Processes, Jobs and Scheduling

Learning Linux - Section 5.12

Processes, Jobs and Scheduling

This section introduces and explains the terms Application, Script, Process, Daemon, Thread and Job. These will be described in more detail in the following sections.

Terms covered

• Application syn. Service
• Script
• Process
• Daemon
• Threads
• Job

Application

An Application or service is a program, that runs on your machine. Applications are very common and a few examples for Applications are:

• NTP
• NFS
• rsyslog
• Apache
• RTRR
• Rsync
• top
• etc.

Script

A script can be a small application or a command, that is written down in a file containing the steps, that the scripts will follow when called. The script includes the algorithm, that explains step by step what the instance calling it has to do in order to execute the script. An example for a script is a shell script, these scripts can be sometimes identified by having the extension .sh. In the case of a shell script, what is written in the script is executed top to bottom, line by line. An simple example for such a script is the following:

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#!/bin/bash
echo -e " the command 'whoami' returns:\n"
whoami
mkdir -p /home/tklein/script-execution
exedir='/home/tklein/script-execution'
cd ${exedir}
cat /home/tklein/cars_are_best > ${exedir}/cars_are_still_best
echo -e "\nThe file 'cars_are_still_best' has that many lines:"
wc -l ${exedir}/cars_are_still_best
echo -e "^@\nFancy new Line, just sayin'^@\n"
cat ${exedir}/cars_are_still_best | nl
echo -e "^@\nFancy new Line, just sayin'^@\n"
echo 'Script executed successfully, bash out!'

Which returns the following output on the command line:

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~ $ bash scripts/real_script.sh
 the command 'whoami' returns:

tklein

The file 'cars_are_still_best' has that many lines and its absolute path is:
5 /home/tklein/script-execution/cars_are_still_best

Fancy new Line, just sayin'

     1  Not just any text file.
     2  The secret car file.
     3  No CAPS, REALLY: This is good stuff.
     4  The faster the car, the quicker it covers distance.
     5  A V12 engine is a twelve-cylinder piston engine. 

Fancy new Line, just sayin'

Script executed successfully, bash out!
~ $ 

Process

An application for example generates a process with a unique process id.

Examples for Process/Services Commands

• systemctl or service
  • systemctl is the new command on Red Hat 7/8 and replaces service
• ps lets the user see what processes are running on their Linux machine.
• top shows the user all processes with dependant processes and information regarding memory and CPU load.
• kill stops a process by terminating the process id or the process.
• crontab is used to schedule the executions of these services, processes or applications on the system.
  • When one of these is executed, it becomes a job.

Daemon

A daemon is a process, that runs continuously in the background. A common use for daemons are updater. These can for example send a HTTP GET request and gets a HTTP response in return, telling it if there is a new firmware available for download from the server.

Threads

Every process could have multiple threads associated with it. A running application could have running multiple threads running in the background.

Job

A job or workorder runs a service or process at a scheduled time.

at.

at. is similar to crontab as it is used to schedule the execution of service, process or application, the difference however is that it is only used to do this once. It is not used for reoccurring jobs.