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Sending commands to an application and reading its output via file descriptor in bash – Debian

bluetoothctl is an application that has a shell and accepts input commands. I would like to send command to it from the shell of the linux arbitrarily and I would also like to read its output without colours. I am on debian based linux (Raspbian buster) and would like to do it from bash shell if possible.

What I have tried so far:

Method one (sending commands via fd)

  • Run the process attached: $ bluetoothctl
  • Then from another terminal: $ echo "my command" > /proc/$(pidof bluetoothctl)/fd/0 and $ echo -e "my commandn" > /proc/$(pidof bluetoothctl)/fd/0 and $ echo -e "my command13" > /proc/$(pidof bluetoothctl)/fd/0 but my command just appears as an output of bluetoothctl and even pressing ‘enter’ key on my keyboard in the terminal of the program does not execute the command at all just gives a new line.

Method two (writing output to a text file)

  • Run the process detached: $ bluetoothctl > my.output &, but it exits from the application immediately

Method three (sending command via pipe)

  • Create a pipe mkfifo mypipe
  • Redirect the pipe to bluetoothctl cat mypipe | bluetoothctl > my.output
  • Write a command to the pipe echo "my command" > mypipe, but it closes the application immediately and the application has no time to process the command asynchronously. Hence, does not work.

I believe the feasible solution would be to start the application detached $ bluetoothctl & and then send command to it via /proc/$(pidof bluetoothctl)/fd/0 and read its stdout and stderr via /proc/$(pidof bluetoothctl)/fd/1 and /proc/$(pidof bluetoothctl)/fd/2 but I am totally lost.

One of the command I would like to execute and read it’s output is ‘scan on’ and ideally one bluetoothctl instance would process multiple commands not only one that the pipe can do.
Can someone please advise how a command can be sent via fd, and how the output can be read via the another fd of the process. All of this in bash shell, if possible.

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2

Answers


  1. Chosen as BEST ANSWER
    0

    For everyones benefit: Following the accepted answer, I have resolved my issue with coprocess, however, I have used the /proc/$(pidof bluetoothctl)/fd/0 to send commands because it better suited to my application due to some of its assynchronous nature. The output was then diverted to a file and the colours were removed with sed upon read.

    #!/bin/bash

readOutput() {
    echo "$(cat $TEMP/bluetoothctl.out | sed 's/x1B[[0-9;]{1,}[A-Za-z]//g'"
}

executeCommandInExistingProcess() {
  if [ -z "$(pidof bluetoothctl)" ]
  then
    exit -1
  else
    echo "$1" >&/proc/$(pidof bluetoothctl)/fd/0
  fi
}

startParallelProcess() {
  output="$TEMP/bluetoothctl.out"
  
  if [ ! -z "$(pidof bluetoothctl)" ]
  then
    echo "Bluetooth process is running..."
    exit -1
  fi
  
  if [ -f "$output" ]
  then
    rm $output
  fi

  coproc bctl { bluetoothctl > $output; }
  [ "$#" -ge 1 -a ! -z "$1" ] && executeCommandInExistingProcess "$1"

  sleep 30

  if [ ! -z $(pidof bluetoothctl) ]
  then
    [ "$#" -ge 2 -a ! -z "$2" ] && executeCommandInExistingProcess "$2"
    executeCommandInExistingProcess "quit"
  fi
}

  2. 0

    A coprocess is the appropriate tool for this job:

    #!/usr/bin/env bash
case $BASH_VERSION in ''|[0-3].*|4.0*) echo "ERROR: bash 4.1+ required" >&2; exit 1;; esac

coproc btctl { bluetoothctl; }

echo "scan on" >&"${btctl[1]}"

while IFS= read -r -u "${btctl[0]}" line; do
  echo "Read line from btctl: $line" >&2
done
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