Crontab in Linux with 20 Useful Examples to Schedule Jobs

The crontab is used for running specific tasks on a regular interval. Linux crontab is similar to windows task schedules. Crontab is very useful for routine tasks like scheduling system scanning, daily backups etc. Crontab executes jobs automatically in the backend on a specified time and interval. In this tutorial, you will learn to uses of crontab with 20 useful examples for scheduling jobs. You can also use crontab for the tasks to run once in future only, but for any tasks to run once we recommends to use Linux at command.

Linux Crontab Syntax

Linux crontab has six fields. 1-5 fields defines the date and time of execution. The 6’th fields are used for command or script to be executed.The Linux crontab syntax are as following:

[Minute] [hour] [Day_of_the_Month] [Month_of_the_Year] [Day_of_the_Week] [command]

◈ Astrics (*) – Matches anything

◈ Define range – You can define range using the hypen like: 1-10 or 20-30 or sun-fri or feb-apr

◈ Define multiple range – You can define multiple ranges with command seprated like: jan-mar,jul-sep

How to Add/Edit Crontab

To add or update job in crontab, use below command. It will open crontab file in the editor where a job can be added/updated.

crontab -e

By default, it will edit crontab entries of current logged in user. To edit other user crontab use command as below

crontab -u username -e

Change EDITOR environment variable to change your default editor.

How to List Crontab

To view crontab entries of current user use the following command.

crontab -l

Use -u followed by username to view crontab entries of the specified user.

crontab -u username -l

20 Useful Crontab Examples

1. Schedule a cron to execute at 2am daily.

This will be useful for scheduling database backup on daily basis.

0 2 * * * /bin/sh backup.sh

◈ are used for matching all the records.

2. Schedule a cron to execute twice a day.

Below example command will execute at 5 AM and 5 PM daily. You can specify multiple time stamp by comma separated.

0 5,17 * * * /scripts/script.sh

3. Schedule a cron to execute on every minutes.

Generally, we don’t require any script to execute on every minute but in some case, you may need to configure it.

* * * * *  /scripts/script.sh

4. Schedule a cron to execute on every Sunday at 5 PM.

This type of cron is useful for doing weekly tasks, like log rotation etc.

0 17 * * sun  /scripts/script.sh

5. Schedule a cron to execute on every 10 minutes.

If you want to run your script on 10 minutes interval, can configure like below. These type of crons are useful for monitoring.

*/10 * * * * /scripts/monitor.sh

*/10: means to run on every 10 minutes. Same as if you want to execute on every 5 minutes use */5.

6. Schedule a cron to execute on selected months.

Sometimes we required scheduling a task to be executed for selected months only. Below example script will run in January, May and August months.

* * * jan,may,aug *  /script/script.sh

7. Schedule a cron to execute on selected days.

If you required scheduling a task to be executed for selected days only. Below example will run on each Sunday and Friday at 5 PM.

0 17 * * sun,fri  /script/script.sh

8. Schedule a cron to execute on first sunday of every month.

To schedule a script to execute a script on first Sunday only is not possible by time parameter, But we can use the condition in command fields to do it.

0 2 * * sun  [ $(date +%d) -le 07 ] && /script/script.sh

9. Schedule a cron to execute on every four hours.

If you want to run a script on 4 hours interval. It can be configured like below.

0 */4 * * * /scripts/script.sh

10. Schedule a cron to execute twice on every Sunday and Monday.

To schedule a task to execute twice on Sunday and Monday only. Use following settings to do it.

0 4,17 * * sun,mon /scripts/script.sh

11. Schedule a cron to execute on every 30 Seconds.

To schedule a task to execute on every 30 seconds is not possible by time parameters, But it can be done by schedule same cron twice like below.

* * * * * /scripts/script.sh

* * * * *  sleep 30; /scripts/script.sh

12. Schedule a multiple tasks in single cron.

To configure multiple tasks with single cron, Can be done by separating tasks by the semicolon ( ; ).

* * * * * /scripts/script.sh; /scripts/scrit2.sh

13. Schedule tasks to execute on yearly ( @yearly ).

@yearly timestamp is similar to “0 0 1 1 *”. It will execute task on the first minute of every year, It may useful to send new year greetings.

@yearly /scripts/script.sh

14. Schedule tasks to execute on monthly ( @monthly ).

@monthly timestamp is similar to “0 0 1 * *”. It will execute a task in the first minute of the month. It may useful to do monthly tasks like paying the bills and invoicing to customers.

@monthly /scripts/script.sh

15. Schedule tasks to execute on Weekly ( @weekly ).

@weekly timestamp is similar to “0 0 1 * mon”. It will execute a task in the first minute of the week. It may useful to do weekly tasks like the cleanup of system etc.

@weekly /bin/script.sh

16. Schedule tasks to execute on daily ( @daily ).

@daily timestamp is similar to “0 0 * * *”. It will execute a task in the first minute of every day, It may useful to do daily tasks.

@daily /scripts/script.sh

17. Schedule tasks to execute on hourly ( @hourly ).

@hourly timestamp is similar to “0 * * * *”. It will execute a task in the first minute of every hour, It may useful to do hourly tasks.

