A service is an application (or set of applications) that runs in the background waiting to be used, or carrying out essential tasks. They are not interactive. It is detached from the keyboard and display of any interactive user. Process is a running program. At a particular instant of time, it can be either running, sleeping, or waiting for it’s parent process. A process is one or more threads of execution together with their shared set of resources.

To get list all the services on your computer, execute below command on terminal

service --status-all

Start Service

A service can be started using systemctl utility. Syntax of the command is

systemctl [action] [service name]

To start a service, open up a terminal window, and enter the following commands.

sudo systemctl start ufw

Above command will start a service ufw (firewall service in Ubuntu). Below example illustrate how to stop and restart the a service.

sudo systemctl stop ufw
sudo systemctl restart ufw

To check the status of service, execute below command

sudo systemctl status ufw

A service can also start, stop, or restart using the service command too. For example

sudo service ufw stop
sudo service ufw start
sudo service ufw restart
sudo service ufw status

Init Scripts

A service can also be managed using init scripts in the /etc/init.d directory. This directory consists of various scripts for different services. To work with these scripts, open up a terminal window and enter the following commands to start a service:

/etc/init.d/ufw start

Similarly for other options use below example

/etc/init.d/ufw stop
/etc/init.d/ufw restart
/etc/init.d/ufw status