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Scheduling tasks with cron
Earlier in this chapter, we worked through starting processes and enabling them to 
run in the background, and ensuring they start as soon as the server boots. In some 
cases, you may need an application to perform a job at a specific time, rather than 
to have it always running in the background. This is where 
cron
 comes in. With 
cron
, you can set a process, program, or script to run at a specific time, down to the 
minute. Each user is able to have their own set of 
cron
 configuration (known as a 
crontab
), which can perform any function that a user would be able to do normally. 
The 
root
 user has a 
crontab
 as well, which allows system-wide administrative tasks 
to be performed. Each 
crontab
 includes a list of 
cron
 jobs (one per line), which we'll 
get into shortly. To view a 
crontab
 for a user, we can use the 
crontab
 command:
crontab -l 
With the 
-l
 option, the 
crontab
 command will show you a list of jobs for the user 
who executed the command. If you execute it as 
root
, you'll see the root account's 
crontab
. If you execute it as user 
jdoe
, you'll see the 
crontab
 for 
jdoe
, and so on. If 
you want to view a 
crontab
 for a user other than yourself, you can use the 
-u
 option 
and specify a user, but you'll need to execute it as 
root
 or with 
sudo
 to view the 
crontab
 for someone other than the user you're logged in as:
sudo crontab -u jdoe -l 
By default, no user has a 
crontab
 until you create one or more jobs. Therefore, you'll 
probably see output such as the following when you check for your current users:
no crontab for jdoe 
To create a 
cron
 job, first log in as the user account you want the task to run under. 
Then, issue the following command:
crontab -e 

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Controlling and Managing Processes