meaning of tilde in linux bash (not home directory)

Question

Hi all, 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?

Please let me know.

Thanks, jbu

Answer 1

Are they the home directories of users in /etc/passwd? Services like postgres, sendmail, apache, etc., create system users that have home directories just like normal users.

Answer 2

These are the home directories of these users. Try 'cd ~(your username)` for example

Answer 3

those are users, check your /etc/passwd

cd ~username

takes you to that users home dir

Answer 4

On my machine, because of the way I have things set up, doing:

cd ~             # /work1/jleffler
cd ~jleffler     # /u/jleffler

The first pays attention to the value of environment variable $HOME; I deliberately set my $HOME to a local file system instead of an NFS-mounted file system. The second reads from the password file (approximately; NIS complicates things a bit) and finds that the password file says my home directory is /u/jleffler and changes to that directory.

The annoying stuff is that most software behaves as above (and the POSIX specification for the shell requires this behaviour). I use some software (and I don't have much choice about using it) that treats the information from the password file as the current value of $HOME, which is wrong.

Applying this to the question - as others have pointed out, 'cd ~x' goes to the home directory of user 'x', and more generally, whenever tilde expansion is done, ~x means the home directory of user 'x' (and it is an error if user 'x' does not exist).

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It might be worth mentioning that:

cd ~-       # Change to previous directory ($OLDPWD)
cd ~+       # Change to current directory ($PWD)

I can't immediately find a use for '~+', unless you do some weird stuff with moving symlinks in the path leading to the current directory.

You can also do:

cd -

That means the same as ~-.

Answer 5

Tilde expansion in Bash:

http://bash-hackers.org/wiki/doku.php/syntax/expansion/tilde

Answer 6

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   (same effect as ./)
~-             $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.