Getting output from executing a terminal command in a java code running inside Cubieboard Platform

Question

The code that I am using for running a terminal command in Linux Debian and getting the output inside a java program is this:

public static String execute(String command) {
    StringBuilder sb = new StringBuilder();
    String[] commands = new String[]{"/bin/sh", "-c", command};
    try {
        Process proc = new ProcessBuilder(commands).start();
        BufferedReader stdInput = new BufferedReader(new
                InputStreamReader(proc.getInputStream()));

        BufferedReader stdError = new BufferedReader(new
                InputStreamReader(proc.getErrorStream()));

        String s = null;
        while ((s = stdInput.readLine()) != null) {
            sb.append(s);
            sb.append("\n");
        }

        while ((s = stdError.readLine()) != null) {
            sb.append(s);
            sb.append("\n");
        }
    } catch (IOException e) {
        return e.getMessage();
    }
    return sb.toString();
}

Now the problem is, it works for normal commands like ls / and gives back the appropriate result. But my goal is to run commands like:

echo 23 > /sys/class/gpio/export

which is, for example, for activating the gpio pin in the CubieBoard platform. (Cubieboard is a mini-pc board like Raspberry Pi).

Now running this command in the terminal of the system itself, works fine and gives me the proper result. But when i am running it from this java code, i cannot get any results back.

The point is that, it works and the command executes well, but just that i cannot get the output message of the command!

For example if the pin was active from the past, then normally it should give me back the result like:

bash: echo: write error: Device or resource busy

But when i run this command through java code above, i do not get any response back. (again it takes effect but just the response of the terminal i cannot get!)

When i run the code, both stdInput and stdError variables in the code are having the value null. :(

Please help me so that i can finish my project. this is the only part that is remaining :(

Thank you.

Answer 1

the code is right, just in the second line, I changed

"/bin/sh" to "/bin/bash"

And everything works!

sh == bash?

For a long time, /bin/sh used to point to /bin/bash on most GNU/Linux systems. As a result, it had almost become safe to ignore the difference between the two. But that started to change recently.

Some popular examples of systems where /bin/sh does not point to /bin/bash (and on some of which /bin/bash may not even exist) are:

1. Modern Debian and Ubuntu systems, which symlink sh to dash by default;

2. Busybox, which is usually run during the Linux system boot time as part of initramfs. It uses the ash shell implementation.

3. BSDs. OpenBSD uses pdksh, a descendant of the Korn shell. FreeBSD's sh is a descendant of the original UNIX Bourne shell.

For more information on this please refer to : Difference between sh and bash

Answer 2

There maybe the childProcess doesn't run to end

Please to try:

proc.waitFor()

and run read stdInput and stdError in other Thread before proc.waitFor().

Example:

public static String execute(String command) {
        String[] commands = new String[] { "/bin/sh", "-c", command };

        ExecutorService executor = Executors.newCachedThreadPool();
        try {
            ProcessBuilder builder = new ProcessBuilder(commands);

            /*-
            Process proc = builder.start();
            CollectOutput collectStdOut = new CollectOutput(
                    proc.getInputStream());
            executor.execute(collectStdOut);

            CollectOutput collectStdErr = new CollectOutput(
                    proc.getErrorStream());
            executor.execute(collectStdErr);
            // */

            // /*-
            // merges standard error and standard output
            builder.redirectErrorStream();
            Process proc = builder.start();
            CollectOutput out = new CollectOutput(proc.getInputStream());
            executor.execute(out);
            // */
            // child proc exit code
            int waitFor = proc.waitFor();

            return out.get();
        } catch (IOException e) {
            return e.getMessage();
        } catch (InterruptedException e) {
            // proc maybe interrupted
            e.printStackTrace();
        }
        return null;
    }

    public static class CollectOutput implements Runnable {
        private final StringBuffer buffer = new StringBuffer();
        private final InputStream inputStream;

        public CollectOutput(InputStream inputStream) {
            super();
            this.inputStream = inputStream;
        }

        /*
         * (non-Javadoc)
         * 
         * @see java.lang.Runnable#run()
         */
        @Override
        public void run() {
            BufferedReader reader = null;
            String line;
            try {
                reader = new BufferedReader(new InputStreamReader(inputStream));
                while ((line = reader.readLine()) != null) {
                    buffer.append(line).append('\n');
                }
            } catch (Exception e) {
                System.err.println(e);
            } finally {
                try {
                    reader.close();
                } catch (IOException e) {
                }
            }

        }

        public String get() {
            return buffer.toString();
        }
    }