How do I listen for a response from shell command in android studio?

Question

In an android terminal emulator I can type the following commands:

> su
> echo $(</sys/class/power_supply/battery/charge_rate)

and depending on how the phone is charging the output will be "None", "Normal" or "Turbo". I would like to be able to retrieve this output and store it as a string value in my program.

So I've done a little bit of research into this and the code I came up with is as follows:

    String chargeRate = "None";
    try {
        Runtime rt = Runtime.getRuntime();
        Process process = rt.exec("su \"\"echo $(</sys/class/power_supply/battery/charge_rate)");

        BufferedReader stdInput = new BufferedReader(new InputStreamReader(process.getInputStream()));

        if ((chargeRate = stdInput.readLine()) == null)
            chargeRate = "None";
    }
    catch (Exception e) {
        // TODO
    }

This is taken from a number of different answers and I'm not quite sure whats wrong with it. I can't step over or past this line while debugging:

if ((chargeRate = stdInput.readLine()) == null)

As soon as the debugger reaches this line it says "The application is running"

Answer 1

UPDATE : Solution is in Unable using Runtime.exec() to execute shell command "echo" in Android Java code :

Runtime.getRuntime.exec() doesn't execute a shell command directly, it executes an executable with arguments. "echo" is a builtin shell command. It is actually a part of the argument of the executable sh with the option -c. Commands like ls are actual executables. You can use type echo and type ls command in adb shell to see the difference.

So final code is:

String[] cmdline = { "sh", "-c", "echo $..." }; 
Runtime.getRuntime().exec(cmdline);

cat is also executable from within Runtime.exec() without invoking sh

This is also analyzed in https://www.javaworld.com/article/2071275/when-runtime-exec---won-t.html?page=2 in paragraph Assuming a command is an executable program

The code in Execute shell commands and get output in a TextView is good although it uses a command that is executable directly (ls, see update above) :

try {
        // Executes the command.
        Process process = Runtime.getRuntime().exec("ls -l");

        // Reads stdout.
        // NOTE: You can write to stdin of the command using
        //       process.getOutputStream().
        BufferedReader reader = new BufferedReader(
                new InputStreamReader(process.getInputStream()));

        int read;
        char[] buffer = new char[4096];
        StringBuffer output = new StringBuffer();
        while ((read = reader.read(buffer)) > 0) {
            output.append(buffer, 0, read);
        }
        reader.close();

        // Waits for the command to finish.
        process.waitFor();

        return output.toString();
    } catch (IOException e) {
        throw new RuntimeException(e);
    } catch (InterruptedException e) {
        throw new RuntimeException(e);
    }