Java Linux Shell Application

Question

So I just received a task for creating a Java Shell App, without using any 3rd party libraries, and without using Runtime.exec() or ProcessBuilder APIs.

I don't want the solution (obviously I want to do this myself) but I do need a hint how to do this? I want the app to open a shell prompt which will accept various commands with usage of JDK 8 (Nashorn?).

Thanks!

Answer 1

Not really clear what you want to achieve. If you want to run a Nashhorn shell you can achieve it like this (Java 8)

import jdk.nashorn.tools.Shell;
public class NashornShell {
    public static void main(String[] args) {
        Shell.main(new String[]{ "-scripting"});
    }
}

When you see the Nashorn prompt jjs> you can execute Linux commands...

jjs> $EXEC("ls");

which will list the current directory (using the Linux ls command).

... or execute Java commands ...

jjs> java.lang.System.out.println("foo");

... or execute JavaScript commands ...

jjs> print("foo");

For more information have a look in the nashorn guide.

edit If you want to pass only yourCommand as parameter.

NashornScriptEngineFactory factory = new NashornScriptEngineFactory();
ScriptEngine engine = factory.getScriptEngine(new String[]{"-scripting"});
String yourCommand = "ls";
Object eval = engine.eval("$EXEC(\"" + yourCommand + "\")");
System.out.println(eval);

edit do you think that instead of Nashorn I could just use raw streams directed to the OS from JVM

Following is possible

Commands.java

class Commands {
    public static void main(String[] args) {
        System.out.println("ls");
        System.out.println("whoami");
    }
}

run.sh

#!/bin/sh
java Commands | while read command; do
  echo
  echo "command: $command"
  $command
done

But obviously this is not to recommend when you want to execute the output of Commands:

• your Java application has no control about the return state of the executed single commands
• if one command wait for user input your Java application don't know it
• your Java application has no access to the output produced by the commands
• all commands are blindly exected
• and some more downsides