/etc/init.d/etc-setserial is in setserial 2.17-46.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 | #! /bin/sh
#
### BEGIN INIT INFO
# Provides:             etc-setserial
# Required-Start:       checkfs
# Required-Stop:        $remote_fs
# Default-Start:        S
# Default-Stop:         0 1 6
# Short-Description:    controls configuration of serial ports
# Description:          Set and/or report the configuration information
#                       associated with a serial port. This information
#                       includes what I/O port and which IRQ a particular
#                       serial port is using.
### END INIT INFO
# This file uses settings from /etc/serial.conf to configure the serial devices.
# This file only exists if you chose to configure the serial devices
# by hand yourself. Otherwise serial configuration happens later on
# using the /etc/init.d/setserial file.
#
SETSERIAL=/bin/setserial
# If the serial executable has been removed abort the configuration
[ -x ${SETSERIAL} ] || exit 0
#
# Support devfs when it arrives.
#
if /bin/ls /dev/tts 2> /dev/null 1>&2 ; then
	ALLDEVS="/dev/tts/*"
else
	# No devfs - old naming scheme
	ALLDEVS="/dev/ttyS?"
	if /bin/ls /dev/ttyS?? 2> /dev/null 1>&2 ; then
		ALLDEVS="$ALLDEVS /dev/ttyS??"
	fi
fi
#
# Handle System V init conventions...
#
case $1 in
start | restart | force-reload )
	action="start";
	;;
stop)
	action="stop";
	;;
modload)
	action="modload";
	;;
modsave)
	action="modsave";
	;;
*)
	action="start";
esac
#
# Is it Start
#
if test $action  = start ; then
  if test -f /etc/serial.conf ; then
        echo "Loading the saved-state of the serial devices from /etc/serial.conf "
        while read device args
        do
               case "$device" in
                   ""|\#*)
                       continue
                       ;;
               esac
           ${SETSERIAL} -z $device $args
           ${SETSERIAL} -bg $device
        done < /etc/serial.conf
  fi
fi
 |