java command execution escapes '|'

Question

I have a function to execute a system command:

public String cmd(String s) {
    String out = "";
    try {
        Runtime run = Runtime.getRuntime();
        Process pr = run.exec(s.split(" "));
        pr.waitFor();
        BufferedReader buf = new BufferedReader(new InputStreamReader(pr.getInputStream()));
        String line = "";
        while ((line=buf.readLine())!=null) {
            out+=line+"\n";
        }
    } catch(Exception e) {
        e.printStackTrace();
    }
    return out;
}

The command passes through:

cmd("nmap -sL -n 192.168.1.0/24 | awk '/Nmap scan report/{print $NF}'");

Expected Output:

192.168.1.0
192.168.1.1
 ...

Actual Output:

Starting Nmap 7.80 ( https://nmap.org ) at 2021-04-12 20:27 EET
Nmap scan report for 192.168.1.0 ...

Answer 1

Similar questions answers this well:

• Using Java ProcessBuilder to Execute a Piped Command
• Java program not getting output from terminal

To execute a pipeline, you have to invoke a shell, and then run your commands inside that shell.

Process p = new ProcessBuilder().command("bash", "-c", command).start();

bash invokes a shell to execute your command and -c means commands are read from string. So, you don't have to send the command as an array in ProcessBuilder.

Adapted to you case

String cmd(String command) {
  ProcessBuilder builder = new ProcessBuilder();
  builder.redirectErrorStream(true); // add stdErr to output

  Process process = builder.command("bash", "-c", command).start();
    
  StringBuilder processOutput = new StringBuilder(); // add lines easier
  // try-with to auto-close resources
  try (BufferedReader processOutputReader = new BufferedReader(new InputStreamReader(process.getInputStream()));) {
    String readLine;
    while ((readLine = processOutputReader.readLine()) != null) {
      processOutput.append(readLine + System.lineSeparator()); // use system's line-break
    }

    process.waitFor();
  }

  return processOutput.toString().trim();
}

Then call as expected:

cmd("nmap -sL -n 192.168.1.0/24 | awk '/Nmap scan report/{print $NF}'");

Note: I enhanced it a bit to

• use try-with-resources to deal cleanly with resources
• add output from StdErr
• use a StringBuilder to concatenate output lines
• use System.lineSeparator for platform-independency (Win/Mac/Linux/Unix)

Inspired by: read the output from java exec

Answer 2

The pipe is interpreted by the shell. It executes one command then passes the output of one command into the next one. You could emulate this in Java starting both commands and then pumping the OutputStream of the first program to the InputStream of the second.

Alternatively if you don't want to do this you can still call something like "sh -c 'command1 | command2"