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bashdeviceread-write

Bash: write on device then read from it

I'm developing a very small program with a weight sensor connected to S0. Here is how I initialize the serial port :

stty -F /dev/ttyS0 9600 min 60 time 1 ignbrk -brkint -icrnl -imaxbel -opost -onlcr -isig -icanon -iexten -echo -echoe -echok -echoctl -echoke parenb -ixon

I have tested several manners to write to the device and then read from it :

echo IDN? >> /dev/ttyS0
read -t1 output < /dev/ttyS0   # waits for ever here.

I've also tried :

echo IDN? >> /dev/ttyS0 && read -t1 output < /dev/ttyS0  # waits too

But unsuccess. In one line, it doesn't change anything.

until read -t1 < /dev/ttyS0; do
    echo IDN? > /dev/ttyS0
done
identity=$(echo $REPLY | tr -d $'\r')

This works when the device is connected.

For my tests, I did this and it worked:

$ cat /dev/ttyS0 &
[1] 9188
$ echo IDN? >> /dev/ttyS0
HBM-xxxyyyzzz$

The three manners I've tried are unsuccessful. Maybe because the device answers quicker than the bash interprets ? If it's the case, how can I dodge this to have a perfect timed answer from the device ?

What I actually am searching for, is if the device doesn't answer within 1 second, then skip the read, but if it's reading at 200ms, then continue without waiting for the 800 last ms

Solution

  • Opening the device in read/write mode should work.

    exec 3<> /dev/ttyS0
echo "IDN?" >&3
read -t1 output <&3

    Opening the file with exec like this means the file stays open across multiple commands, rather than being opened and closed by each command.