SystemD - Managing System Services (part 2)

2018-01-30

Linux Debian

Previous versions of Linux distributions like Debian or Ubuntu, which were distributed with SysV init or Upstart, used init scripts located in the /etc/rc.d/init.d/ directory. These init scripts were typically written in Bash, and allowed the system administrator to control the state of services and daemons in their system. With systemd these init scripts have been replaced with service units.

Service units end with the .service file extension and serve a similar purpose as init scripts. To view, start, stop, restart, enable, or disable system services, use the systemctl command as described in Table 2.1 - systemctl utility

Table 2.1 - systemctl utility

systemctl	descritption
`systemctl start name.service`	Starts a service.
`systemctl stop name.service`	Stop a service
`systemctl restart name.service`	Restart a service
`systemctl try-restart name.service`	Restarts a service only if it is running.
`systemctl reload name.service`	Reloads configuration.
`systemctl status name.service`	Checks if a service is running.
`systemctl list-units --type service --all`	Displays the status of all services.

systemctl	descritption
`systemctl enable name.service`	Enables a service
`systemctl disable name.service`	Disables a service
`systemctl status name.service`	Checks if a service is enabled.
`systemctl is-enabled name.service`	Checks if a service is enabled.
`systemctl list-unit-files --type service`	Lists all services and checks if they are enabled.
`systemctl list-dependencies --after`	Lists services that are ordered to start before the specified unit.
`systemctl list-dependencies --before`	Lists services that are ordered to start after the specified unit.

2.1 Specifying Service Units

For clarity, all command examples in the rest of this section use full unit names with the .service file extension, for example:

~]$ systemctl stop apache2.service

However, the file extension can be omitted, in which case the systemctl utility assumes the argument is a service unit. The following command is equivalent to the one above:

~]$ systemctl stop apache2

Additionally, some units have alias names. Those names can have shorter names than units, which can be used instead of the actual unit names. To find all aliases that can be used for a particular unit, use:

~]$ systemctl show apache2.service -p Names

The file extension can be omitted, in which case the systemctl utility assumes the argument is a service unit

2.2 Listing Services

To list all currently loaded service units, type the following at a shell prompt:

root@hostname:~$ systemctl list-units --type service

UNIT                               LOAD   ACTIVE SUB     DESCRIPTION                           
-----------------------------------------------------------------------------------------------------
apache2.service                    loaded active running The Apache HTTP Server                                            
console-setup.service              loaded active exited  Set console font and keymap                                       
cron.service                       loaded active running Regular background program processing daemon                      
dbus.service                       loaded active running D-Bus System Message Bus
.
.
.

For each service unit file, this command displays its full name (UNIT) followed by a note whether the unit file has been loaded (LOAD), its high-level (ACTIVE) and low-level (SUB) unit file activation state, and a short description (DESCRIPTION).

By default, the systemctl list-units command displays only active units. If you want to list all loaded units regardless of their state, run this command with the --all or -a command line option:

~]$ systemctl list-units --type service --all

  UNIT                                 LOAD      ACTIVE   SUB     DESCRIPTION
--------------------------------------------------------------------------------------------------------                                                       
  apache2.service                      loaded    active   running The Apache HTTP Server                                            
● apparmor.service                     not-found inactive dead    apparmor.service                                                  
  apt-daily-upgrade.service            loaded    inactive dead    Daily apt upgrade and clean activities                            
  apt-daily.service                    loaded    inactive dead    Daily apt download activities                                     
● auditd.service                       not-found inactive dead    auditd.service
.
.
.

You can also list all available service units to see if they are enabled. To do so, type:

~]$ systemctl list-unit-files --type service

UNIT FILE                              STATE
-----------------------------------------------    
apache-htcacheclean.service            disabled 
apache-htcacheclean@.service           disabled 
apache2.service                        enabled  
apache2@.service                       disabled 
apt-daily-upgrade.service              static   
apt-daily.service                      static   
autovt@.service                        enabled  
bootlogd.service                       masked 
.
.
.

