$ cat modem.sh
#!/bin/bash
function get_response
{
        local ECHO
        # cat will read the response, then die on timeout
        cat <&5 >$TMP &
        echo "$1" >&5
        # wait for cat to die
        wait $!
        # Check if the modem echoed our command
        read ECHO <$TMP
        [ "$ECHO" != "$1" ] && return 1
        return 0
}

TMP="./response"

# Clear out old response
: > $TMP

# Set modem with timeout of 5/10 a second.  Things reading will read until
# data stops, wait 1/2 second, then quit.
stty -F /dev/ttyS0 9600 -echo igncr -icanon onlcr ixon min 0 time 5

# Open modem on FD 5
exec 5<>/dev/ttyS0

get_response "AT+CGMI" || echo "Bad response"
echo "Response was:"    ;       cat $TMP

get_response "AT" || echo "Bad response"
echo "Response was:"    ;       cat $TMP

exec 5<&-

$ ./modem.sh
Response was:
AT+CGMI

ERROR
Response was:
AT

OK
$

#!/bin/bash

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

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

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

TMP="./response"

# Clear out old response
: > $TMP

# Set modem with timeout of 5/10 a second
stty -F /dev/ttyS0 9600 -echo igncr -icanon onlcr ixon min 0 time 5

# Open modem on FD 5
exec 5<>/dev/ttyS0

get_response "AT+CGMI" || echo "Bad response"
echo "Response was '${RESPONSE}'"       ;       cat $TMP

echo

get_response "AT" || echo "Bad response"
echo "Response was '${RESPONSE}'"       ;       cat $TMP

exec 5<&-

$ ./modem.sh
Response was 'ERROR'
AT+CGMI

ERROR

Response was 'OK'
AT

OK
$