@Adi Tiwari, I've found the cause. 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 $BOOTCLASSPATH" }; 
Runtime.getRuntime().exec(cmdline);

The first thing you should do is to log the stderr stream that is available for your process as well. This will give you information about what is wrong with your command.

Your command is not correctly processed as it is seen as one command. The reason is explained in this answer.

The solution is to use a String[] as an argument of exec and explicitly execute the command with the shell. I wrote some code that executes your command, but it is in Java on an unrooted device. Still, it generates output and grep works.

String[] arrayCommand = {"sh", "-c","dumpsys telephony.registry | grep \"permission\""};
            
Runtime r = Runtime.getRuntime();
Process process = r.exec(arrayCommand);

String stdoutString = convertInputStreamToString(process.getInputStream());
String stderrString = convertInputStreamToString(process.getErrorStream());

If you are using service to check every 5 seconds then you must return START_NOT_STICKY from override method onStartCommand() if you want to prevent it start service automatically when it crashes.

First, you should wrap calls to exec in a try-catch-clause to catch IOExceptions.

Second, use exec(java.lang.String[]) to execute a command with parameters. For example, similar to

Runtime.getRuntime().exec(new String[]{ "shell", "/system/bin/chmod", "0777", sdCard.getAbsolutePath() + "/" + nativeFile });

I find solution, API29+ have more "feature": Removed execute permission for app home directory

The workaround is to rename the executable from "iperf" to "libiperf.so", and create "lib" directory, make "arm64-v8a" subdirectory and put "libiperf.so" there.

Then you can execute iperf

Java.Lang.Process processIperf = Java.Lang.Runtime.GetRuntime().Exec(ApplicationInfo.NativeLibraryDir + "/libiperf.so -v");  
  
Java.IO.BufferedReader bufferedReader = new Java.IO.BufferedReader(new Java.IO.InputStreamReader(processIperf.InputStream));  

For more details and discussions, please go to the internet. No decent solution, there are only more workarounds;)

why not use p.getInputStream().read() to read what you need to send while using out.write() to send data accordingly.

here is an example taken from: http://www.rgagnon.com/javadetails/java-0014.html

String line;
OutputStream stdin = null;
InputStream stderr = null;
InputStream stdout = null;

  // launch EXE and grab stdin/stdout and stderr
  Process process = Runtime.getRuntime ().exec ("/folder/exec.exe");
  stdin = process.getOutputStream ();
  stderr = process.getErrorStream ();
  stdout = process.getInputStream ();

  // "write" the parms into stdin
  line = "param1" + "\n";
  stdin.write(line.getBytes() );
  stdin.flush();

  line = "param2" + "\n";
  stdin.write(line.getBytes() );
  stdin.flush();

  line = "param3" + "\n";
  stdin.write(line.getBytes() );
  stdin.flush();

  stdin.close();

  // clean up if any output in stdout
  BufferedReader brCleanUp =
    new BufferedReader (new InputStreamReader (stdout));
  while ((line = brCleanUp.readLine ()) != null) {
    //System.out.println ("[Stdout] " + line);
  }
  brCleanUp.close();

  // clean up if any output in stderr
  brCleanUp =
    new BufferedReader (new InputStreamReader (stderr));
  while ((line = brCleanUp.readLine ()) != null) {
    //System.out.println ("[Stderr] " + line);
  }
  brCleanUp.close();