systemd-activate — Test socket activation of daemons
/usr/lib/systemd/systemd-activate  [OPTIONS...]  daemon  [OPTIONS...]
systemd-activate can be used to launch a socket activated daemon from the command-line for testing purposes. It can also be used to launch single instances of the daemon per connection (inetd-style).
The daemon to launch and its options should be specifed after options intended for systemd-activate.
If the -a option is given, file descriptor
    of the connection will be used as the standard input and output of
    the launched process. Otherwise, standard input and output will be
    inherited, and sockets will be passed through file descriptors 3
    and higher. Sockets passed through $LISTEN_FDS
    to systemd-activate will be passed through to
    the dameon, in the original positions. Other sockets specified
    with --listen will use consecutive descriptors.
    
--help, -h¶Prints a short help text and exits.
--version¶Prints a short version string and exits.
-l address, --listen=address¶Listen on this address.
        Takes a string like 2000 or
        127.0.0.1:2001.
-a, --accept¶Launch a separate instance of daemon per connection and pass the connection socket as standard input and standard output.
-E VAR[=VALUE], --environment=VAR[=VALUE]¶Add this variable to the environment of the
        launched process. If VAR is
        followed by = assume that it is a
        variable–value pair. Otherwise obtain the value from the
        environment of systemd-activate itself.
        
$LISTEN_FDS, $LISTEN_PID¶See sd_listen_fds(3).
$SYSTEMD_LOG_TARGET, $SYSTEMD_LOG_LEVEL, $SYSTEMD_LOG_COLOR, $SYSTEMD_LOG_LOCATION¶Same as in systemd(1).
$ /usr/lib/systemd/systemd-activate -l 19531 /usr/lib/systemd/systemd-journal-gatewayd
This runs a socket activated instance of systemd-journal-gatewayd(8).