Shell Programming and Scripting

Check what it actually sends, esp. the terminator/delimiter. cat the device and pipe that through an octal or hex dumper.

You might also want to consider using:

Code:

printf 'add read 0\n\r'

instead of:

Code:

echo "add read 0" > /dev/ttyACM0
echo -e '\r' > /dev/ttyACM0

Note that the two echo commands each open and close the device and each prints a <newline> character that might neither be needed nor desirable from the second echo.

Alternatively, you might want to consider opening the device once at the start of your script using exec to open (and hold open) your device:

Code:

exec <> /dev/ttyACM0

(replacing standard input for the entire script in this case, but a different file descriptor can also be specified if some other file is being fed into your script). Then you could consider using the stty utility to set appropriate parameters for your device such as icrnl or -icrnl and igncr or -igncr (depending on whether your device sends <carriage-return> and/or <newline> characters as line terminators when you read data from your device) and ocrnl or -ocrnl, onlcr or -onlcr, and onocr or -onocr depending on what it wants to see as line terminators in commands it receives from your script. (And, of course, check the stty man page on your system for other operands that might simplify using your device.)

I thought about proposing using the stty utility as well but wasn't sure if it would work/apply, as we are dealing with a USB GPIO device.

Using stty might or might not work, but it is really easy to find out...

If the command stty < /dev/ttyACM0 works (printing the current settings for the device), it will work. If it exits with a non-zero exit status and prints a diagnostic similar to:

Code:

stty: stdin isn't a terminal

then it won't work.

Thanks Don. I will try your suggestions today. One question: If I use

Code:

exec <> /dev/ttyACM0

to open the port for the duration of the script, what is the command for closing it when I exit the script?

There is no need, it will close itself on exit, but:

Code:

exec <&-