
`head -cNUM ... | tr -dc SET` might generate output containing fewer than NUM characters. Given the limited alphabet, this could result in a fairly weak passphrase. The construction `tr </dev/urandom | head -cNUM`, however, is sure to give us the full `NUM`.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.