Objectives

After completing this section, you should be able to list system daemons and network services started by systemd service and socket units.

Introduction to systemd

The systemd daemon manages startup for Linux, including service startup and service management in general. It activates system resources, server daemons, and other processes both at boot time and on a running system.

Daemons are processes that either wait or run in the background, performing various tasks. Generally, daemons start automatically at boot time and continue to run until shutdown or until they are manually stopped. It is a convention for names of many daemon programs to end in the letter d.

A service in the systemd sense often refers to one or more daemons, but starting or stopping a service may instead make a one-time change to the state of the system, which does not involve leaving a daemon process running afterward (called oneshot).

In Red Hat Enterprise Linux, the first process that starts (PID 1) is systemd. A few of the features provided by systemd include:

Describing Service Units

systemd uses units to manage different types of objects. Some common unit types are listed below:

The systemctl command is used to manage units. For example, display available unit types with the systemctl -t help command.

Listing Service Units

You use the systemctl command to explore the current state of the system. For example, the following command lists all currently loaded service units, paginating the output using less.

[root@host ~]# systemctl list-units --type=service
UNIT                               LOAD   ACTIVE SUB     DESCRIPTION
atd.service                        loaded active running Job spooling tools
auditd.service                     loaded active running Security Auditing Service
chronyd.service                    loaded active running NTP client/server
crond.service                      loaded active running Command Scheduler
dbus.service                       loaded active running D-Bus System Message Bus
...output omitted...

The above output limits the type of unit listed to service units with the --type=service option. The output has the following columns:

By default, the systemctl list-units --type=service command lists only the service units with active activation states. The --all option lists all service units regardless of the activation states. Use the --state= option to filter by the values in the LOAD, ACTIVE, or SUB fields.

[root@host ~]# systemctl list-units --type=service --all
UNIT                          LOAD      ACTIVE   SUB     DESCRIPTION
  atd.service                 loaded    active   running Job spooling tools
  auditd.service              loaded    active   running Security Auditing ...
  auth-rpcgss-module.service  loaded    inactive dead    Kernel Module ...
  chronyd.service             loaded    active   running NTP client/server
  cpupower.service            loaded    inactive dead    Configure CPU power ...
  crond.service               loaded    active   running Command Scheduler
  dbus.service                loaded    active   running D-Bus System Message Bus
● display-manager.service     not-found inactive dead    display-manager.service
...output omitted...

The systemctl command without any arguments lists units that are both loaded and active.

[root@host ~]# systemctl
UNIT                                 LOAD   ACTIVE SUB       DESCRIPTION
proc-sys-fs-binfmt_misc.automount    loaded active waiting   Arbitrary...
sys-devices-....device               loaded active plugged   Virtio network...
sys-subsystem-net-devices-ens3.deviceloaded active plugged   Virtio network...
...
-.mount                              loaded active mounted   Root Mount
boot.mount                           loaded active mounted   /boot
...
systemd-ask-password-plymouth.path   loaded active waiting   Forward Password...
systemd-ask-password-wall.path       loaded active waiting   Forward Password...
init.scope                           loaded active running   System and Servi...
session-1.scope                      loaded active running   Session 1 of...
atd.service                          loaded active running   Job spooling tools
auditd.service                       loaded active running   Security Auditing...
chronyd.service                      loaded active running   NTP client/server
crond.service                        loaded active running   Command Scheduler
...output omitted...

The systemctl list-units command displays units that the systemd service attempts to parse and load into memory; it does not display installed, but not enabled, services. To see the state of all unit files installed, use the systemctl list-unit-files command. For example:

[root@host ~]# systemctl list-unit-files --type=service
UNIT FILE                                   STATE
arp-ethers.service                          disabled
atd.service                                 enabled
auditd.service                              enabled
auth-rpcgss-module.service                  static
autovt@.service                             enabled
blk-availability.service                    disabled
...output omitted...

In the output of the systemctl list-units-files command, valid entries for the STATE field are enabled, disabled, static, and masked.

Viewing Service States

View the status of a specific unit with systemctl status name.type. If the unit type is not provided, systemctl will show the status of a service unit, if one exists.

[root@host ~]# systemctl status sshd.service
● sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2019-02-14 12:07:45 IST; 7h ago
 Main PID: 1073 (sshd)
   CGroup: /system.slice/sshd.service
           └─1073 /usr/sbin/sshd -D ...

Feb 14 11:51:39 host.example.com systemd[1]: Started OpenSSH server daemon.
Feb 14 11:51:39 host.example.com sshd[1073]: Could not load host key: /et...y
Feb 14 11:51:39 host.example.com sshd[1073]: Server listening on 0.0.0.0 ....
Feb 14 11:51:39 host.example.com sshd[1073]: Server listening on :: port 22.
Feb 14 11:53:21 host.example.com sshd[1270]: error: Could not load host k...y
Feb 14 11:53:22 host.example.com sshd[1270]: Accepted password for root f...2
...output omitted...

This command displays the current status of the service. The meaning of the fields are:

Several keywords indicating the state of the service can be found in the status output:

Verifying the Status of a Service

The systemctl command provides methods for verifying the specific states of a service. For example, use the following command to verify that the a service unit is currently active (running):

[root@host ~]# systemctl is-active sshd.service
active

The command returns state of the service unit, which is usually active or inactive.

Run the following command to verify whether a service unit is enabled to start automatically during system boot:

[root@host ~]# systemctl is-enabled sshd.service
enabled

The command returns whether the service unit is enabled to start at boot time, which is usually enabled or disabled.

To verify whether the unit failed during startup, run the following command:

[root@host ~]# systemctl is-failed sshd.service
active

The command either returns active if it is properly running or failed if an error has occurred during startup. In case the unit was stopped, it returns unknown or inactive.

To list all the failed units, run the systemctl --failed --type=service command.