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 ...

hc_dev · Accepted Answer · 2021-04-12 19:16:08Z

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

Jens Baitinger · Accepted Answer · 2021-04-12 18:41:00Z

1

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"

answered Apr 12, 2021 at 18:41

Jens Baitinger

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1 Comment

مراد على Over a year ago

Could you please show an example of emulating the passing of the output of cmd1 to cmd2 ? Thanks.

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