Systemd Summary
================


* Systemd Units: https://man7.org/linux/man-pages/man5/systemd.unit.5.html
* ``systemctl``: https://man7.org/linux/man-pages/man1/systemctl.1.html



Systemd User mk units:
    ~/.config/systemd/user/*


Systemd Programs:
    * ``systemctl``
    * ``loginctl``


User Linger:
    This means that a systemd user service is going to be started upon reboot
    Even if user is not logged in yet. This is enabled by ``loginctl enable-linger mk``




Some useful snippets

.. code-block:: bash

    systemctl --user list-units --type=target

    systemctl list-unit-files

    systemctl --user list-unit-files

    sudo systemctl daemon-reload

    systemctl --user start nexus
    systemctl --user enable nexus
    systemctl --user disable nexus
    



Systemd User Unit base Template::

    # man pages in webpage
    # https://man7.org/linux/man-pages/man5/systemd.service.5.html
    # https://man7.org/linux/man-pages/man5/systemd.unit.5.html
    # https://www.freedesktop.org/software/systemd/man/systemd.directives.html

    [Unit]
    Description=Redis Lastest (6.2.6) Server

    After=network-online.target
    # Before=

    # weak dependency
    Wants=network-online.target systemd-networkd-wait-online.service

    # stronger dependency on other units
    # Requires=

    # will fail unless the listed unit(s) are already started
    # can be conbined with 'After='
    # Requisite=

    [Service]
    # https://www.freedesktop.org/software/systemd/man/systemd.service.html#Type=
    # In general, most services will be good with simple
    Type=simple

    Environment="SHELL=/bin/bash" "BASH_ENV=/bits/_env.sh"

    WorkingDirectory=/bits/data/redis

    # This relies on the BASH_ENV environment file
    ExecStart=/bin/bash -c "/zbin/redis/src/redis-server /bits/stargate/common/red/redis.conf"

    # https://www.freedesktop.org/software/systemd/man/systemd.service.html#Restart=
    # Restart=always
    Restart=on-failure
    RestartSec=10

    [Install]
    # this is primarily used when the service is "enabled" and "disabled" by
    # systemctl --user enable service

    # default target is a symlink to graphical.target on desktop
    # or multi-user.target on a server
    WantedBy=default.target

    # WantedBy=graphical.target
    # WantedBy=multi-user.target

Systemd Unit with auto recovery::

    [Unit]
    Description=Your Daemon
    After=network-online.target
    Wants=network-online.target systemd-networkd-wait-online.service

    StartLimitIntervalSec=500
    StartLimitBurst=5

    [Service]
    Restart=on-failure
    RestartSec=5s

    ExecStart=/path/to/daemon

    [Install]
    WantedBy=multi-user.target

Targets
-------

achieved when the system reached certain states, or run-levels::

    Run Lvl Target Units                        Description
    0       runlevel0.target, poweroff.target   Shut down and power off
    1       runlevel1.target, rescue.target     Set up a rescue shell
    2,3,4   runlevel[234].target,               Set up a non-gfx multi-user shell
            multi-user.target
    5       runlevel5.target, graphical.target  Set up a gfx multi-user shell
    6       runlevel6.target, reboot.target     Shut down and reboot the system

The ``default.target`` is a sym link to one of the targets above. Use this to get the default

``systemctl get-default``

On a desktop, that would be the ``graphical.target``
On a server, that would be ``multi-user.target``

https://www.landoflinux.com/linux_runlevels_systemd.html

* ``network-online.target`` When the system have gone online (IPs established, DHCP, etc)


.. code-block:: bash

    systemctl list-dependencies network-online.target
    systemctl list-dependencies network.target
    systemctl list-units | grep network