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Lets explore crontab, its commands, and various operations associated with Crontab.
Origin
Crontab stands for cron table.
The word cron has been taken from the Greek wordChronosmeaning time.
Crontab is a system daemon process that will automatically perform some tasks and jobs as per the scheduled time.
Using cron, an administrator can schedule a job to run on a specific time and day.
So lets see how we can set this up.
Crontab Syntax
To execute a list of commands, cron utility reads the configuration files.
To interpret the lines in thecrontabconfiguration tables, it follows a Crontab syntax.
The last field, i.e., 6th field, could be a username and/or task /job/command/script to be executed.
Schedule a cron to execute at 5 AM daily
2.
Schedule a cron to execute twice a day at 6 AM and 6 PM
3.
Schedule a cron to execute on every minute
4.
Schedule a cron to execute on every Monday at 7 PM.
Schedule a cron to execute every 15 minutes.
Schedule a cron to execute on selected months
- trigger the shell script/home/script/backup.shon March 4 at 7:25 A.M
8.
The above task will stop the execution for 30 seconds, and it will resume again.
It gives the ability to system crontab to run commands as any user.
The User Crontab
User can also install their own cron jobs using the crontab command.
We need to use crontab command for editing or setting up our own cron jobs.
How does it work?
It also checks the/var/spool/cron/directory.
It will not only save us time but will also improve readability.
Crontab Special String Examples
1.
Crontab to schedule tasks to execute on a monthly basis.
Similarly, we can use other Crontab special string for weekly, daily, and hourly purposes.
This job will run at12.45 AMon a daily basis.
We need to mention the user name to perform this job.
In the above example,geekflareuser is performing cron job operation.
Dont worry we have a solution to this problem.
There are several Cron formatting tools available online.
Lets take a real-time example where I have to execute/home/processMonitor.shat every 5 minutes between Monday to Friday.
# 3.Crontab Guru
Crontab Guruis a simple online cron schedule expression editor.
In addition, it provides a useful means of monitoring your cronjob.
All we need to do is copy a command provided and append at the end of the crontab definition.
In case cron job fails or doesnt even start, you will receive an alert email.
In the below example, Job will execute at04:05, every day, every month and every week.
I hope the above helps you to get a better understanding of Crontab and setting up cron jobs.
Scheduling jobs through Cron is one of the many tasks performed by Linux Admin.
