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?

~X simply returns the literal "~X" but ~ returns $HOME and ~+ returns $PWD - they are not special other than that they fetch shell variables. i.e. they do not store the values or execute a command to look them up. They are already defined. Have you tried ~- ? I can't think of a use for it, but again, it returns a shell variable $OLDPWD — SDsolar, Jul 31 '17 at 02:00

tylerl · Answer 1 · 2022-02-04T17:50:34.710

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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.

Edited to add:

As Sean Bright pointed out in a comment, the baseline tilde behavior regarding home directories is codified as standard behavior for POSIX-compliant shells. Additionally, the wordexp() C API function is specified to implement this behavior. Though, obviously, use with caution.

edited Feb 04 '22 at 17:50

answered Nov 02 '09 at 08:41

tylerl

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as a note [tag:fish] doesn't support any of them except the plain tilde (~) – Feb 18 '17 at 10:03
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why they came up with `~+`? seems rather useless. – cregox May 11 '17 at 09:06
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~+ is roughly the same as . However, to use something like ./file it has to go to the file system and figure out where . is located - it takes a few cycles. Using ~+ saves it the work by simply returning the contents of the shell variable $PWD which is already defined as you traverse the file system. – SDsolar Jul 31 '17 at 02:02
The `~` and `~username` forms are [specified by POSIX](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_01), so it's not specifically a bash thing. It may have been added to POSIX since this answer was written though. – Sean Bright Feb 03 '22 at 15:37
@SeanBright Fair enough; my point was that it was a shell thing, and different shells do different things. And other non-shell things (like GUIs or other programs) may do nothing at all. POSIX, after all, is just a standard saying "please implement these features in your stuff." But you're right, it's worth pointing out the standards involved. – tylerl Feb 04 '22 at 17:29
You can add `=~`when doing string comparison in a test. – Timo Mar 31 '22 at 12:05

score 36 · Answer 2 · edited Jul 17 '14 at 16:53

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Those are the home directories of the users. Try cd ~(your username), for example.

edited Jul 17 '14 at 16:53

Sam

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answered Jun 15 '09 at 22:01

Ana Betts

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They're not symlinks, service accounts don't have home directories under /home – Ana Betts Mar 05 '14 at 18:24
Best answer in my case. Also, in my particular case, it was the root directory/folder, not the home folder. – wlwl2 Dec 27 '17 at 09:22

score 19 · Answer 3 · answered Jun 15 '09 at 22:00

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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.

answered Jun 15 '09 at 22:00

Sean Bright

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Indeed they are ~username is the home directory of the specified user – Steve Weet Jun 15 '09 at 22:02

score 15 · Answer 4 · edited Feb 14 '20 at 08:36

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Those are users. Check your /etc/passwd.

cd ~username takes you to that user's home directory.

edited Feb 14 '20 at 08:36

Aulis Ronkainen

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answered Jun 15 '09 at 22:02

Joakim Elofsson

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FTW. Upvote. TNX for clearing that up. I wonder how many cycles and perhaps even a subshell are involved in that one. Just imagine if your system has 3K users, like at a small ISP. Yikes. – SDsolar Jul 31 '17 at 02:05

Jonathan Leffler · Answer 5 · 2009-06-15T23:08:00.060

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).

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 ~-.

About `cd ~+`: you could potentially use it to check if a directory you're currently in still exists - even if it is a pretty unorthodox way of doing so. — professorsloth, Mar 20 '13 at 08:52

score 5 · Answer 6 · answered Jun 15 '09 at 23:03

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Tilde expansion in Bash:

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

answered Jun 15 '09 at 23:03

TheBonsai

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New link: https://wiki.bash-hackers.org/syntax/expansion/tilde – Engineer Jan 03 '23 at 23:01

score 1 · Answer 7 · answered Nov 21 '14 at 18:43

If you're using autofs then the expansion might actually be coming from /etc/auto.home (or similar for your distro). For example, my /etc/auto.master looks like:

/home2 auto.home --timeout 60

and /etc/auto.home looks like:

mgalgs -rw,noquota,intr space:/space/mgalgs

JS. · Answer 8 · 2016-04-29T17:52:52.347

It's possible you're seeing OpenDirectory/ActiveDirectory/LDAP users "automounted" into your home directory.

In *nix, ~ will resolve to your home directory. Likewise ~X will resolve to 'user X'.

Similar to automount for directories, OpenDirectory/ActiveDirectory/LDAP is used in larger/corporate environments to automount user directories. These users may be actual people or they can be machine accounts created to provide various features.

If you type ~Tab you'll see a list of the users on your machine.

Meaning of tilde in Linux bash (not home directory)

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