Extract characters from a serial port

Question

I have a dummy question for you, I am trying to extract some lines from a file beginning by a special string of characters and to copy them into a file. I tried :

cat /dev/ttyACM0 | grep "something" > essai

The file is created but it contains nothing!

Does the cat /dev/ttyACM0 part work right? Often with serial ports you need to run stty to set the speed of the port to match the speed of the device attached to the port. — Mark Plotnick, Oct 30 '15 at 21:37
Yep, there is terminal outputs when i am not using the '>'. It shows exactly whatbi want to save — Nicolas, Oct 30 '15 at 21:40
It may be a matter of buffering: see similar questions Tail -f piped through grep not outputing to file, but outputs to console and Turn off buffering in pipe for example — steeldriver, Oct 30 '15 at 21:45
It's probably a buffering issue, then, since grep isn't going to stop reading until the serial port goes offline and will be storing up its output until either a few kilobytes has been amassed or it exits. On many systems you can give grep a -m numberofmatches argument so it'll quit after that number of matched lines. Can you describe any conditions for which grep should consider the file completely read, so it can quit? — Mark Plotnick, Oct 30 '15 at 21:46
Using dd instead of cat would take care of the buffering issue. — mdpc, Oct 30 '15 at 21:55
Try grep --line-buffered < /dev/ttyACM0 > essai - it should be line-buffered on both input and output. — Mark Plotnick, Oct 30 '15 at 23:30
Thank you mark Plot'ick it does work now. could you explain me what was the problem with the buffer? — Nicolas, Oct 31 '15 at 19:18

Mark Plotnick · Answer 1 · 2015-11-01T00:24:39.597

For efficiency, grep and many other commands use buffered I/O, that is, they read large blocks of data at once (rather than, say, one character at a time), and do not output data until a certain amount has been amassed (rather than, say, writing a line at a time or a character at a time)

But, when a program's input is from a terminal (such as your serial port), the operating system does the program a favor and returns a line at a time (the program can change this by putting the terminal into raw or noncanonical mode; most don't).

And when a program uses the stdio library, standard output to a terminal (the documentation calls this an "interactive device") is, by default, line buffered, but output to a file or pipe is fully buffered.

A program using the stdio library can choose full buffering, line buffering, or no buffering by calling setvbuf. It can also call fflush to force a write whenever it wishes.

GNU grep takes a --line-buffered option, which will run the following code in the function that outputs a matching line:

if (line_buffered)
  fflush (stdout);

Putting all the pieces together:

With this command:

cat /dev/ttyACM0 | grep "something" > essai

cat will read a line at a time from /dev/ttyACM0. After it has amassed a few kilobytes of output, it will write to the pipe. It will repeat this until the read returns a count of zero or fails (which probably won't happen until the serial port goes offline).

grep will read a few kilobytes at a time from the pipe, and after it has amassed a few kilobytes of output, it will write to the essai fle. It will repeat this until the read returns a count of zero or fails, which will happen if and when the cat process exits.

So you won't see anything appear in the essai file until grep has found a few kilobytes' worth of matching lines.

To get output written to the file more promptly, you can give GNU grep that option:

grep --line-buffered < /dev/ttyACM0 > essai

Since it's reading from a serial port, it will be line buffered, and the --line-buffered option will make output line buffered as well.

Extract characters from a serial port

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