Cron configuration on Linux OS

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CRON is Scheduler for Unix based system, used in all unix flavors such as IBM-AIX,HP-UX, Sun Solaris, Linux (RedHat, Suse, Mandrake, Debian etc.).

CRON is a service which comes in the RAM at the time of bootup and remains there till system shutdowns.

It gets activated every one minute and on activation it does three things.

1.     It reads its configuration file /etc/crontab and does as per instructed.

Environmental Variables for crond

SHELL=/bin/bash

PATH=/sbin:/bin:/usr/sbin:/usr/bin

MAILTO=root

HOME=/

Time	As User	Instruction	Argument (Optional)
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

2.     It goes into /etc/cron.d and opens each file and does as per instruction.

3.     It goes into /var/spool/cron and opens each file and does as per instruction.

Cron utility is an effective way to schedule a routine background job at a specific time and/or day on an on-going basis.

4.     Linux Crontab Format

MIN HOUR DOM MON DOW CMD

Field	Description	Allowed Value
MIN	Minute field	0 to 59
HOUR	Hour field	0 to 23
DOM	Day of Month	1 to 31
MON	Month field	1 to 12
DOW	Day Of Week	0 to 6
CMD	Command	Any command to be executed with absolute path of execution

5.     Scheduling a Job For a Specific Time Every Day

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/samual/full-backup

Where,

30  - 30th Minute

08  - 08 AM

10  - 10th Day

06  - 6th Month (June)

*   - Every day of the week

6.     Schedule a Job For More Than One Instance (e.g. Twice a Day)

The following script take an incremental backup twice a day every day. This example executes the specified incremental backup shell script (incremental-backup) at 11:00 and 16:00 on every day. The comma separated value in a field specifies that the command needs to be executed in all the mentioned time.

00 11,16 * * * /home/samual/bin/incremental-backup

Where,

00 - 0th Minute (Top of the hour)

11,16  - 11 AM and 4 PM

* - Every day

* - Every month

* - Every day of the week

7.     Schedule a Job for Specific Range of Time (e.g. Only on Weekdays)

If you wanted a job to be scheduled for every hour with in a specific range of time then use the following.

Below example checks the status of the database everyday (including weekends) during the working hours 9 a.m – 6 p.m

00 09-18 * * * /home/samual/bin/check-db-status

Where,

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

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/samual/bin/check-db-status

Where,

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)

8.     How to View Crontab Entries?

View Current Logged-In User’s Crontab entries

To view your crontab entries type crontab -l from your unix account as shown below.

[samual@localhost ~]$ crontab -l

30 08 10 06 * /home/samual/full-backup

00 11,16 * * * /home/samual/bin/incremental-backup

00 09-18 * * * /home/samual/bin/check-db-status

00 09-18 * * 1-5 /home/samual/bin/check-db-status

[Note: This displays crontab of the current logged in user]

Login as root user (su - root) and do crontab -l as shown below.

[root@localhost ~]# crontab -l

no crontab for root

To view crontab entries of other Linux users, login to root and use -u {username} -l as shown below.

[root@localhost ~]# crontab -u test -l

00 09-18 * * * /home/test/check-db-status

9.     How to edit Crontab entries?

To edit a crontab entries, use crontab -e as shown below. By default this will edit the current logged-in users crontab.

[samual@localhost ~]$ crontab -e

30 08 10 06 * /home/samual/full-backup

00 11,16 * * * /home/samual/bin/incremental-backup

00 09-18 * * * /home/samual/bin/check-db-status

00 09-18 * * 1-5 /home/samual/bin/check-db-status

~

~

"/tmp/crontab.XXXX8oXiYl" 5L, 193C

[Note: This will open the crontab file in Vim editor for editing. Please note cron created a temporary /tmp/crontab.XX... ]

Login as root user (su – root) and do crontab -e as shown below.

[root@localhost ~]# crontab -e

To edit crontab entries of other Linux users, login to root and use -u {username} -e as shown below.

[root@localhost ~]# crontab -u test -e

10.  Schedule a Job for Every Minute Using Cron.

Ideally you may not have a requirement to schedule a job every minute.

But this example will help you understand the other examples mentioned below

* * * * * CMD

The * means all the possible unit i.e every minute of every hour throughout the year. More than using this * directly, you will find it very useful in the following cases.

When you specify */5 in minute field means every 5 minutes.

When you specify 0-10/2 in minute field mean every 2 minutes in the first 10 minute.

Thus the above convention can be used for all the other 4 fields.

