 4:55pm Friday
 echo '5 p.m. meeting with U' | mail abh
 ^D
 Job c00ceb7fb.01 will be executed using /bin/sh

22. Listing pending job
 To get a list of your pending at jobs, enter:
 $ atq
 If you are superuser, atq shows you the pending at
jobs of all users.
 To delete a job, enter:
 $ atrm job_number where job_number is the job
number returned by atq.
 The superuser can also remove other user's jobs.

23. Thank You