====== Service ======

three ways to interact with a service
  - directly with the init.d script
  - with the service wrapper script
  - with the systemctl script

  $ sudo /etc/init.d/ssh stop
  $ sudo /etc/init.d/networking restart
  $ sudo systemctl restart ssh

The word "service" is used in different ways.

A service is a daemon that is started by systemd-init during startup and can be stopped, restarted, and status checked by an administrator.  The service may come with additional daemons, control programs and configuration files.

In this article we will answer the question: "How to start a program at boot?"

We will walk through four ways to run a program:  foreground, background, daemon, service.


In the following series, open three shell terminal windows.
  - Run the commands listed here.
  -  $ tail -f out.txt
  -  $ ps -Hao ppid,pid,user,stat,command  

  $ mkdir work
  $ cd work

===== Foreground =====

  $ ping google.com
  $ (press ctl-c)   # cancel (kill) the process

  $ ping google.com >out.txt  # redirect the output 
  In window 3
    $ tail -f out.txt

This program is running in the foreground.  
The terminal window is tied up; you cannot use it as long as this program is running in the program.
This implies that terminal session can have only one foreground job.

Run ps in the other window and notice you have started two new processes: bash and ping.

  $ (press ctl-c)   # cancel (kill) the process

Run ps and notice both the bash and ping processes are gone.

test table

^ test commands                    ^ Commentary                     |
| <code>$ sudo cat >out.txt</code> | Create a new file name out.txt |
| <code>$ (ctl-a)          </code> | Kill the job                   |

===== Background =====

  $ ping google.com >out.txt
  $ (press ctl-z)     # stop (suspend) the process  

Notice the log output has ceased, because this process is suspended.
In the ps window, both the bash and ping processes are listed with state T for stopped. 

  $ jobs    # list the background jobs for this terminal
  $ bg %1   # put job 1 into the background
  $ jobs   

This process is listed as a running job, and it's process is state S.

  $ kill %1    # kill job 1  

  $ ping google.com >out.txt &    # run in the background
  $ jobs
  $ (close the terminal window)

When the terminal session is killed, the background jobs are also killed. 

===== Daemon =====

  $ nohup ping google.com >out.txt &    # suppress the Hangup signal 
  $ jobs
  $ (close the terminal window)

This time, the program keeps running, even though its associated terminal no longer exists.

Create a shell script with our test command.

  $ echo 'nohup ping google.com >out.txt &' >~/.local/bin/myping.sh 
  $ chmod +x ~/.local/bin/myping.sh 
  $ myping.sh    # test it

  $ sudo mv ~/.local/bin/myping.sh /etc/rc.local/

Now it will run at boot as root.
Executables in this directory are run at boot by root.

This process now meets the qualifications to be considered a daemon.
  * running in the background
  * not associated with a terminal
  * requires no human interaction
  * started during startup

===== Service =====

Copy the script to a system executable directory.
  $ sudo cp pingoog.sh /usr/local/bin/

Create a service file.
  $ sudo cat >/etc/systemd/system/pingoog.service
  [Unit]
  Description=Skateboard monitor webservice.
  
  [Service]
  Type=simple
  ExecStart=/usr/local/bin/pingoog.sh
  
  [Install]
  WantedBy=multi-user.target graphical.target

Mark the service file executable.
  $ sudo chmod 644 /etc/systemd/system/pingoog.service

Test your service with systemctl.
  $ sudo systemctl start pingoog      # start it
  $ sudo systemctl status pingoog     # show the status
  $ sudo systemctl stop pingoog       # stop it

Finally, enable it to make it start at boot.
  $ sudo systemctl enable pingoog     # enable it to start at boot