Meaning of tilde in Linux bash (not home directory)

Question

First off, I know that ~/ is the home directory. CDing to ~ or ~/ takes me to the home directory.

However, cd ~X takes me to a special place, where X seems to be anything.

In bash, if I hit "cd ~" and hit tab, it shows a bunch of possible ~X options like ~mail and ~postgres and ~ssh. Going to those folders and doing a pwd shows me that these folders are not in the home directory; they're all over the place.

They are not aliases. I've checked. They're not env. variables, or else they'd require a $.

What is setting these links, and where can I find where these are being set?

Answer 1

It's a Bash feature called "tilde expansion". It's a function of the shell, not the OS. You'll get different behavior with csh, for example.

To answer your question about where the information comes from: your home directory comes from the variable $HOME (no matter what you store there), while other user's homes are retrieved real-time using getpwent(). This function is usually controlled by NSS; so by default values are pulled out of /etc/passwd, though it can be configured to retrieve the information using any source desired, such as NIS, LDAP or an SQL database.

Tilde expansion is more than home directory lookup. Here's a summary:

~              $HOME
~fred          (freds home dir)

~+             $PWD       (your current working directory)
~-             $OLDPWD    (your previous directory)
~1             `dirs +1`
~2             `dirs +2`
~-1            `dirs -1`

dirs and ~1, ~-1, etc., are used in conjunction with pushd and popd.