A pathname consisting of a single slash shall resolve to the root directory of the process. A null pathname shall not be successfully resolved. A pathname that begins with two successive slashes may be interpreted in an implementation-defined manner, although more than two leading slashes shall be treated as a single slash.
大多数实现，包括 bash，都是把 // 当 / 来处理，但是仍然显示 //。但 zsh 不是，zsh 把 // 当 / 处理并显示。