How to modify input stream of netcat?

Question

I'm creating a TCP/IP interface to a serial device on a redhat linux machine. netcat in a bash script was used to accomplish this with out to much trouble.

nc -l $PORT < $TTYDEVICE > $TTYDEVICE

The problem is that the serial device uses carriage returns('\r') for line ends in its responses. I want to translate this to ("\r\n") so windows machines telneting in can view the response without any trouble. I'm trying to figure out how to go about this with a simple bash solution. I also have access to stty to configure the serial device, but there is no "\r" to "\r\n" translate on the input side(from what I can tell).

I did try to use tr on the input side of netcat, but it didn't work.

#cat $TTYDEVICE | tr '\r' '\r\n' | nc -l $PORT > $TTYDEVICE

Any ideas?

`tr` is useful for character-to-character translation, but it will not translate one character into multiple characters. (Specifying a longer target string simply causes the excess characters to be ignored. Perhaps `tr` should display a warning in this case.). Use e.g. Perl instead. — tripleee, Jan 27 '12 at 16:32

score 2 · Answer 1 · answered Jan 27 '12 at 16:25

Your problem is that the client that connects to $PORT probably does not have a clue that it is working with a tty on the other side, so you will experience issues with tty-specific "features", such as ^C/^D/etc. and CRLF.

That is why

socat tcp-listen:1234 - | hexdump -C
telnet localhost 1234
[enter text]

will show CRLFs, while

ssh -t localhost "hexdump -C"
[enter text]

yields pure LFs. Subsequently, you would e.g. need

ssh -t whateverhost "screen $TTYDEVICE"

tl;dr: netcat won't do it.

This is true, but I don't think my application need to many control characters. — richmb, Jan 27 '12 at 19:16

score 1 · Answer 2 · answered Jan 27 '12 at 14:59

1

There are a number of versions of netcat (GNU and BSD) but you might want to try:

 -C      Send CRLF as line-ending

answered Jan 27 '12 at 14:59

stsquad

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just tried it, it just adds newlines to to output of netcat. I need to add them on the input stream side. – richmb Jan 27 '12 at 15:32

William Pursell · Accepted Answer · 2012-01-27T15:10:49.683

1

This is overly difficult with standard tools, but pretty easy in perl (although perl is pretty much a standard tool these days):

perl -pe 's/\r/\r\n/g'

The version above will likely read the entire input into memory before doing any processing (it will read until it finds '\n', which will be the entire input if the input does not contain '\n'), so you might prefer:

perl -015 -pe '$\="\n"'

edited Jan 27 '12 at 15:10

answered Jan 27 '12 at 14:59

William Pursell

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sorry my perl is a little rusty. so are you suggesting I do something like this. cat $TTYDEVICE | perl -015 -pe '$\="\n"' | nc -l $PORT > $TTYDEVICE – richmb Jan 27 '12 at 15:39
well..it certainly works on a regular file. but its acting the same way the tr command did and i'm not getting a response back. – richmb Jan 27 '12 at 15:55
1

Probably a buffering issue. Try adding $| = 1 to enable auto flushing. – William Pursell Jan 27 '12 at 16:00
To clarify, do: perl -015 -pe 'BEGIN{ $\="\n"; $|=1 }' – William Pursell Jan 27 '12 at 16:01

How to modify input stream of netcat?

3 Answers3