How to make /dev/ttyACM0 (and friends) exclusive?

[valdis.kletnieks at vt.edu]

On Sat, 02 Mar 2019 14:36:12 -0500, Jeffrey Walton said:

> I feel like I am missing something... Does Linux consider the modem a
> shared resource instead of an exclusive resource? What use cases
> support two different programs sending commands to the modem at the
> same time?

The Linux kernel has exactly zero clue what a "modem" is.  It's talking to a
serial port, and doesn't care where the other end of the serial cable is. If
you have a onboard modem, that cable may be all of 2 mm long and consist of a
bunch of traces between two chips on a PCB, or even internal connections
between two sides of a chip, but it's still there.

So the correct question is "what use cases have two programs talking to the
same serial port"?

And the answer is:  A lot.  For a long time, there were these things called
"terminals", that the younger folk may not have encountered. And a very common
use case was to login via a terminal.  At that point, you usually had a login
shell like bash or similar running and often doing I/O to the terminal - and if
you ran any sub-processes, they also would do I/O to the terminal. So consider
the following bash one-liner:

% for i in `seq 1 10`; do echo "Loop number $i"; date; sleep 1; done

How and why does this work? (Hint 1:  'echo' is a bash builtin, Hint 2:
think about how a shell handles stdin/out for child processes)