Error while executing adb command programmatically

I am trying to execute adb commands programmatically

Here is my code:

File f = new File(Environment.getExternalStorageDirectory(), "screen" + System.currentTimeMillis() + ".png");

new ExecuteCommand(MainActivity.this).execute("adb shell screencap -p "
        + Environment.getExternalStorageDirectory().getPath() +
        "/" + "screen" + System.currentTimeMillis() + ".png");

ExecuteCommand Class:

public class ExecuteCommand extends AsyncTask<String, String, String> {

    Context mContext=null;
    public ExecuteCommand(Context _ctx)
    {
        mContext =_ctx;
    }

    ProgressDialog progressdailog;
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        progressdailog = ProgressDialog.show(mContext,
                "Executing", "Please Wait");
    }
    @Override
    protected String doInBackground(String... params) {
        Process p;
        StringBuffer output = new StringBuffer();
        try {
            p = Runtime.getRuntime().exec(params[0]);
            BufferedReader reader = new BufferedReader(
                    new InputStreamReader(p.getInputStream()));
            String line = "";
            while ((line = reader.readLine()) != null) {
                output.append(line + "\n");
                p.waitFor();
            }
        } catch (IOException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        String response = output.toString();
        return response;
    }

    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        progressdailog.dismiss();
        Log.e("Output", result);
    }
}

Problem in logs:

07-31 15:26:31.832 18716-18716/com.example E/Output﹕ cannot bind 'tcp:5038' * daemon not running. starting it now on port 5038 *

I am not able to take a screenshot, but if I ran the same command on command prompt then this is working fine.

But if I execute

new ExecuteCommand(MainActivity.this).execute("ls");

this works fine. Where is the problem in command?