How to redirect output from Groovy script?

Try this using Binding

public void exec(File file, OutputStream output) throws Exception {
    Binding binding = new Binding()
    binding.setProperty("out", output) 
    GroovyShell shell = new GroovyShell(binding);
    shell.evaluate(file);
}

After comments

public void exec(File file, OutputStream output) throws Exception {
    Binding binding = new Binding()
    binding.setProperty("out", new PrintStream(output)) 
    GroovyShell shell = new GroovyShell(binding);
    shell.evaluate(file);
}

Groovy Script

def name='World'
out << "Hello $name!"

How about using javax.script.ScriptEngine? You can specify its writer.

ScriptEngine engine = new ScriptEngineManager().getEngineByName("Groovy");
PrintWriter writer = new PrintWriter(new StringWriter());
engine.getContext().setWriter(writer);
engine.getContext().setErrorWriter(writer);
engine.eval("println 'HELLO'")