This gist is a notes on systemd which I have noted down while reading a online documnet https://n0where.net/understanding-systemd/ and through man pages.Will keep updating it.
socket-based activation bus-based activation path-based activation device-based activation implicit dependency mapping instances and templates easy security hardening drop-ins and snippets - Override the Vanila units.
/lib/systemd/system/ - The systemd unit files are stored.
/etc/systemd/system/ - overrides above services if the same service name.
/run/systemd/system/ - runtime units.
units:
.service - for daemons and applications. (man systemd.service)
.socket - for socket activation, encodes info about IPC or network socket. (man systemd.socket)
.device - for device unit configuration. This may be used to Configures dependency between device and other units. (man systemd.device)
.mount - encodes info about file system mount points. (man systemd.mount)
.automount - encodes info about file system automount points. (man systemd.automount)
.swap - encodes info about swap device and file for memory paging. (man systemd.swap)
.target - system states. (man systemd.target)
.path - path based activation 'inotify'. (man systemd.path)
.timer - manage jobs with time. cron type. (man systemd.timer)
.snapshot - system current state snapshot. (man systemd.snapshot)
.slice - Resource isolation with cgroups for users. (man systemd.slice)
.scope - not configured as a unit file, but creates programatically. (man systemd.scope)
UNIT File structure:
Description=: Stting which describes the unit.
Documentation=: URL reference of documentation of the unit.
Requires=: This directive lists any units upon which this unit essentially depends. If the current unit is activated, the units listed here must successfully activate as well, else this unit will fail. These units are started in parallel with the current unit by default.
Wants=: This directive is similar to Requires=, but less strict. Systemd will attempt to start any units listed here when this unit is activated. If these units are not found or fail to start, the current unit will continue to function.
BindsTo=: This directive is similar to Requires=, but also causes the current unit to stop when the associated unit terminates.
Before=: The units listed in this directive will not be started until the current unit is marked as started if they are activated at the same time.
After=: The units listed in this directive will be started before starting the current unit. This does not imply a dependency relationship and one must be established through the above directives if this is required.
Conflicts=: This can be used to list units that cannot be run at the same time as the current unit.
Condition=: Before starting a unit verify that the specified condition is true. There are many conditions, please check man systemd.unit Before starting a unit verify that the specified condition is true. There are many conditions, please check man systemd.unit Example: ConditionPathIsReadWrite, ConditionFileIsExecutable etc.
Only units that can be enabled will have this section.
WantedBy=:
RequiredBya=:
Alias=: A list of additional names for a unit.
Also=:
DefaultInstance=:
The [Service] section is used to provide configuration that is only applicable for services.
Type= directive:
simple: exestart is the main process.
forking: exestart is the parent process which might exit by launching child process.
oneshot: wait till comeples this process, its very short term ;).
dbus: wait untill the bus name is created.
notify: notifies systemd after started successfully.
idle: This indicates that the service will not be run until all jobs are dispatched.
RemainAfterExit=:
PIDFile=:
BusName=:
NotifyAccess=:
ExecStart=: full path to the binary which needs to be executed.
ExecStartPre=: path to the app, to run before the main process.
ExecStartPost=: path to the app, to run after the main process created.
ExecReload=: path to the app, to reload the service.
ExecStop=: path to the app, to stop the process.
ExecStopPost=: execute after stop.
RestartSec=: time to wait before restarting the process.
Restart=: restart the process on event.
TimeoutSec=: time of waiting to declare the process is failed to start/stop.
Common directives:
ListenStream=: stream socket address.
ListenDatagram=: datagram socket address.
ListenSequentialPacket=: sequential, reliable communication with max length datagrams that preserves message boundaries
ListenFIFO:
Additional Directives:
Accept=: to control creating instances for each connections.
SocketUser=: root user if left unset.
SocketGroup=: group owner of the socket, root if unset.
SocketMode=: permissions
What=: Takes an absolute path of a device node
Where=: The absolute path of the mount point where the resource should be mounted.
Type=: Filesystem type.
Options=: Any mount options that need to be applied. This is a comma-separated list.
SloppyOptions=:
DirectoryMode=:
TimeoutSec=: Configures the amount of time the system will wait until the mount operation is marked as failed.
Where=:
DirectoryMode=:
What=: Absolute path to the location of swap space.
Priority=:
Options=: options sets in /etc/fstab
TimeoutSec=:
PathExists=:
PathExistsGlob=:
PathChanged=:
PathModified=:
DirectoryNotEmpty=:
Unit=:
MakeDirectory=:
DirectoryMode=:
onActiveSec=:
OnBootSec=:
OnStartupSec=:
OnUnitActiveSec=:
OnUnitInactiveSec=:
OnCalendar=:
AccuracySec=:
Unit=: This directive is used to specify the unit that should be activated when the timer elapses. If unset, systemd will look for a .service unit with a name that matches this unit.
Persistent=:
Template Specifiers:
%n: Full resulting unit name.
%N:
%p: Unit name prefix.
%P:
%i: This references the instance name, which is the identifier following the @ in the instance unit.
%I:
%f:
%c: Control group of the unit.
%u: Name of the user configured to run the unit.
%U: Name of the user, but as UID - numeric.
%H: Hostname of the running system.
%%: To insert the literal percentage.
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systemctl start
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systemctl stop
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systemctl restart
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systemctl reload
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systemctl enable
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systemctl disable
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systemctl list-units
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systemctl list-units --all
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systemctl list-unit-files
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journalctl
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journalctl -b #Current boot log.
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journalctl -k #Kernel messages.
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systemctl status
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journalctl -u
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systemctl cat
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systemctl list-unit-files --type=target
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systemctl get-default
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systemctl set-default multi-user.target
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systemctl list-dependencies multi-user.target
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systemctl isolate multi-user.target
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systemctl show sshd.service -p Conflicts
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systemctl poweroff #poweroff the Server
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systemctl reboot #reboot the system.
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systemctl rescue #boot to rescue mode.
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systemctl start application.service
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systemctl start applications
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systemctl stop application.service
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systemctl restart application.service
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systemctl reload application.service
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systemctl reload-or-restart application.service
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systemctl enable application.service
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systemctl disable application.service
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systemctl status application.service
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systemctl is-active application.service
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systemctl is-enabled application.service
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systemctl is-failed application.service
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systemctl list-units --all --state=inactive
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systemctl mask application.service
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systemctl unmask application.service
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journalctl --list-boots
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journalctl --since yesterday
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journalctl _PID=<PID_NUMBER>
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man systemd.journal-fields
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journalctl -F _PID
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journalctl /usr/bin/bash
[Priority]
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journalctl -p err -b
0: emerg 1: alert 2: crit 3: err 4: warning 5: notice 6: info 7: debug
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journalclt --no-pager #output to stdout
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journalctl -b -u shhd -o json
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journalctl -n #display last 10 lines.
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journalctl -n 20 #display last 20 lines.
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journalctl -f #following logs.
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journalctl --disk-usage
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journalctl --vacuum-size=1G
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journalctl --vacuum-time=1years