What is the double slash (//) directory?

I mistakenly typed cd // instead of cd /. To my surprise, current directory showed up as //.

What is that directory? Why does it exist?

apple@snipped $ pwd
/home/apple
apple@snipped $ cd /
apple@snipped $ pwd
/
apple@snipped $ cd //
apple@snipped $ pwd
//
apple@snipped $ cd ///
apple@snipped $ pwd
/

Solution 1:

// is usually the same as /. /// must be the same as /.

ls would have shown you that cd // took you to the root directory, the same as cd / does.

$ cd /
$ ls
bin
boot
dev
...
$ cd //
$ ls
(same as above)

The technical way to confirm they are definitely the same directory is:

$ cd /
$ stat -c "%i" .
2
$ cd //
$ stat -c "%i" .
2

they will print the same inode number, meaning they are the same thing.

The gory details are documented in the POSIX Pathname Resolution specification:

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.