Some terminal commands run via Java don't display output in Linux

Question

I'm trying to write a Java program to run terminal command. Googling and SO got me to here:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

public class Detector {
    public static void main(String[] args) {

        String[] cmd = {"ls", "-la"};
        try {
            Process p = Runtime.getRuntime().exec(cmd);

            BufferedReader reader = new BufferedReader(
                new InputStreamReader(p.getInputStream()));

            String line = "";
            while ((line = reader.readLine()) !=null){
                System.out.println(line);
            }
            p.waitFor();

        } catch (IOException  | InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }
}

So far, so good. The problem is if I try to run a command like "python -V" via

String[] cmd = {"python", "-V"};

The program will run, but no output is actually printed out. Any ideas?

python -V by default writes to STDERR, not STDOUT. Changing it to python -V 2>&1 should work, but I am sure there is someway to do this in Java without the 2>&1 part — Munir
– Munir, Commented Mar 14, 2016 at 22:20

childofsoong · Accepted Answer · 2016-03-14 22:19:21Z

1

The output you see on your command line when running python -V is actually being printed to standard error. To capture this, you need to use a different InputStream, such as this:

BufferedReader errorReader = new BufferedReader(
    new InputStreamReader(p.getErrorStream()));

The rest of your code is fine.

answered Mar 14, 2016 at 22:19

childofsoong

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1 Comment

Matter Cat Over a year ago

Hey, that did the trick! Thanks very much. Strange how it does that.

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