Crontab in Linux with Examples

The crontab could be a list of commands that you just need to run on a regular schedule. Crontab stands for “cron table”. The cron is that the system process which will automatically perform tasks for you in keeping with a set schedule.

Linux Crontab Format

MIN HOUR DOM MON DOW CMD

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.

To List all Active Cron Jobs Running, you can run below command

crontab -l

To list cron jobs that belong to a particular user, run the following command:

sudo crontab -u username -l

Replace username with the actual username that you want to list cron jobs

Examples of Cron jobs

1.Scheduling a Cron job For a Specific Time

For Eg :- To execute the Full backup shell script (full-backup) on 10th May 08:30 PM

The time field uses 24 hours format. So, for 8 PM use 20 (for 8 AM use 08)

30 20 10 05 * /home/testuser/full-backup

30 – 30th Minute , 20 – 08 PM, 10 – 10th Day, 05 – 5th Month (May) and * – Every day of the week

2. To schedule a Cronjob for every minute

* * * * *  command-that-you-want-to-execute

• * all the possible unit means every minute of every hour through out the year.
• */5 in minute field means every 5 minutes.
• 0-10/2 in minute field mean every 2 minutes in the first 10 minute.

3. To schedule a Cronjob for more than one time (e.g. Twice a Day)

For Eg :- To take a incremental backup twice a day every day (10:00 and 15:00 on every day)

00 10,15 * * * /home/testuser/incremental-backup

00 – 0th Minute (Top of the hour), 10,15 – 10 AM and 3 PM, * – Every day, * – Every month and * – Every day of the week

4.To schedule a Cronjob for certain range of time (e.g. Only on Weekdays)

For eg :-

a) To checks the status of a service everyday (including weekends) during the working hours 10 a.m – 7 p.m

00 10-19 * * * /home/testuser/check-service-status

00 – 0th Minute (Top of the hour), 10-19 – 10 am, 11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm, 7 pm, * – Every day, * – Every month and * – Every day of the week

b) To checks the status of a service every weekday (i.e excluding Sat and Sun) during the working hours 10 a.m – 7 p.m.

00 10-19 * * 1-5 /home/testuser/check-service-status

00 – 0th Minute (Top of the hour), 10-19 – 10 am, 11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm, 7 pm, * – Every day, * – Every month and 1-5 -Mon, Tue, Wed, Thu and Fri (Every Weekday)

5. To schedule a Cron job for every 10 minutes.

For Eg:- To check the disk space usage every 10 minutes.

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

Cron special keywords and its meaning

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

1. To schedule a Cronjob for first minute of every year using @yearly

@yearly /home/testuser/annual-maintenance-check

This will execute the scheduled task at 00:00 on Jan 1st for every year.

2. To schedule a Cronjob beginning of every month using @monthly

@monthly /home/testuser/take-backup

This will execute the scheduled task at 00:00 on 1st of every month.

3. To schedule a Cronjob every day using @daily

@daily /home/testuser/cleanup-logs "day started"

This will execute the scheduled task at 00:00 on every day.

4. To execute a linux command after every reboot using @reboot

@reboot command

This will execute the given command once after the server got booted every time.