How to escape strings for terminal in Ruby?

Shellwords should work for you :)

exec "/usr/bin/mplayer %s" % Shellwords.escape(song.file)

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In ruby 1.9.x, it looks like you have to require it first

require "shellwords"

But in ruby 2.0.x, I didn't have to explicitly require it.

Please never use the "single command line" form of exec, that leaves you open to all the usual quoting and injection issues and pointlessly launches a shell. From the fine manual:

exec(cmdname, arg1, ...)

command name and one or more arguments (no shell)

So instead of mucking around with quoting and escaping and what not, just use the shell-less version:

exec '/usr/bin/mplayer', song.file

and bypass the shell completely. Similarly for system.