java – How to redirect Process Builder's output to a string?-ThrowExceptions

Exception or error:

I am using the following code to start a process builder.I want to know how can I redirect its output to a String.

ProcessBuilder pb = new ProcessBuilder(System.getProperty("user.dir")+"/src/generate_list.sh", filename);
Process p = pb.start();

I tried using ByteArrayOutputStream but it didn’t seem to work.

How to solve:

Read from the InputStream. You can append the output to a StringBuilder:

BufferedReader reader = 
                new BufferedReader(new InputStreamReader(process.getInputStream()));
StringBuilder builder = new StringBuilder();
String line = null;
while ( (line = reader.readLine()) != null) {
   builder.append(line);
   builder.append(System.getProperty("line.separator"));
}
String result = builder.toString();

Answer:

Using Apache Commons IOUtils you can do it in one line:

ProcessBuilder pb = new ProcessBuilder("pwd");
String output = IOUtils.toString(pb.start().getInputStream(), StandardCharsets.UTF_8);

Answer:

Java 8 example:

public static String runCommandForOutput(List<String> params) {
    ProcessBuilder pb = new ProcessBuilder(params);
    Process p;
    String result = "";
    try {
        p = pb.start();
        final BufferedReader reader = new BufferedReader(new InputStreamReader(p.getInputStream()));

        StringJoiner sj = new StringJoiner(System.getProperty("line.separator"));
        reader.lines().iterator().forEachRemaining(sj::add);
        result = sj.toString();

        p.waitFor();
        p.destroy();
    } catch (Exception e) {
        e.printStackTrace();
    }
    return result;
}

Usage:

List<String> params = Arrays.asList("/bin/sh", "-c", "cat /proc/cpuinfo");
String result = runCommandForOutput(params);

I use this exact code and it works well for single or multiple line results. You could add an error stream handler as well.

Answer:

You might do something like this:

private static BufferedReader getOutput(Process p) {
    return new BufferedReader(new InputStreamReader(p.getInputStream()));
}

private static BufferedReader getError(Process p) {
    return new BufferedReader(new InputStreamReader(p.getErrorStream()));
}
...
Process p = Runtime.getRuntime().exec(commande);
BufferedReader output = getOutput(p);
BufferedReader error = getError(p);
String ligne = "";

while ((ligne = output.readLine()) != null) {
    System.out.println(ligne);
}

while ((ligne = error.readLine()) != null) {
 System.out.println(ligne);
}

Answer:

For Java 7 and 8 this should work:

private String getInputAsString(InputStream is)
{
   try(java.util.Scanner s = new java.util.Scanner(is)) 
   { 
       return s.useDelimiter("\\A").hasNext() ? s.next() : ""; 
   }
}

Then in your code, do this:

String stdOut = getInputAsString(p.getInputStream());
String stdErr = getInputAsString(p.getErrorStream());

I shamelessly stole that from: How to redirect Process Builder's output to a string?

Answer:

Just add .inheritIO(); to the process builder line.

IE:

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

Answer:

In java 8 there is a nice lines() stream you can combine with String.join and System.lineSeparator():

    try (BufferedReader outReader = new BufferedReader(new InputStreamReader(p.getInputStream()))
    {
        return String.join(System.lineSeparator(), outReader.lines().collect(toList()));
        \\ OR using jOOλ if you like reduced verbosity
        return Seq.seq(outReader.lines()).toString(System.lineSeparator())
    }

Answer:

After trying to handle different cases (handling both stderr and stdout and not blocking any of these, terminate process after timeout, properly escaping slashes, quotation marks, special characters, spaces, …. ) I gave up and found Apache Commons Exec https://commons.apache.org/proper/commons-exec/tutorial.html that seems to be doing all these things pretty well.

I do recommend everyone who needs to invoke external process in java to use Apache Commons Exec library instead of reinventing it again.

Answer:

As of Java 9, we finally have a one liner:

ProcessBuilder pb = new ProcessBuilder("pwd");
Process process = pb.start();
String result = new String(process.getInputStream().readAllBytes());

Answer:

Solutions

  • This code is a running example for the general solution to your question:

How to redirect Process Builder’s output to a string?

  • Credits go to Greg T, after trying multiple solutions to run various commands and capture their outputs Greg T’s answer contained the essence of the particular solution. I hope the general example is of use for someone combining multiple requirements whilst capturing the output.
  • To obtain your particular solution you can uncomment ProcessBuilder pb = new ProcessBuilder(System.getProperty("user.dir")+"/src/generate_list.sh", filename);, uncomment the line, and comment out: ProcessBuilder processBuilder = new ProcessBuilder(commands);.

Functionality

  • It is a working example that executes command echo 1 and returns the output as a String.
  • I also added setting a working path and an environment variable, which is not required for your particular example so you can delete it.

