Introduction to Systemd on CentOS 7

A few terms we need to grasp:

1. Daemons are processes that wait or run in the background performing various tasks.
2. To listen for connections, a daemon uses a socket.
3. service often refers to one or more daemons

If you are moving from CentOS 6 to CentOS 7, you may be wondering why the need to move to systemd. Here are the features

1. Parallelization capabilities, which increase the boot speed of a system
2. Automatic service dependency management which can prevent long time-out.
3. A method of tracking related processes together by using Linux control groups
4. On-demand starting of daemons without requiring a separate service

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Listing unit files with systemctl

1. Query the state of all units

   # systemctl

2. Query the state of selected service

   # systemctl --type=service

3. List full output of the selected service. Useful for detailed check and investigation

   # systemctl status sshd.service -l

4. To check whether the particular is active and enabled to start at boot time

   # systemctl is-active sshd
   # systemctl is-enabled sshd

5. List the active state of all loaded units. --all will include inactive units

   # systemctl list-units --type=service
   # systemctl list-units --type=service --all

6. View the enabled and disabled settings for all units

   # systemctl list-unit-files --type=service
   

7. View only failed services.

   # systemctl --failed --type=service
   

    

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Controlling System Services a. Status of a Service

# systemctl status sshd.service

b. Disable of a Service

# systemctl disable sshd.service

c. Enable and verify the status of a Service

# systemctl enable sshd.service

# systemctl is-enabled sshd.service

d. Reload configuration file of a running service

# systemctl is-enabled sshd.service