Beginners Guide to Managing Services Using systemd

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 CentOS/RHEL, the first process that starts (PID 1) is systemd. A few of the features provided by systemd include:

• Parallelization capabilities (starting multiple services simultaneously), which increase the boot speed of a system.
• On-demand starting of daemons without requiring a separate service.
• Automatic service dependency management, which can prevent long timeouts. For example, a network-dependent service will not attempt to start up until the network is available.
• A method of tracking related processes together by using Linux control groups.

Describing Service Units

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

• Service units have a .service extension and represent system services. This type of unit is used to start frequently accessed daemons, such as a web server.
• Socket units have a .socket extension and represent inter-process communication (IPC) sockets that systemd should monitor. If a client connects to the socket, systemd will start a daemon and pass the connection to it. Socket units are used to delay the start of a service at boot time and to start less frequently used services on demand.
• Path units have a .path extension and are used to delay the activation of a service until a specific file system change occurs. This is commonly used for services which use spool directories such as a printing system.

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.
crond.service         loaded   active   running    Command Scheduler
dbus.service          loaded   active   running    D-Bus System Message
...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:

Columns in the systemctl list-units Command Output

• UNIT - The service unit name.
• LOAD - Whether systemd properly parsed the unit’s configuration and loaded the unit into memory.
• ACTIVE - The high-level activation state of the unit. This information indicates whether the unit has started successfully or not.
• SUB - The low-level activation state of the unit. This information indicates more detailed information about the unit. The information varies based on unit type, state, and how the unit is executed.
• DESCRIPTION - The short description of the unit.

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.device  loaded   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
[email protected]                             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:

FIELD	DESCRIPTION
Loaded	Whether the service unit is loaded into memory.
Active	Whether the service unit is running and if so, how long it has been running.
Main PID	The main process ID of the service, including the command name.
Status	Additional information about the service.

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

KEYWORD	DESCRIPTION
loaded	Unit configuration file has been processed.
active (running)	Running with one or more continuing processes.
active (exited)	Successfully completed a one-time configuration.
active (waiting)	Running but waiting for an event.
inactive	Not running.
enabled	Is started at boot time.
disabled	Is not set to be started at boot time.
static	Cannot be enabled, but may be started by an enabled unit automatically.

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 command:

# systemctl --failed --type=service