For each service unit, this command displays its full name (UNIT FILE) followed by information whether the service unit is enabled or not (STATE). For information on how to determine the status of individual service units, see Table 2, "Available Service Unit Information"

2.3 Displaying Service Status

To display detailed information about a service unit that corresponds to a system service, type the following at a shell prompt:

~]$ systemctl status name.service

Replace name with the name of the service unit you want to inspect (for example, apache2). This command displays the name of the selected service unit followed by its short description, one or more fields described in Table 2, "Available Service Unit Information" , and if it is executed by the root user, also the most recent log entries.

Table 2 - Available Service Unit Information

Field	Description
`Loaded`	Information whether the service unit has been loaded, the absolute path to the unit file, and a note whether the unit is enabled.
`Active`	Information whether the service unit is running followed by a time stamp.
`Main Pid`	The PID of the corresponding system service followed by its name.
`Status`	Additional information about the corresponding system service.
`Process`	Additional information about related processes.
`CGroup`	Additional information about related Control Groups (cgroups).

To only verify that a particular service unit is running, run the following command:

~]$ systemctl is-active name.service

Similarly, to determine whether a particular service unit is enabled, type:

~]$ systemctl is-enabled name.service

Note that both systemctl is-active and systemctl is-enabled return an exit status of 0 if the specified service unit is running or enabled. For information on how to list all currently loaded service units, see 2.2 Listing Services

Example Displaying Service Status

The service unit for the apache2 web server is named apache2.service. To determine the current status of this service unit, type the following at a shell prompt:

~]$ systemctl status apache2.service
● apache2.service - The Apache HTTP Server
   Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2018-01-31 14:48:36 CET; 1h 22min ago
  Process: 11914 ExecStop=/usr/sbin/apachectl stop (code=exited, status=0/SUCCESS)
  Process: 10215 ExecReload=/usr/sbin/apachectl graceful (code=exited, status=0/SUCCESS)
  Process: 11922 ExecStart=/usr/sbin/apachectl start (code=exited, status=0/SUCCESS)
 Main PID: 11926 (apache2)
    Tasks: 11 (limit: 4915)
   CGroup: /system.slice/apache2.service
           ├─11926 /usr/sbin/apache2 -k start
           ├─11946 /usr/sbin/apache2 -k start
           ├─12761 /usr/sbin/apache2 -k start
           ├─13010 /usr/sbin/apache2 -k start
           ├─13362 /usr/sbin/apache2 -k start
           ├─13393 /usr/sbin/apache2 -k start
           ├─13481 /usr/sbin/apache2 -k start
           ├─13509 /usr/sbin/apache2 -k start
           ├─13510 /usr/sbin/apache2 -k start
           ├─13527 /usr/sbin/apache2 -k start
           └─13528 /usr/sbin/apache2 -k start

Jan 31 14:48:35 websystem-test systemd[1]: Stopped The Apache HTTP Server.
Jan 31 14:48:35 websystem-test systemd[1]: Starting The Apache HTTP Server...
Jan 31 14:48:36 websystem-test systemd[1]: Started The Apache HTTP Server.

Example Displaying Services Ordered to Start Before a Service

To determine what services are ordered to start before the specified service, type the following at a shell prompt:

~]$ systemctl list-dependencies --after apache2.service
apache2.service
● ├─-.mount
● ├─system.slice
● ├─systemd-journald.socket
● ├─basic.target
● │ ├─-.mount
● │ ├─tmp.mount
● │ ├─paths.target
● │ │ ├─systemd-ask-password-console.path
● │ │ └─systemd-ask-password-wall.path
● │ ├─slices.target
● │ │ ├─-.slice
● │ │ ├─system.slice
● │ │ └─user.slice
● │ ├─sockets.target
● │ │ ├─dbus.socket
● │ │ ├─syslog.socket
● │ │ ├─systemd-initctl.socket
.
.
. And more

