Crontab and cron job

Introduction

Cron jobs are added tasks to the cron daemon, for it to run them periodically, at specific times.

You usually want your jobs or tasks executed every, Monday, or every day at 5 pm, etc. Cron is very powerful and flexible, so you can tweak it a lot. Let's see what cron is.

Cron is a time-based job scheduler in Unix-like computer operating systems. Cron enables users to schedule jobs (commands or shell scripts) to run periodically at certain times or dates. It is commonly used to automate system maintenance or administration, though its general-purpose nature means that it can be used for other purposes, such as connecting to the Internet and downloading email

Now under Linux, Cron Jobs can be added in two ways.

• Adding them to the user cron job script
• Adding them to the root's @hourly, @dayly, @monthly or @weekly folders

Add a Cron Job in GNU/Linux, or Mac or any Unix based operating system

Let's see now the first way to add a job to the cron. You'll need to run this command.

crontab -e

This will edit the actual user cron script, the file you'll see could be empty or already have some jobs in it.

If you want to edit some other's cron script, enter this command (you need to be root):

crontab -u [user] -e

If you want to list your scheduled jobs

crontab -l

Syntax

The syntax of this file is:

Min Hour Day Month DoW /path/to/command arg1 arg2 … arg

Where:

• Min = Minute, should be numeric value from 0 to 59
• Hour = Hour at which the command is executed, also a numeric value from 0 to 23
• Day = The day of the month when the command is executed, numeric value from 1 to 31
• Month = The month when the command is executed, numeric value from 1 to 12, where 1 is January
• DoW = Day of week, when the command is executed, also a numeric value from 0 to 7 where 0 and 7 is Sunday

You can put an '*' asterisk on any or all field to indicate that at all Hours, minutes, etc. should the command be executed.

Example:

30 * * * * /home/user/backup.sh

And backup.sh may look like this:

rsync --progress --partial -avz /folder/to/copy/ user@remote.server:/remote/folder

You can also use intervals, in any field, example:

30 23 * * 1-5 /home/user/script.sh

This will execute in all week days at 23:30 the scrip.sh

Now the second way is, if you want to execute a task hourly, weekly, monthly or daily, just insert your scrip in one of these folders:

• /etc/cron.daily/
• /etc/cron.hourly/
• /etc/cron.monthly/
• /etc/cron.weekly/

Depending on what you need or want, you should make your script executable once in that folder, and root will execute it, you will need to have permissions to write on those folders.

Some other examples are:

Schedule a task for an specific moment one time run

12 02 2 11 * /home/user/my-task.pl So, at 2:12 am on November the 2nd the program /home/user/my-task.pl will run.

Run a command twice in a day, month, or any other time frame

You can use commas to tell the Operating System you want to run the program twice. Let’s suppose in the above example, you want to run on November and December the same program.

12 02 02 11,12 * /home/user/my-task.pl

Run a command in time intervals, like weekdays

00 14 * * 1-5 /home/user/my-task.pl

What about only on weekends and in the morning

06-12 * * 6,7 /home/user/my-task.pl

Run once every year, month, day

You can use these special words for this:

@yearly -Every year January first 00:00 @daily -Every day at 00:00 @hourly -Every hour at 0 minutes @reboot -As soon as you reboot your computer

Run a program every X minutes

Let’s say you want to run a command every 15 minutes, enter this line the crontab file:

*/15 * * * * command

Conclusion

Linux Cron Jobs, should be used everytime you need to execute a task periodically, do not forget to read the man pages of crontab and crond