OT?: break up an incoming data stream into fixed length lines? in bash?

[Karl Auer]

On Fri, 2018-04-06 at 11:04 -0700, Dave Stevens wrote:
> I'm using U16.04 to read data from a sensor. The incoming serial data
> stream has no newlines, just continuous bytes. In the datastream the
> letters B and M occur together at the start of every 24 byte
> subsequence. I'm reading the data using getserial with appropriate
> speed and parity parms. I don't see a simple way to break it up into
> lines with each line having 24 bytes. Does anyone care to suggest a
> method? I'd use a bash function if there is one.

Sounds like you need a little state machine.

You can do it in bash. Read the doco for the "read" function; it has
options for setting the number of characters to read. By default it
reads a line; to read a single char you may have to change some tty
settings - a bit of googling will find you what you need.

Your state machine would look something like:

   clear buffer
   state = waiting_for_B
   while not EOF
      get char
      if state is:
         waiting_for_B 
            if char == B
               append char to buffer
               state = waiting_for_M
         waiting_for_M
            if char == M
               append char to buffer
               state = collecting
            else
               clear buffer
               state = waiting_for_B
         collecting
            save char in buffer
            if buffer length > 23
               output buffer, newline
               clear buffer
               state = waiting_for_B
      
The above discards any data not inside a 24-byte sequence starting with
"BM". You can use the same state machine on data from a file.

Regards, K.

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Karl Auer (kauer at biplane.com.au)
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