Usage & verification

  • You can copy paste this code as a class, compile it to jar and run it.
  • It is verified in WSL Ubuntu 16.04.
  • Setting the workdirectory is verified by setting binaryCommand[0]="touch";and binaryCommand[1]="1";, re-compiling and running the .jar file.

Limitations

  • If the pipe is full (due to a “too large” output), the code hangs.

Code

import java.io.BufferedReader;
import java.io.File;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Map;
import java.util.StringJoiner;

public class GenerateOutput {

    /**
     * This code can execute a command and print the output accompanying that command.
     * compile this project into a .jar and run it with for example:
     * java -jar readOutputOfCommand.jar
     * 
     * @param args
     * @throws Exception 
     */
    public static void main(String[] args) throws Exception {
        boolean answerYes = false; // no yes answer to any command prompts is needed.

        // to execute a command with spaces in it in terminal, put them in an array of Strings.
        String[] binaryCommand = new String[2];

        // write a command that gives a binary output:
        binaryCommand[0] = "echo";
        binaryCommand[1] = "1";

        // pass the commands to a method that executes them
        System.out.println("The output of the echo command = "+executeCommands(binaryCommand,answerYes));
    }

    /**
     * This executes the commands in terminal. 
     * Additionally it sets an environment variable (not necessary for your particular solution)
     * Additionally it sets a working path (not necessary for your particular solution)
     * @param commandData
     * @param ansYes
     * @throws Exception 
     */
    public static String executeCommands(String[] commands,Boolean ansYes) throws Exception {
        String capturedCommandOutput = null;
        System.out.println("Incoming commandData = "+Arrays.deepToString(commands));
        File workingDirectory = new File("/mnt/c/testfolder b/");

        // create a ProcessBuilder to execute the commands in
        ProcessBuilder processBuilder = new ProcessBuilder(commands);
        //ProcessBuilder processBuilder = new ProcessBuilder(System.getProperty("user.dir")+"/src/generate_list.sh", "a");

        // this is not necessary but can be used to set an environment variable for the command
        processBuilder = setEnvironmentVariable(processBuilder); 

        // this is not necessary but can be used to set the working directory for the command
        processBuilder.directory(workingDirectory);

        // execute the actual commands
        try {

             Process process = processBuilder.start();

             // capture the output stream of the command
             BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
            StringJoiner sj = new StringJoiner(System.getProperty("line.separator"));
            reader.lines().iterator().forEachRemaining(sj::add);
            capturedCommandOutput = sj.toString();
            System.out.println("The output of this command ="+ capturedCommandOutput);

             // here you connect the output of your command to any new input, e.g. if you get prompted for `yes`
             new Thread(new SyncPipe(process.getErrorStream(), System.err)).start();
             new Thread(new SyncPipe(process.getInputStream(), System.out)).start();
            PrintWriter stdin = new PrintWriter(process.getOutputStream());

            //This is not necessary but can be used to answer yes to being prompted
            if (ansYes) {
                System.out.println("WITH YES!");
            stdin.println("yes");
            }

            // write any other commands you want here

            stdin.close();

            // this lets you know whether the command execution led to an error(!=0), or not (=0).
            int returnCode = process.waitFor();
            System.out.println("Return code = " + returnCode);
        } catch (IOException e1) {
            e1.printStackTrace();
        }
        return capturedCommandOutput;
    }


    /**
     * source: https://stackoverflow.com/questions/7369664/using-export-in-java
     * @param processBuilder
     * @param varName
     * @param varContent
     * @return
     */
    private static ProcessBuilder setEnvironmentVariable(ProcessBuilder processBuilder){
        String varName = "variableName";
        String varContent = "/mnt/c/testfolder a/";

        Map<String, String> env = processBuilder.environment();
         System.out.println("Setting environment variable "+varName+"="+varContent);
         env.put(varName, varContent);

         processBuilder.environment().put(varName, varContent);

         return processBuilder;
    }
}


class SyncPipe implements Runnable
{   
    /**
     * This class pipes the output of your command to any new input you generated
     * with stdin. For example, suppose you run cp /mnt/c/a.txt /mnt/b/
     * but for some reason you are prompted: "do you really want to copy there yes/no?
     * then you can answer yes since your input is piped to the output of your
     * original command. (At least that is my practical interpretation might be wrong.)
     * @param istrm
     * @param ostrm
     */
    public SyncPipe(InputStream istrm, OutputStream ostrm) {
        istrm_ = istrm;
        ostrm_ = ostrm;
    }
    public void run() {

      try
      {
          final byte[] buffer = new byte[1024];
          for (int length = 0; (length = istrm_.read(buffer)) != -1; )
          {
              ostrm_.write(buffer, 0, length);                
              }
          }
          catch (Exception e)
          {
              e.printStackTrace();
          }
      }
      private final OutputStream ostrm_;
      private final InputStream istrm_;
}

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