How to redirect ProcessBuilder's output to a string?

Java Problem Overview

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.

Java Solutions

Solution 1 - Java

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();

Solution 2 - Java

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);

Solution 3 - Java

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());

Solution 4 - Java

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.

Solution 5 - Java

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);
}

Solution 6 - Java

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

IE:

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

Solution 7 - Java

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: https://stackoverflow.com/questions/16714127/how-to-redirect-process-builders-output-to-a-string

Solution 8 - Java

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())
    }

Solution 9 - Java

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.