How to call public static String method from jar from command-line?

You just need to create a main method that passes a command line parameter to the method and then does a System.out.println(result).

A more general solution would require a wrapper that used reflection to invoke the right method and print out the result.

You could do that via something like this:

public class InvokeMethod {
    public static void main(String[] args) throws ClassNotFoundException, SecurityException, NoSuchMethodException, IllegalArgumentException, IllegalAccessException, InvocationTargetException {
        String clazz = args[0];
        String method = args[1];
        String[] methodArgs = new String[args.length - 2];
        Class<?>[] methodParam = new Class[args.length - 2];
        for (int i = 2; i<args.length; i++) {
            methodArgs[i-2] = args[i];
            methodParam[i-2] = String.class;
        }

        Class<?> c = Class.forName(clazz);
        Method m = c.getMethod(method, methodParam);
        String result = (String) m.invoke(methodParam);
        System.out.println(result);
    }
}

Compile that and package it into invoke.jar and then you can just do java -cp invoke.jar:your_real_jar.jar InvokeMethod some.class.Here method_name arg1 arg2 arg3...

You can pass the output of the wrapper to your shell script.

Java

// -- Test.java ----
public class Test
{
    public static void main(String args[])
    {
        // Call the method from Foo.jar here
        System.out.println("Hello" + " World");
    }
}

Shell

# ----- test.sh ------
#!/bin/sh
grep "$1" input.txt

input.txt

# Sample Input
Line 1 : Hello World
Line 2 : Here is Hello World Again
Line 3 : This is not Hello + World
Line 4 : AB Hello World CD

Commands

javac Test.java
./test.sh "`java Test`" # Note the `

Sample o/p

Line 1 : Hello World
Line 2 : Here is Hello World Again
Line 4 : AB Hello World CD