How can I trigger a shell script and run in background (async) in Ruby?

Try this:

Process.fork { system "./test.sh" }

Won't work on windows, for which you can use threading.

@TanzeebKhalili's answer works, but you might consider Kernel.spawn(), which doesn't wait for the process to return:

pid = spawn("./test.sh")
Process.detach(pid)

Note that, according to the documentation, whether you use spawn() or manually fork() and system(), you should grab the PID and either Process.detach() or Process.wait() before exiting.

Regarding redirecting standard error and output, that's easy with spawn():

pid = spawn("./test.sh", :out => "test.out", :err => "test.err")
Process.detach(pid)