why does /var/mail/{username} = /Users/{username}?

Question

My basic question is why do both of these paths point to home (i.e., ~)? Is it pointing to the same home or is it duplicated?

I doubt it's duplicated, so if not, how does cd .. decide which directory to take me back to?

Answer 1

How cd .. behaves will depend on the shell, the shell settings, and whether (as is likely in this case) symlinks are involved.

bash-4.1$ cd /var/tmp
bash-4.1$ mkdir -p real/cats
bash-4.1$ ln -s real/cats dogs
bash-4.1$ cd dogs/
bash-4.1$ pwd
/var/tmp/dogs
bash-4.1$ pwd -P
/var/tmp/real/cats
bash-4.1$ cd ..
bash-4.1$ pwd
/var/tmp
bash-4.1$ set -o physical
bash-4.1$ cd dogs
bash-4.1$ pwd
/var/tmp/real/cats
bash-4.1$ cd ..
bash-4.1$ pwd
/var/tmp/real
bash-4.1$ 

Investigating the /var/mail directory with ls should reveal if there are any symlinks done by I'm guessing Apple.

Answer 2

On macOS (which I'm guessing this is since you mention /Users), the /var/mail/username is the mbox-formatted inbox mailbox for user username, and /Users/username is the home directory for the same user.

On an ordinary, non-modified installation of macOS, /var/mail/username will not be a symbolic link to the user's home directory, nor will /Users/username be a link to /var/mail/username.

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cd .. will by default work like cd -L .., i.e. it will take you up to the logical parent directory rather than the physical parent directory (cd -P ..).

From the ksh-manual on macOS:

By default, symbolic link names are treated literally when finding the directory name. This is equivalent to the -L option. The -P option causes symbolic links to be resolved when determining the directory. The last instance of -L or -P on the command line determines which method is used.