Control Services and Daemons

Question 1

What is a daemon?

Answer 1

Daemons are processes that either wait or run in the background, to perform various tasks.

Question 2

What does the systemd daemon do?

Answer 2

systemd manages the startup process for Linux, including service startup and service management in general.

Question 3

What is a service within the systemd context?

Answer 3

A service in systemd often refers to one or more daemons.

Question 4

What does PID stand for?

Answer 4

Process Identifier

Question 5

In Red Hat Enterprise Linux, what is the first process that starts? (PID 1)

Answer 5

systemd

Question 6

What features does systemd provide in Linux?

Answer 6

1. Parallelization (starting multiple services simultaniously).
2. On-demand starting of daemons.
3. Automatic service dependency management.
4. A way of tracking related processes using control groups.

Question 7

What is a unit in systemd?

Answer 7

A unit is an abstact concept to define objects that the system knows how to manage.

Question 8

What are unit files?

Answer 8

Configuration files which encapsulate information about system services, listening sockets, and other relevant objects for the systemd init system.

Question 9

Name the three types of systemd units and their file extensions.

Answer 9

1. Service units (.service)
2. Socket units (.socket)
3. Path units (.path)

Question 10

What are .service units?

Answer 10

.service units represent system services, and are used to start often accessed daemons, such as a web server.

Question 11

What are .socket units.

Answer 11

.sockets represent inter-process communications (IPC) that systemd should monitor.

Question 12

What is a .path unit?

Answer 12

.path units delay the activation of a service until a specific file system change occurs.

Question 13

What CLI command is used to manage systemd units.

Answer 13

systemctl

Question 14

What CLI command will list all currently loaded service units?

Answer 14

systemctl list-units --type=service

Question 15

What CLI command wil list all units regardless of activation state?

Answer 15

systemctl list-units --all

Question 16

What CLI command will list units that are both loaded and active?

Answer 16

systemctl

Question 17

What CLI command will list all installed unit files and their state?

Answer 17

systemctl list-unit-files

Question 18

What CLI command will show the status of a unit?

Answer 18

systemctl status unit.type

Question 19

What CLI comand will manualy start a system service?

Answer 19

systemctl start unit.type

If the .type is omited from a unit’s name, systemd will assume it’s a . service

Question 20

What CLI command is used to stop a running service?

Answer 20

systemctl stop name.type

Question 21

What CLI command will “restart” a running service?

Answer 21

systemctl restart unit.type

Restart will completely stop and restart a service resulting in the service getting a new PID assigned.

Question 22

What CLI command will “reload” a running system service?

Answer 22

systemctl reload unit.type

Reload will reload a running service without stopping it first, this results in it keeping it’s PID.

Question 23

What CLI command should you use to reload a service if you are not sure if the service has the option to reload?

Answer 23

systemctl reload-or-restart unit.type

If the service has the ability to reload itself it will, otherwise it will restart.

Question 24

What CLI command is used to list a systemd unit’s dependencies?

Answer 24

systemctl list-dependencies unit.type

To list the services that depend on a unit add the --reverse option fl

Question 25

What CLI command would you use to disable a service and prevent an administrator from accidentaly restarting it?

Answer 25

systemctl mask unit.type

Use systemctl unmask unit.type to allow a unit to be enabled.

Question 26

What CLI command would you use to prevent a service from starting at boot?

Answer 26

systemctl disable unit.type

Use systemctl enable unit.type to make a service start at boot.