• Validate Systemd Service File
• List Running Services
• Editing Service Config
• Tracing
• Enabling Services

How to validate systemd service file
Check or lint a service for syntax errors!
	sudo systemd-analyze verify foobar.service




List Running Services

In order to visualize the running services in our system, we can execute
 the following command:
	$ systemctl --type=service

Which gives us (cut off for just diaplaying purpose):


  UNIT                                      LOAD   ACTIVE SUB    
 DESCRIPTION        
  alsa-restore.service                      loaded active exited 
 Save/Restore Sound Card State
  bolt.service                              loaded active running
 Thunderbolt system service
  dbus.service                              loaded active running D-Bus
 System Message Bus
  iio-sensor-proxy.service                  loaded active running IIO
 Sensor Proxy service
  NetworkManager.service                    loaded active running Network
 Manager
  sddm.service                              loaded active running Simple
 Desktop Display Manager
  systemd-journald.service                  loaded active running Journal
 Service
  systemd-logind.service                    loaded active running User
 Login Management
  user@1000.service                         loaded active running User
 Manager for UID 1000
  wpa_supplicant.service                    loaded active running WPA
 supplicant
  ...

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.
36 loaded units listed. Pass --all to see loaded but inactive units, too.
We also get a piece of help if we want to see the installed ones:
	$ systemctl list-unit-files




Editing service configuration
Individually, per service, we can edit its configuration. In this example we 
are using NetworkManager.service which is one of the main ones for handling
our network connection. So let’s see its configuration:
	$ sudo systemctl edit --full NetworkManager.service

And the output:
	[Unit]
Description=Network Manager
Documentation=man:NetworkManager(8)
Wants=network.target
After=network-pre.target dbus.service
Before=network.target 

[Service]
Type=dbus
BusName=org.freedesktop.NetworkManager
#ExecReload=/bin/kill -HUP $MAINPID
ExecStart=/usr/bin/NetworkManager --no-daemon
Restart=on-failure
# NM doesn't want systemd to kill its children for it
KillMode=process

ProtectSystem=true
ProtectHome=read-only

# We require file descriptors for DHCP etc. When activating many interfaces,
# the default limit of 1024 is easily reached.
LimitNOFILE=65536

[Install]
WantedBy=multi-user.target
Also=NetworkManager-dispatcher.service

# We want to enable NetworkManager-wait-online.service whenever this service
# is enabled. NetworkManager-wait-online.service has
# WantedBy=network-online.target, so enabling it only has an effect if
# network-online.target itself is enabled or pulled in by some other unit.
Also=NetworkManager-wait-online.service



#Enabling Services
After modifying a service file, execute:
sudo systemctl daemon-reload      # Informs daemong services have changed
sudo systemctl start  xyz.service   # Start new (or reconfigged) service



Tracing
It turns out that systemd services have a list of permitted system calls.  If a 
service attempts a system call that isn't permitted, the process is terminated 
with a SIGSYS signal.

To tell systemd to allow the process to execute any system call, you need to 
update the .service file and add the following under the [Service] section:
	SystemCallFilter=@known

Then, just do a 
	systemctl daemon-reload followed by 
systemctl start pulseaudio.service
(omit --user for system services) and it should start right up.

For more information on the SystemCallFilter parameter, refer to 
man 5 systemd.exec.