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Created January 16, 2021 15:09
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Systemd cheatsheet
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Linux - Systemd cheatsheet.md

Linux - Systemd cheatsheet

systemctl

Activates a service immediately:

systemctl start foo.service

Deactivates a service immediately:

systemctl stop foo.service

Restarts a service:

systemctl restart foo.service

Shows status of a service including whether it is running or not:

systemctl status foo.service

Enables a service to be started on bootup:

systemctl enable foo.service

Disables a service to not start during bootup:

systemctl disable foo.service

Check whether a service is already enabled or not:

0 indicates that it is enabled. 1 indicates that it is disabled

systemctl is-enabled foo.service; echo $?

Change ad-hoc runlevel with systemctl isolate command:

Switch to another target (in this case multi-user/runlevel 3 in old SysV):

systemctl isolate multi-user.target

Switch to graphical target (in this case graphical/runlevel 5 in old SysV):

systemctl isolate graphical.target

Change permanently change default.target:

Remove configured default target

rm /etc/systemd/system/default.target

Create default.target as symbolic link to multi-user.target

ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target

Set default target through systemctl

To find a [new target]:

- To get active targets, 
  systemctl --type=target list-units
- To get all targets, (active/inactive)
  systemctl --type=target --all list-units
- To list available targets, (ls -l /usr/lib/systemd/system/*.target)
  systemctl --type=target list-unit-files


systemctl set-default [new target]

List-units by pattern:

systemctl list-units *etwok*.service

Example content:
UNIT                   LOAD   ACTIVE SUB     DESCRIPTION
network.service        loaded active exited  LSB: Bring up/down networking
NetworkManager.service loaded active running Network Manager

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.

2 loaded units listed. Pass --all to see loaded but inactive units, too.

Display a content of unit file

systemctl cat network.service

Displayed content:

# /run/systemd/generator.late/network.service
# Automatically generated by systemd-sysv-generator
[Unit]
Documentation=man:systemd-sysv-generator(8)
SourcePath=/etc/rc.d/init.d/network
Description=LSB: Bring up/down networking
Before=runlevel2.target runlevel3.target runlevel4.target runlevel5.target shutdown.target network-online.target network.target
After=iptables.service ip6tables.service NetworkManager-wait-online.service NetworkManager.service
Wants=network-online.target
Conflicts=shutdown.target
[Service]
Type=forking
Restart=no
TimeoutSec=5min
IgnoreSIGPIPE=no
KillMode=process
GuessMainPID=no
RemainAfterExit=yes
ExecStart=/etc/rc.d/init.d/network start
ExecStop=/etc/rc.d/init.d/network stop
ExecReload=/etc/rc.d/init.d/network reload

Suppress non zero exit code (service after stop will be displayed as inactive, not failed):

[Service]
SuccessExitStatus=143

Link:

How to put PID of service as variable to service systemd file:

ExecReload=/bin/kill -s HUP $MAINPID

Link

List all services with pattern

<pattern> - httpd e.g

systemctl list-units -t service --full| cut -d' ' -f1 | grep <pattern> | while read s; do systemctl status $s; done

Link

Reference links:

Pid Eins:

Digital Ocean:

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