Ruby 2.3.0 リファレンスマニュアル > ライブラリ一覧 > 組み込みライブラリ > File::Statクラス > world_writable?
world_writable? -> Integer | nil
[permalink][rdoc]If stat is writable by others, returns an integer representing the file permission bits of stat. Returns nil otherwise. The meaning of the bits is platform dependent; on Unix systems, see stat(2).
m = File.stat("/tmp").world_writable? # => 511 sprintf("%o", m) # => "777"