If combined,
           the system manager will use unit names and description in status
           messages. When specified, overrides the system manager configuration
           file option StatusUnitFormat=, see systemd‐system.conf(5).

       systemd.log_color, systemd.log_level=, systemd.log_location,
       systemd.log_target=, systemd.log_time, systemd.log_tid
           Controls log output, with the same effect as the $SYSTEMD_LOG_COLOR,
           $SYSTEMD_LOG_LEVEL, $SYSTEMD_LOG_LOCATION, $SYSTEMD_LOG_TARGET,
           $SYSTEMD_LOG_TIME, and $SYSTEMD_LOG_TID environment variables
           described above.  systemd.log_color, systemd.log_location,
           systemd.log_time, and systemd.log_tid= can be specified without an
           argument, with the same effect as a positive boolean.

       systemd.default_standard_output=, systemd.default_standard_error=
           Controls default standard output and error output for services and
           sockets. That is, controls the default for StandardOutput= and
           StandardError= (see systemd.exec(5) for details). Takes one of
           inherit, null, tty, journal, journal+console, kmsg, kmsg+console. If
           the argument is omitted systemd.default-standard-output= defaults to
           journal and systemd.default-standard-error= to inherit.

       systemd.setenv=
           Takes a string argument in the form VARIABLE=VALUE. May be used to
           set default environment variables to add to forked child processes.
           May be used more than once to set multiple variables.

       systemd.machine_id=
           Takes a 32 character hex value to be used for setting the
           machine-id. Intended mostly for network booting where the same
           machine-id is desired for every boot.

       systemd.set_credential=
           Sets a system credential, which can then be propagated to system
           services using the LoadCredential= setting, see systemd.exec(5) for
           details. Takes a pair of credential name and value, separated by a
           colon. Note that the kernel command line is typically accessible by
           unprivileged programs in /proc/cmdline. Thus, this mechanism is not
           suitable for transferring sensitive data. Use it only for data that
           is not sensitive (e.g. public keys/certificates, rather than private
           keys), or in testing/debugging environments.

           For further information see System and Service Credentials[8]
           documentation.

       systemd.import_credentials=
           Takes a boolean argument. If false disables importing credentials
           from the kernel command line, the DMI/SMBIOS OEM string table, the
           qemu_fw_cfg subsystem or the EFI kernel stub.

       quiet
           Turn off status output at boot, much like systemd.show_status=no
           would. Note that this option is also read by the kernel itself and
           disables kernel log output. Passing this option hence turns off the
           usual output from both the system manager and the kernel.

       debug
           Turn on debugging output. This is equivalent to
           systemd.log_level=debug. Note that this option is also read by the
           kernel itself and enables kernel debug output. Passing this option
           hence turns on the debug output from both the system manager and the
           kernel.

       emergency, rd.emergency, -b
           Boot into emergency mode. This is equivalent to
           systemd.unit=emergency.target or rd.systemd.unit=emergency.target,
           respectively, and provided for compatibility reasons and to be
           easier to type.

       rescue, rd.rescue, single, s, S, 1
           Boot into rescue mode. This is equivalent to
           systemd.unit=rescue.target or rd.systemd.unit=rescue.target,
           respectively, and provided for compatibility reasons and to be
           easier to type.

       2, 3, 4, 5