monochrome. Additionally, the variable can take one of the following
           special values: "16", "256" to restrict the use of colors to the
           base 16 or 256 ANSI colors, respectively. This can be specified to
           override the automatic decision based on $TERM and what the console
           is connected to.

       $SYSTEMD_URLIFY
           The value must be a boolean. Controls whether clickable links should
           be generated in the output for terminal emulators supporting this.
           This can be specified to override the decision that systemd makes
           based on $TERM and other conditions.

       $LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES
           Set by systemd for supervised processes during socket-based
           activation. See sd_listen_fds(3) for more information.

       $NOTIFY_SOCKET
           Set by systemd for supervised processes for status and start-up
           completion notification. See sd_notify(3) for more information.

       For further environment variables understood by systemd and its various
       components, see Known Environment Variables[7].

KERNEL COMMAND LINE
       When run as the system instance, systemd parses a number of options
       listed below. They can be specified as kernel command line arguments
       which are parsed from a number of sources depending on the environment
       in which systemd is executed. If run inside a Linux container, these
       options are parsed from the command line arguments passed to systemd
       itself, next to any of the command line options listed in the Options
       section above. If run outside of Linux containers, these arguments are
       parsed from /proc/cmdline and from the "SystemdOptions" EFI variable (on
       EFI systems) instead. Options from /proc/cmdline have higher priority.
       The following variables are understood:

       systemd.unit=, rd.systemd.unit=
           Overrides the unit to activate on boot. Defaults to default.target.
           This may be used to temporarily boot into a different boot unit, for
           example rescue.target or emergency.service. See systemd.special(7)
           for details about these units. The option prefixed with "rd."  is
           honored only in the initrd, while the one that is not prefixed only
           in the main system.

       systemd.dump_core
           Takes a boolean argument or enables the option if specified without
           an argument. If enabled, the systemd manager (PID 1) dumps core when
           it crashes. Otherwise, no core dump is created. Defaults to enabled.

       systemd.crash_chvt
           Takes a positive integer, or a boolean argument. Can be also
           specified without an argument, with the same effect as a positive
           boolean. If a positive integer (in the range 1–63) is specified, the
           system manager (PID 1) will activate the specified virtual terminal
           when it crashes. Defaults to disabled, meaning that no such switch
           is attempted. If set to enabled, the virtual terminal the kernel
           messages are written to is used instead.

       systemd.crash_shell
           Takes a boolean argument or enables the option if specified without
           an argument. If enabled, the system manager (PID 1) spawns a shell
           when it crashes, after a 10s delay. Otherwise, no shell is spawned.
           Defaults to disabled, for security reasons, as the shell is not
           protected by password authentication.

       systemd.crash_reboot
           Takes a boolean argument or enables the option if specified without
           an argument. If enabled, the system manager (PID 1) will reboot the
           machine automatically when it crashes, after a 10s delay. Otherwise,
           the system will hang indefinitely. Defaults to disabled, in order to
           avoid a reboot loop. If combined with systemd.crash_shell, the
           system is rebooted