If you have the Mirth application already listening on port 10004 then you can't really have netcat also listening on that same port (the -l flag denotes listen). Perhaps you intended to write to the port rather than listen on it:

# Connect once and write many messages
( while :; do echo MSH....; sleep 3; done ) | nc -vvv localhost 10004

or

# Connect for each message
( while :; do echo MSH.... | nc -vvv localhost 10004; sleep 3; done )

Be aware that there are several different implementations of netcat. Every implementation seems to handle a different subset of flags...

Based on the new information in your question, here's another possible solution:

MESG=
while read -r LINE || test -n "$MESG"
do
    if test -n "$LINE"
    then
        # Build the message block
        test -z "$MESG" && MESG="$LINE" || MESG=$(printf "%s\n%s" "$MESG" "$LINE")
        continue
    fi

    # Send the message block to the service
    echo "$MESG" | nc -vvv localhost 10004
    MESG=
    sleep 3
done < /path/to/your/messagefile.txt

If you have the Mirth application already listening on port 10004 then you can't really have netcat also listening on that same port (the -l flag denotes listen). Perhaps you intended to write to the port rather than listen on it:

# Connect once and write many messages
( while :; do echo MSH....; sleep 3; done ) | nc -vvv localhost 10004

or

# Connect for each message
( while :; do echo MSH.... | nc -vvv localhost 10004; sleep 3; done )

Be aware that there are several different implementations of netcat. Every implementation seems to handle a different subset of flags...

Based on the new information in your question, here's another possible solution:

MESG=
while IFS= read -r LINE || test -n "$MESG"
do
    if test -n "$LINE"
    then
        # Build the message block
        test -z "$MESG" && MESG="$LINE" || MESG=$(printf "%s\n%s" "$MESG" "$LINE")
        continue
    fi

    # Send the message block to the service
    echo "$MESG" | nc -vvv localhost 10004
    MESG=
    sleep 3
done < /path/to/your/messagefile.txt

If you have the Mirth application already listening on port 10004 then you can't really have netcat also listening on that same port (the -l flag denotes listen). Perhaps you intended to write to the port rather than listen on it:

# Connect once and write many messages
( while :; do echo MSH....; sleep 3; done ) | nc -vvv localhost 10004

or

# Connect for each message
( while :; do echo MSH.... | nc -vvv localhost 10004; sleep 3; done )

Be aware that there are several different implementations of netcat. Every implementation seems to handle a different subset of flags...

If you have the Mirth application already listening on port 10004 then you can't really have netcat also listening on that same port (the -l flag denotes listen). Perhaps you intended to write to the port rather than listen on it:

# Connect once and write many messages
( while :; do echo MSH....; sleep 3; done ) | nc -vvv localhost 10004

or

# Connect for each message
( while :; do echo MSH.... | nc -vvv localhost 10004; sleep 3; done )

Be aware that there are several different implementations of netcat. Every implementation seems to handle a different subset of flags...

Based on the new information in your question, here's another possible solution:

MESG=
while read -r LINE || test -n "$MESG"
do
    if test -n "$LINE"
    then
        # Build the message block
        test -z "$MESG" && MESG="$LINE" || MESG=$(printf "%s\n%s" "$MESG" "$LINE")
        continue
    fi

    # Send the message block to the service
    echo "$MESG" | nc -vvv localhost 10004
    MESG=
    sleep 3
done < /path/to/your/messagefile.txt