Example Displaying Services Ordered to Start After a Service

To determine what services are ordered to start after the specified service, type the following at a shell prompt:

~]$ systemctl list-dependencies --before apache2.service
apache2.service
● ├─multi-user.target
● │ ├─systemd-update-utmp-runlevel.service
● │ └─graphical.target
● │   └─systemd-update-utmp-runlevel.service
● └─shutdown.target

2.4 Starting a Service

To start a service unit that corresponds to a system service, type the following at a shell prompt as root:

~]$ systemctl start name.service

Replace name with the name of the service unit you want to start (for example, apache2).

2.5 Stopping a Service

To stop a service unit that corresponds to a system service, type the following at a shell prompt as root:

~]$ systemctl stop name.service

Replace name with the name of the service unit you want to stop (for example, apache2).

2.6 Restarting a Service

To restart a service unit that corresponds to a system service, type the following at a shell prompt as root:

~]$ systemctl restart name.service

Replace name with the name of the service unit you want to stop (for example, apache2).

This command stops the selected service unit in the current session and immediately starts it again.

Importantly, if the selected service unit is not running, this command starts it too. To tell systemd to restart a service unit only if the corresponding service is already running, run the following command as root:

~]$ systemctl try-restart name.service

Certain system services also allow you to reload their configuration without interrupting their execution. To do so, type as root:

~]$ systemctl reload name.service

Note that system services that do not support this feature ignore this command altogether. For convenience, the systemctl command also supports the reload-or-restart and reload-or-try-restart commands that restart such services instead. For information on how to determine the status of a certain service unit, see Section Displaying Service Status .

2.7 Enabling a service

To configure a service unit that corresponds to a system service to be automatically started at boot time, type the following at a shell prompt as root:

~]$ systemctl enable name.service

Replace name with the name of the service unit you want to enable (for example, apache2). This command reads the [Install] section of the selected service unit and creates appropriate symbolic links to the /usr/lib/systemd/system/name.service file in the /etc/systemd/system/ directory and its subdirectories.

Above command does not, however, rewrite links that already exist. If you want to ensure that the symbolic links are re-created, use the following command as root: systemctl reenable name.service

This command disables the selected service unit and immediately enables it again. For information on how to determine whether a certain service unit is enabled to start at boot time, see Section Displaying Service Status .

2.8. Disabling a Service

To prevent a service unit that corresponds to a system service from being automatically started at boot time, type the following at a shell prompt as root:

~]$ systemctl disable name.service

Replace name with the name of the service unit you want to disable (for example, postfix). This command reads the [Install] section of the selected service unit and removes appropriate symbolic links to the /usr/lib/systemd/system/name.service file from the /etc/systemd/system/ directory and its subdirectories. In addition, you can mask any service unit to prevent it from being started manually or by another service. To do so, run the following command as root:

~]$ systemctl mask name.service

This command replaces the /etc/systemd/system/name.service file with a symbolic link to /dev/null, rendering the actual unit file inaccessible to systemd. To revert this action and unmask a service unit, type as root:

~]$ systemctl unmask name.service

For prevent any service unit being started manually or by another service type as root: systemctl mask name.service. To revert this action and unmask a service unit, type as root systemctl unmask name.service

2.9 Starting a Conflicting Service

In systemd, positive and negative dependencies between services exist. Starting particular service may require starting one or more other services (positive dependency) or stopping one or more services (negative dependency).

When you attempt to start a new service, systemd resolves all dependencies automatically. Note that this is done without explicit notification to the user.

If you are already running a service, and you attempt to start another service with a negative dependency, the first service is automatically stopped.

For example, if you are running the postfix service, and you try to start the sendmail service, systemd first automatically stops postfix, because these two services are conflicting and cannot run on the same port.