11.  Schedule a Background Cron Job For Every 10 Minutes.

Use the following, if you want to check the disk space every 10 minutes.

*/10 * * * * /home/samual/check-disk-space

It executes the specified command check-disk-space every 10 minutes throughout the year.

But you may have a requirement of executing the command only during office hours or vice versa. The above examples shows how to do those things.

Instead of specifying values in the 5 fields, we can specify it using a single keyword as mentioned below.

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.

Keyword	Equivalent
@yearly	0 0 1 1 *
@daily	0 0 * * *
@hourly	0 * * * *
@reboot	Run at startup.

12.  Schedule a Job For First Minute of Every Year using @yearly

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/samual/red-hat/bin/annual-maintenance

13.  Schedule a Cron Job Beginning of Every Month using @monthly

It is as similar as the @yearly as above. But executes the command monthly once using @monthly cron keyword. This will execute the shell script tape-backup at 00:00 on 1st of every month.

@monthly /home/samual/suse/bin/tape-backup

14.  Schedule a Background Job Every Day using @daily

Using the @daily cron keyword, this will do a daily log file cleanup using cleanup-logs shell scriptat 00:00 on every day.

@daily /home/samual/arch-linux/bin/cleanup-logs "day started"

15.  How to execute a Linux Command After Every Reboot using @reboot

Using the @reboot cron keyword, this will execute the specified command once after the machine got booted every time.

@reboot CMD

16.  How to Disable/Redirect the Crontab Mail Output using MAIL keyword

By default crontab sends the job output to the user who scheduled the job. If you want to redirect the output to a specific user, add or update the MAIL variable in the crontab as shown below.

[samual@localhost ~]$ crontab -l

MAIL="samual"

30 08 10 06 * /home/samual/full-backup

00 11,16 * * * /home/samual/bin/incremental-backup

00 09-18 * * * /home/samual/bin/check-db-status

00 09-18 * * 1-5 /home/samual/bin/check-db-status

Note: Crontab of the current logged in user with MAIL variable

If you wanted the mail not to be sent to anywhere, i.e to stop the crontab output to be emailed, add or update the MAIL variable in the crontab as shown below.

MAIL=""

17.  How to Execute a Linux Cron Jobs Every Second Using Crontab.

You cannot schedule an every-second cronjob. Because in cron the minimum unit you can specify is minute. In a typical scenario, there is no reason for most of us to run any job every second in the system.

18.  Specify PATH Variable in the Crontab

All the above examples we specified absolute path of the Linux command or the shell-script that needs to be executed.

For example, instead of specifying /home/samual/tape-backup, if you want to just specify tape-backup, then add the path /home/samual to the PATH variable in the crontab as shown below.

[samual@localhost ~]$ crontab -l

MAIL="samual"

PATH=/bin:/sbin:/usr/bin:/usr/sbin:/home/samual

30 08 10 06 * /home/samual/full-backup

00 11,16 * * * /home/samual/bin/incremental-backup

00 09-18 * * * /home/samual/bin/check-db-status

00 09-18 * * 1-5 /home/samual/bin/check-db-status

Note: Crontab of the current logged in user with PATH variable

19.  Installing Crontab From a Cron File

Instead of directly editing the crontab file, you can also add all the entries to a cron-file first. Once you have all those entries in the file, you can upload or install them to the cron as shown below.

[samual@localhost ~]$ crontab -l

MAIL="samual"

PATH=/bin:/sbin:/usr/bin:/usr/sbin:/home/samual

30 08 10 06 * /home/samual/full-backup

00 11,16 * * * /home/samual/bin/incremental-backup

00 09-18 * * * /home/samual/bin/check-db-status

00 09-18 * * 1-5 /home/samual/bin/check-db-status

@daily /home/samual/arch-linux/bin/cleanup-logs "day started"

@monthly /home/samual/suse/bin/tape-backup

@yearly /home/samual/red-hat/bin/annual-maintenance

[samual@localhost ~]$ cat cron-file.txt

*/10 * * * * /home/samual/cron-file

[samual@localhost ~]$ crontab cron-file.txt

[samual@localhost ~]$ crontab -l

*/10 * * * * /home/samual/cron-file

Note: This will install the cron-file.txt to your crontab, which will also remove your old cron entries.

So, please be careful while uploading cron entries from a cron-file.txt.

20.  ANACRON

Anacron is also a scheduler but only in RedHat Linux. It is an extension to cron. Whatever jobs is left by cron is completed by anacron. Anacron's configuration file is /etc/anacrontab

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