Java ProcessBuilder Output to String

Question

How to redirect or get the system output to String?

ProcessBuilder pb = new ProcessBuilder().inheritIO();
...

for (...){
    pb.command(...);
    pb.start();

    //here >>> assign output string to variable
}

Answer 1

Here is an opinion on how to capture the standard output of a system command process into a string container.

Adapted from the web:

try {
  ProcessBuilder pb = new ProcessBuilder("echo", "dummy io");
  final Process p=pb.start();
  BufferedReader br=new BufferedReader(new InputStreamReader(p.getInputStream()));
  String line;
  StringBuilder sb = new StringBuilder();
  while((line=br.readLine())!=null) sb.append(line);
}

System.out.println(sb.toString());

Answer 2

In congruence with my original comment on what would be a good example of Basic I/O. I hacked out some code, with a few more features than basic.

---

Extras

• An environment shell for variables and
• A working directory

These features add "profile-style" execution to your System commands.

---

Foundational Work

Java Threading and Joining by Oracle.

---

Code

import java.io.BufferedReader;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.Map;

/**
 * Created by triston on 11/2/17.
 */

public class Commander {

  private Commander(){} // no construction

  public static class StreamHandler implements Runnable {

    Object source;
    Object destination;

    StreamHandler(Object source, Object oDestination) {
      this.source = source; this.destination = oDestination;
    }

    public void run() {
      if (source instanceof InputStream) {
        BufferedReader br = new BufferedReader(new InputStreamReader((InputStream) source));
        String line;
        try {
          while ((line = br.readLine()) != null) ((StringBuilder) destination).append(line + '\n');
        } catch (IOException oE) {
        }
      } else {
        PrintWriter pw = new PrintWriter((OutputStream)destination);
        pw.print((String)source);
        pw.flush(); pw.close();
      }
    }

    public static Thread read(InputStream source, StringBuilder dest) {
      Thread thread = new Thread(new StreamHandler(source, dest));
      (thread).start();
      return thread;
    }

    public static Thread write(String source, OutputStream dest) {
      Thread thread = new Thread(new StreamHandler(source, dest));
      (thread).start();
      return thread;
    }

  }

  static Map<String, String> environment = loadEnvironment();

  static String workingDirectory = ".";

  static Map<String, String> loadEnvironment() {
    ProcessBuilder x = new ProcessBuilder();
    return x.environment();
  }

  static public void resetEnvironment() {
    environment = loadEnvironment();
    workingDirectory = ".";
  }

  static public void loadEnvirons(HashMap input) {
    environment.putAll(input);
  }

  static public String getEnviron(String name) {
    return environment.get(name);
  }

  static public void setEnviron(String name, String value) {
    environment.put(name, value);
  }

  static public boolean clearEnviron(String name) {
    return environment.remove(name) != null;
  }

  static public boolean setWorkingDirectory(String path) {
    File test = new File(path);
    if (!test.isDirectory()) return false;
    workingDirectory = path;
    return true;
  }

  static public String getWorkingDirectory() {
    return workingDirectory;
  }

  static public class Command {

    ProcessBuilder processBuilder = new ProcessBuilder();
    Process process;

    public Command(String... parameters) {
      processBuilder.environment().putAll(environment);
      processBuilder.directory(new File(workingDirectory));
      processBuilder.command(parameters);
    }

    public int start(String input, StringBuilder output, StringBuilder error) throws IOException {

      // start the process
      process = processBuilder.start();

      // start the error reader
      Thread errorBranch = StreamHandler.read(process.getErrorStream(), error);

      // start the output reader
      Thread outputBranch = StreamHandler.read(process.getInputStream(), output);

      // start the input
      Thread inputBranch = StreamHandler.write(input, process.getOutputStream());

      int rValue = 254;
      try {
        inputBranch.join(); rValue--;
        outputBranch.join(); rValue--;
        errorBranch.join(); rValue--;
        return process.waitFor();
      } catch (InterruptedException oE) {
        oE.printStackTrace();
        return rValue;
      }

  }

}

---

Testing

@Test public void foo() {
  Command cmd = new Command("sh", "--");
  StringBuilder output = new StringBuilder();
  StringBuilder error = new StringBuilder();
  int pValue = 127;
  try {
    pValue = cmd.start("echo well done > /dev/stderr\n\necho oh, wow; false", output, error);
  } catch (IOException oE) {
  }
  System.out.println("output: "+output.toString());
  System.out.println("error: "+error.toString());
  System.out.println("\nExit code: "+pValue);
  System.exit(pValue);
}

Bring your own package and JUnit annotations. This sample code demonstrates return value, command input, command standard output, and command error output.

My original design, called for the main thread to perform the standard output processing.

Have a great day.