Suppose I have cmd=["echo", "*", "*"], I want to unroll this cmd so
that when I call system, it calls, in effect, system("echo", "*",
"*").

The number of argument to echo is known only at run-time. Currently
I'm concat-ing them into a string with each argument double-quoted to
prevent shell expansion: cmd="echo \"#{arg1}\"". Any better way?

YS.