Introduction

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.

This article reviews some awesome examples of crontab job scheduling.

Linux crontab format

The following displays what crontab consists of:

* * * * * CMD
1 2 3 4 5  6
| | | | |  |
| | | | |  +- Command to run
| | | | +---- Day of the Week   (range: 0-6, 1 standing for Monday)
| | | +------ Month of the Year (range: 1-12)
| | +-------- Day of the Month  (range: 1-31)
| +---------- Hour              (range: 0-23)
+------------ Minute            (range: 0-59)

The time specification consists of six fields. They are Minute, Hours, Day of the Month, Month, and Day of the Week. After the time specification, you provide the command to be executed.

Awesome crontab examples

1. 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/robin/full-backup

• 30 - 30th Minute
• 08 - 08 AM
• 10 - 10th Day
• 06 - 6th Month, June
• * - Every day of the week

2. 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.

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/robin/bin/incremental-backup

• 00 - 0th Minute (Top of the hour)
• 11,16 - 11 AM and 4 PM
• * - Every day
• * - Every month
• * - Every day of the week

3. 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.

Cron Job everyday during working hours.

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/robin/bin/check-db-status

• 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

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/bin/check-db-status

• 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)

4. How to View Crontab Entries?

4.1 View current logged-in user's crontab entries

To view your crontab entries type crontab -l into the terminal like below:

robin@servername$ crontab -l
@daily /home/robin/daily-cmd

4.2 View root's crontab entries

You need to login as root user by type su - root and run crontab -l like below

root@servername# crontab -l
no crontab for root

4.3 View other linux user's crontab entries

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

root@servername# crontab -u robin -l
@daily /home/robin/daily-cmd