CronTab (Job scheduling in linux )

Cron Job or Job Scheduling in Linux

Linux Crontab :

A Linux System Administrator 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.

Linux Crontab Format:

MIN HOUR DOM MON DOW CMD                                      

Table: Crontab Fields and Allowed Ranges (Linux Crontab Syntax)
Field Description Allowed Value
MIN Minute field 0 to 59
HOUR Hour field 0 to 23
DOM Day of Month 1-31
MON Month field 1-12
DOW Day Of Week 0-6
CMD Command Any command to be executed.
  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 22th June 09:45 AM.

Please note that the time field uses 24 hours format. So, for 9 AM use 9, and for 9 PM use 21.

45 09 22 06 * /home/intzar/full-backup

  • 45 – 45th Minute
  • 09 – 09 AM
  • 22 – 22nd Day  
  • 06 – 6th Month (June)
  • * – Every day of the week
  1. Schedule a Job for More Than One Instance (e.g. Twice a 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/intzar/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
  1. 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 every day 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/intzar/check-database-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 Saturday and Sunday) during the working hours 9 A.M. – 6 P.M.

00 09-18 * * 1-5 /home/intzar/check-database-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)
  1. How to View Crontab Entries?

View Current Logged-In User’s Crontab entries:

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

intzar@dev-box$ crontab –l

@yearly /home/intzar/annual-maintenance

*/10 * * * * /home/intzar/check-database-status

How to View Other Linux User’s Crontabs entries:

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

root@dev-box$ crontab –u noor -l

@yearly /home/noor/monthly-backup

00 09-18  * * * /home/noor/check-database-status

  1. How to Edit Crontab Entries?

Edit Current Logged-In User’s Crontab entries.

To edit a crontab entry, use crontab -e as shown below. By default this will edit the current logged-in user’s crontab.

intzar@dev-box$ crontab –e

@yearly /home/intzar/annual-maintenance

*/10 * * * * /home/intzar/check-diskspace

When you save the above temporary file with: wq, it will save the crontab and display the following message indicating that the crontab is successfully modified.

crontab: installing new crontab

Edit Root Crontab entries

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

root@dev-box$ crontab -e

Edit Other Linux User’s Crontab File entries

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

root@dev-box$ crontab –u noor -e

@yearly /home/noor/monthly-backup

00 09-18  * * * /home/noor/check-database-status

  1. Schedule a Job for Every Minute Using Cron.

Ideally you may not have a requirement to schedule a job every minute. But understanding of this example will help you to understand the other examples mentioned below in this article.

* * * * * cmd

The * means the entire 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’s 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.

  1. 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/intzar/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 example 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.

Table: Cron special keywords and its meaning

Keyword Equivalent
@yearly 0 0 1 1 *
@daily 0 0 * * *
@hourly 0 * * * *
@reboot Run at startup.
  1. 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/intzar/annual-maintenance

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

It is as similar as the @yearly as above. But execute 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/intzar/files-backup

  1. Schedule a Background Job Every Day using @daily

Using the @daily cron keyword, you can clean up log on daily basis using cleanup-logs shell script at 00:00 on every day.

@daily /home/intzar/cleanup-logs

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