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 the pager exits. Nevertheless, this
               prevents some pager functionality from working, in particular
               paged output cannot be scrolled with the mouse.

           See less(1) for more discussion.

       $SYSTEMD_LESSCHARSET
           Override the charset passed to less (by default "utf-8", if the
           invoking terminal is determined to be UTF-8 compatible).

       $SYSTEMD_PAGERSECURE
           Takes a boolean argument. When true, the "secure" mode of the pager
           is enabled; if false, disabled. If $SYSTEMD_PAGERSECURE is not set
           at all, secure mode is enabled if the effective UID is not the same
           as the owner of the login session, see geteuid(2) and
           sd_pid_get_owner_uid(3). In secure mode, LESSSECURE=1 will be set
           when invoking the pager, and the pager shall disable commands that
           open or create new files or start new subprocesses. When
           $SYSTEMD_PAGERSECURE is not set at all, pagers which are not known
           to implement secure mode will not be used. (Currently only less(1)
           implements secure mode.)

           Note: when commands are invoked with elevated privileges, for
           example under sudo(8) or pkexec(1), care must be taken to ensure
           that unintended interactive features are not enabled. "Secure" mode
           for the pager may be enabled automatically as describe above.
           Setting SYSTEMD_PAGERSECURE=0 or not removing it from the inherited
           environment allows the user to invoke arbitrary commands. Note that
           if the $SYSTEMD_PAGER or $PAGER variables are to be honoured,
           $SYSTEMD_PAGERSECURE must be set too. It might be reasonable to
           completely disable the pager using --no-pager instead.

       $SYSTEMD_COLORS
           Takes a boolean argument. When true, systemd and related utilities
           will use colors in their output, otherwise the output will be
           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.