Some of the variables understood by systemd:

       $SYSTEMD_LOG_LEVEL
           The maximum log level of emitted messages (messages with a higher
           log level, i.e. less important ones, will be suppressed). Either one
           of (in order of decreasing importance) emerg, alert, crit, err,
           warning, notice, info, debug, or an integer in the range 0...7. See
           syslog(3) for more information.

           This can be overridden with --log-level=.

       $SYSTEMD_LOG_COLOR
           A boolean. If true, messages written to the tty will be colored
           according to priority.

           This can be overridden with --log-color=.

       $SYSTEMD_LOG_TIME
           A boolean. If true, console log messages will be prefixed with a
           timestamp.

           This can be overridden with --log-time=.

       $SYSTEMD_LOG_LOCATION
           A boolean. If true, messages will be prefixed with a filename and
           line number in the source code where the message originates.

           This can be overridden with --log-location=.

       $SYSTEMD_LOG_TID
           A boolean. If true, messages will be prefixed with the current
           numerical thread ID (TID).

       $SYSTEMD_LOG_TARGET
           The destination for log messages. One of console (log to the
           attached tty), console-prefixed (log to the attached tty but with
           prefixes encoding the log level and "facility", see syslog(3), kmsg
           (log to the kernel circular log buffer), journal (log to the
           journal), journal-or-kmsg (log to the journal if available, and to
           kmsg otherwise), auto (determine the appropriate log target
           automatically, the default), null (disable log output).

           This can be overridden with --log-target=.

       $XDG_CONFIG_HOME, $XDG_CONFIG_DIRS, $XDG_DATA_HOME, $XDG_DATA_DIRS
           The systemd user manager uses these variables in accordance to the
           XDG Base Directory specification[6] to find its configuration.

       $SYSTEMD_UNIT_PATH, $SYSTEMD_GENERATOR_PATH,
       $SYSTEMD_ENVIRONMENT_GENERATOR_PATH
           Controls where systemd looks for unit files and generators.

           These variables may contain a list of paths, separated by colons
           (":"). When set, if the list ends with an empty component ("...:"),
           this list is prepended to the usual set of paths. Otherwise, the
           specified list replaces the usual set of paths.

       $SYSTEMD_PAGER
           Pager to use when --no-pager is not given; overrides $PAGER. If
           neither $SYSTEMD_PAGER nor $PAGER are set, a set of well-known pager
           implementations are tried in turn, including less(1) and more(1),
           until one is found. If no pager implementation is discovered no
           pager is invoked. Setting this environment variable to an empty
           string or the value "cat" is equivalent to passing --no-pager.

           Note: if $SYSTEMD_PAGERSECURE is not set, $SYSTEMD_PAGER (as well as
           $PAGER) will be silently ignored.

       $SYSTEMD_LESS
           Override the options passed to less (by default "FRSXMK").

           Users might want to change two options in particular:

           K
               This option instructs the pager to exit immediately when Ctrl+C
               is pressed. To allow less to handle Ctrl+C itself to switch back
               to the pager command prompt, unset this option.

               If the value of $SYSTEMD_LESS does not include "K", and the
               pager that is invoked is less, Ctrl+C will be ignored by the
               executable, and needs to be handled by the pager.

           X
               This option instructs the pager to not send termcap
               initialization and deinitialization strings to the terminal. It
               is set by default to allow command output to remain visible in
               the terminal even after