java shell for executing/coordinating processes?

Question

I know about using Runtime.exec, you pass it a native program to run + arguments. If it's a regular program, you can run it directly. If it's a shell script, you have to run an external shell program like sh or csh or cmd.exe.

Is there some Java class (either standard or open-source) that implements a shell, meaning a program that you pass a command string or a script into, that executes commands and redirects standard I/O/err accordingly, so that you could pass a string like foo | bar > baz.out in, and it would run the foo and bar programs w/o having to run another executable outside of Java?

(and by shell I don't mean BeanShell or the standalone Rhino Javascript interpreter, those are Java implementations to execute Java and Javascript code. I'm talking about Java implementations to execute non-Java executables and handle the plumbing of redirecting I/O.)

Answer 1

Since JDK 1.5 there is java.lang.ProcessBuilder which handles std and err streams as well. It's sort of the replacement for java.lang.Runtime

Answer 2

You've always been able to handle streams with Runtime.exec

e.g.

String cmd = "ls -al";
 Runtime run = Runtime.getRuntime();
 Process pr = run.exec(cmd);
 pr.waitFor();
 BufferedReader buf = new BufferedReader(new InputStreamReader(pr.getInputStream()));
 String line = "";
 while ((line=buf.readLine())!=null) {
  System.out.println(line);
 }

However, if you want to put shell characters such as pipe and redirect in there you'd have to write your own command line parser which links up the streams. As far as I know there hasn't one been written. That being said, could you just invoke bash from Java with a -c "ls | sort" for example and then read the input. Hmm time to do some testing.

Answer 3

Ok, I've worked it out:

Basically, you need to invoke bash with a "-s" and then write the full command string to it.

public class ShellExecutor {

private String stdinFlag;
private String shell;

public ShellExecutor(String shell, String stdinFlag) 
{
 this.shell = shell;
 this.stdinFlag = stdinFlag;
}

public String execute(String cmdLine) throws IOException 
{
 StringBuilder sb = new StringBuilder();
    Runtime run = Runtime.getRuntime();
    System.out.println(shell);
    Process pr = run.exec(cmdLine);
    BufferedWriter bufWr = new BufferedWriter(new OutputStreamWriter(pr.getOutputStream()));
    bufWr.write(cmdLine);
    try 
    {
  pr.waitFor();
 } catch (InterruptedException e) {}
    BufferedReader buf = new BufferedReader(new InputStreamReader(pr.getInputStream()));
    String line = "";
    while ((line=buf.readLine())!=null) 
    {
        sb.append(line + "\n");
    }
    return sb.toString();
}

}

Then use it like this:

     ShellExecutor excutor = new ShellExecutor("/bin/bash", "-s");
 try {
  System.out.println(excutor.execute("ls / | sort -r"));
 } catch (IOException e) {
  e.printStackTrace();
 }

Obviously, you aught to do something with the error string but this is a working example.