@hourly /scripts/script.sh

18. Schedule tasks to execute on system reboot ( @reboot ).

@reboot is useful for those tasks which you want to run on your system startup. It will be same as system startup scripts. It is useful for starting tasks in the background automatically.

@reboot /scripts/script.sh

19. Redirect Cron Results to specified email account.

By default, cron sends details to the current user where cron is scheduled. If you want to redirect it to your other account, can be done by setup MAIL variable like below

# crontab -l

MAIL=bob

0 2 * * * /script/backup.sh

20. Taking backup of all crons to plain text file.

I recommend keeping a backup of all jobs entry in a file. This will help you to recover crons in case of accidental deletion.

Check current scheduled cron:

# crontab -l

MAIL=rahul

0 2 * * * /script/backup.sh

Backup cron to text file:

# crontab -l > cron-backup.txt

# cat cron-backup.txt

MAIL=rahul

0 2 * * * /script/backup.sh

Removing current scheduled cron:

# crontab -r

# crontab -l

no crontab for root

Restore crons from text file:

# crontab cron-backup.txt

# crontab -l

MAIL=rahul

0 2 * * * /script/backup.sh

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Add Job to Cron (Crontab Command Examples) - Unix / Linux

Unix or Linux operating system provides a feature for scheduling the jobs. You can setup command or scripts which will run periodically at the specified time. The Crontab is command used to add or remove jobs from the cron. The cron service is a daemon runs in the background and checks for /etc/crontab file, /etc/con.*/ directories and /var/spool/cron/ directory for any scheduled jobs.

Each user has a separate /var/spool/cron/crontab file. Users are not allowed directly to modify the files. The crontab command is used for setting up the jobs in the cron.

The format of crontab command is

* * * * * command to be executed

You can easily remember this command in the below format

MI HH DOM MON DOW command

The field descriptions of the crontab are explained below:

MI  : Minutes      from 0 to 59
HH  : Hours        from 0 to 23
DOM : Day of month from 0 to 31
MON : Months       from 1 to 12
DOW : Day of week  from 0 to 7   (0 or 7 represents Sunday)

Command: Any command or script to be scheduled

Let see the usage of crontab command with examples.

1. List crontab entries

You can list out all the jobs which are already scheduled in cron. Use "crontab -l" for listing the jobs.

crontab -l
0 0 * * *  /usr/local/bin/list_unix_versions.sh

The above contab command displays the cron entries. Here the shell script for listing the unix versions (list_unix_version.sh) is scheduled to run daily at midnight.

2. List crontab entries of other users

To list the corntab entries of other user in the unix, use the -u option with crontab. The syntax is shown below:

crontab -u username -l

3. Removing all crontab entries

You can un-schedule all the jobs by removing them from the crontab. The syntax for removing all the crontab entries is

crontab -r

For removing other user’s crontab entries:
crontab -u username -r

4. Editing the crontab

You can edit the crontab and add a new job to it. You can also remove an existing job from the crontab. Use the -e option for editing the crontab.

crontab -e

For editing other user’s crontab entries:
crontab -u username -e

This will open a file in VI editor. Now use the VI commands for adding, removing the jobs and for saving the crontab entries.

5. Schedule a job to take oracle backup on every Sunday at midnight

Edit crontab using "crontab -e" and append the following entry in the file.

0 0 * * 0 /usr/local/bin/oracle_backup.sh

6. Schedule a job to run every six hours in a day

You can schedule a job to run more than once in a day. As an example the following crontab entry takes the mysql backup more than once in a day.

0 0,6,12,18 * * * /usr/bin/mysql_backup.sh

Here the list 0,6,12,18 indicates midnight, 6am, 12pm and 6pm respectively.

7. Schedule job to run for the first 15 days of the month.

You can schedule a job by specifying the range of values for a field. The following example takes the sql server backup daily at midnight for the first 15 days in a month.

0 0 * 1-15 * /usr/bin/sql_server_backup.sh

8. Schedule job to run every minute.

The following crontab command runs the command to send emails to group of users for every minute.

* * * * * /bin/batch_email_send.sh

9. Taking backup of cron entries

Before editing the cron entries, it is good to take backup of the cron entries. So that even if you do mistake you can get back those entries from the backup.

crontab -l > /var/tmp/cron_backup.dat

10. Restoring the cron entries

You can restore the cron entries from the backup as

crontab cron_backup.dat

Understanding the Operators:

There are three operators allowed for specifying the scheduling times. They are:

◈ Asterisk (*) : Indicates all possible values for a field. An asterisk in the month field indicates all possible months (January to December).
◈ Comma (,) : Indicates list of values. See example 6 above.
◈ Hyphen (-): Indicates range of values. See example 7 above.

Disabling Emails:

By default the crontab sends emails to the local user if the commands or scripts produce any output. To disable sending of emails redirect the output of commands to /dev/null 2>&1.

0 0 * 20 * /usr/bin/online_backup.sh > /dev/null 2>&1

Note: you cannot schedule a job to run at second’s level as the minimum allowed scheduling is at minute level.

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