unexpected behavior of directory renaming in macos (other posix flavors?)

Question

Basically, in Python or bash I can rename a directory to a different name from within that directory: at that point, from within the directory, the old name still shows, but the rename has in fact already taken place.

On macos with APFS, this came about in a Python script I was writing to rename directories as per a specific naming convention I use and I noticed this behavior.

I am posting this as posix/shell/macos as I half expect this to happen as well under Linux and I got the same general behavior out of the zshell.

Let's say I have a directory, foo :

(venv) jluc@test$ tree
.
└── foo

I rename it to bar with mv foo bar

(venv) jluc@test$ tree
.
└── bar

But now, let's cd into that directory and perform the rename there.

$cd bar
$pwd
/Users/jluc/kds2/wk/explore/test/bar
$ mv ../bar ../zoom
$ pwd
/Users/jluc/kds2/wk/explore/test/bar  👈 still the old name

So, now, within bar , I've renamed it to zoom . It did not error out. Locally, a pwd shows that I am still in the same directory. I can do an ls and I am not in a invalid directory which some commands can sometime put me in.

However, a tree one level up tells a different story.

(venv) jluc@bar$ tree ..
..
└── zoom  👈 but here I see the new name

and cd to the current directory fails

cd `pwd`
-bash: cd: /Users/jluc/kds2/wk/explore/test/bar: No such file or directory

What, in the name of Schrodinger's, is going on? Does the inode scheme underlying the file system, APFS, provide this? Would different filesystems, like ext4 for example also exhibit the same behavior?

Update: if I have a separate text file in my testing directory, I can cat the contents of the file before and after the rename locally, so it's not just the shell - the file system collaborates as well. The current directory is still valid and operational (which would fit with @that other guy's answer).

Answer 1

There are two things at play:

1. On Unix, any handle to an open file or directory will generally be unaffected by renames and deletions.
2. The shell remembers the directory you're in, it doesn't re-query every time.

#1 means that for the most part, you can delete or move a file/dir that's still in use, and the using process can continue using it until it's done. #2 means that the shell's pwd will just return the old name (though external /bin/pwd which has to requery will fail).