Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

So I have a standard RS232 serial port that is looped back to itself by simply running a wire from Tx to Rx. I'm testing loopback by running echo and cat in two separate terminals:

cat /dev/ttyS1
echo "hi" > /dev/ttyS1

My issue is with the output. I would expect to see one "hi" come back on the terminal running cat but instead I get this:

[2 newlines]
[4 newlines]
[8 newlines]
[16 newlines]
[32 newlines]

...and so on until I ctrl+c cat.

After interrupting cat, if I run it again it will not output "hi"s until I run echo a second time.

Is this normal? Any idea why I'm seeing this behavior?

Edit: By newline, I mean ASCII 0x0A. There are no carriage returns in this output.

share|improve this question
Could it be caused by having two processes opening the same device? What if you run tip /dev/ttyS1 (~. to exit) and tried typing data there? It should be displayed in your terminal when the wire is connected, as it receives what it has transmitted. – mrb Jul 13 '12 at 14:41
Are you really getting newlines, or a carriage-return/newline pair? The distinction is important at the level you're working. Try "cat /dev/ttyS1 > somefile" then do "od -x somefile" to see exactly what bytes are coming out of the TTY device file. Also, do an "stty -F /dev/ttyS1 -a". Read the man page for "stty" and look at what the output of stty tells you, for every little setting. RS232 serial comms are tricky. – Bruce Ediger Jul 13 '12 at 15:26
Thanks Bruce -- I figured it out based on your help! – Ryan Jul 13 '12 at 17:17
up vote 8 down vote accepted

Thanks to the second comment by Bruce, I was able to figure out the problem on my own.

After running stty -a -F /dev/ttyS1, there were 3 options I found to contribute to the problem: "echo", "onlcr", and "icrnl".

Since this serial port is looped back to itself, here is what happened after running echo "hi" > /dev/ttyS1:

  1. The echo command appends a newline to the end of the message by default, so "hi" + LF is sent out to /dev/ttyS1
  2. Because "onlcr" was set, the serial device converted the LF to CRLF so the physical message sent out the Tx line was "hi" + CRLF
  3. Because "icrnl" was set, the physical messaged received on the Rx line converted the CR to LF. So the message outputted by 'cat' was "hi" + LFLF.
  4. Because "echo" was set, the message received on the Rx ("hi" + LFLF), was then sent back out on the Tx line.
  5. Because of onlcr, "hi" + LFLF became "hi" + CRLFCRLF.
  6. Because of icrnl, "hi" + CRLFCRLF became "hi" + LFLFLFLF
  7. Because of echo, "hi" + LFLFLFLF was then sent out the Tx

And so on...

In order to fix this problem, I ran the following command:

stty -F /dev/ttyS1 -echo -onlcr

Disabling "echo" prevents an infinite loop of messages and disabling "onlcr" prevents the serial device from converting LF to CRLF on output. Now cat receives one "hi" (with a single newline!) for each time I run echo.

CR = carriage return (ASCII 0x0D); LF = line feed or newline (ASCII 0x0A)

share|improve this answer

I had a similar issue as well with concatenating files into a serial tty for testing. In addition to the accepted answer:

If you're testing serial output by doing something like: cat somefile.txt > /dev/ttyS0, it will have a good amount of unexpected byte data if you're testing for exact byte values.

With stty doing a simple stty raw -F /dev/ttyS0 will stop the terminal from inserting/replacing characters (e.g. [...] 0x0A [...] -> [...] 0x0D 0x0A [...]). The raw flag changes the modes of the terminal so no input and output processing is performed.

share|improve this answer
Hmm... doesn't look like stty raw will disable echoing by default. You may need to do stty raw -echo. – BMiner Dec 23 '13 at 23:43

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.