Strange behavior of mv/cp for directory “.” on Linux

Question

I note a strange behavior of mv/cp on Linux. The directories A and A/. should be equivalent but as arguments for the source they seem to behave differently. Is that a bug? Of course you could ask why should I use A/. instead of A as source but sometimes one wants "." as source and the same strange things happen... Maybe someone knows an answer.

Let's start with

$ mkdir A A/A1 B

and now

variant 1)

$ cp -r A B
$ ls B
A

All right.

variant 2)

$ cp -r A/. B
$ ls B
A1

Why "A1" and not "A"?

variant 3)

$ mv A B
$ ls B
A

All right.

variant 4)

$ mv -i A/. B
mv: overwrite `B/.'? y
mv: cannot move `A/.' to `B/.': Device or resource busy

strange...

Answer 1

"A" and "A/." are both directory entries that point to the same thing, but they are themselves different things. "A" is an entry in the current directory, and "A/." is an entry named "." in the A directory. Think of it as operating on a box from the outside, vs. operating on the box from the inside.

Specifically,

cp -r A B

Copies A into B.

cp -r A/. B

Goes into the directory A and copies the contents of the current directory (i.e. ./A1) into B.

mv A B

Moves the directory entry named "A" from the current directory into B.

mv -i A/. B

Tries to move the directory entry "." out of A into B. However, there's already an entry named "." in B. In general, you can't move "." (that's like trying to move the box from the inside).