Granting access to a file or directory to another user but not all users

Question

The question is how to allow one user (but not all!) to access files of another user.

I have a directory /home/alice/dir owned by alice:

$ cd /home/alice
$ ls -l
drwxr-x---  2 alice alice       4096 Feb 10 21:24 dir

i.e., owner alice and group alice have read/execute access.

I want to allow user bob access to this directory, but any other user eve should not have access.

What I did so far was:

$ sudo adduser bob alice

and now

$ getent group alice
alice:x:1001:bob

so now bob is in the group alice.

However, still bob cannot access the directory:

$ whoami
bob
$ cd /home/alice
$ pwd
/home/alice
$ ls -l
drwxr-x---  2 alice alice       4096 Feb 10 21:24 dir
$ getent group alice
alice:x:1001:bob
$ cd dir
bash: cd: test: Permission denied

What's wrong?

Users were created with

sudo adduser alice
sudo adduser bob
sudo adduser eve

Answer 1

The way of doing it as described in the question is correct.

However, when you update the groups, the permissions of the current process, including your shell, are not updated. So in my case groups (current shell started by bob before the groups were changed) and groups bob (current permissions of user bob) showed different results:

$whoami
bob
$ groups bob # current membership of bob
bob alice
$ groups     # shell run by bob BEFORE the groups changed
bob
$ cd /home/alice/dir
bash: cd: test: Permission denied

So I had to close the shell, and even close the tunnel (I was connected to the Ubuntu box via a SSH tunnel) and open a new shell (after I connected to SSH again) and then I saw

$whoami
bob
$ groups bob
bob alice
$ groups     # current shell run by bob AFTER the groups changed
bob alice
$ cd /home/alice/dir
$ pwd
/home/alice/dir

If bob is a service, that service is to be restarted!