#!/bin/bash

MODEM=$2
TMP=$(mktemp)
trap 'rm -f $TMP' EXIT

function get_response
{
        local ECHO
        # cat will read the response, then die on timeout
        cat <&5 >$TMP &
        echo -e "$1\r" >&5
        # wait for cat to die
        wait $!

        exec 6<$TMP
        read ECHO <&6
        if [ "$ECHO" != "$1" ]
        then
                exec 6<&-
                return 1
        fi

        read RESPONSE <&6
        exec 6<&-
        return 0
}

RESPONSE=""

# Set modem with timeout of 5/10 a second
stty -F "$MODEM" 9600 -echo igncr -icanon onlcr ixon min 0 time 8

# Open modem on FD 5
exec 5<>"$MODEM"

get_response "$1" || echo "Bad response"
echo ${RESPONSE}
exec 5<&-

# sh at.sh PSTMNMEAONOFF /dev/ttyS1
at.sh: line 7: function: not found
at.sh: line 27: 5: Bad file descriptor
at.sh: line 27: 5: Bad file descriptor