从Bash常见问题解答：
E10) Why does `cd //' leave $PWD as `//'?POSIX.2, in its description of `cd', says that *three* or more leadingslashes may be replaced with a single slash when canonicalizing thecurrent working directory.This is, I presume, for historical compatibility. Certain versions ofUnix, and early network file systems, used paths of the form//hostname/path to access `path' on server `hostname'.
