3. Example with permissions

In this example we will set up a shared directory between the users "bill" and "karen", where they can store their music files.

Most of the commands in this part need root permissions, so let's switch first to the superuser:
```
sudo su
whoami
```

Let's create the users "bill" and "karen":

useradd -m -s /bin/bash bill

ls /home/

useradd -m -s /bin/bash karen

ls /home/

tail /etc/passwd

We also need to create a group for these two users:

groupadd music

tail /etc/group

adduser bill music

adduser karen music

tail /etc/group

Now let's create a directory:
```
mkdir -p /usr/local/share/Music
```
```
ls -ld /usr/local/share/Music
```
To make this directory shareable we need to change the group ownership and the group permissions to allow writing:
```
chown :music /usr/local/share/Music
```
```
chmod 775 /usr/local/share/Music
```
```
ls -ld /usr/local/share/Music
```
Now we have a directory that is owned by root and allows read and write access to the group music. Users bill and karen are members of the group music, so they can create files in this directory. Other users can only list the contents of the directory but cannot create files there.
But we still have a problem. Let's try to create a file as user bill:
```
su -l bill
```
Now let's create an empty file, just for testing:
```
> /usr/local/share/Music/test
```
```
ls -l /usr/local/share/Music
```
The group of the created file is bill (which is the primary group of the user bill). Actually we want the group of the created file to be music, otherwise karen won't be able to access it properly.

We can fix it by giving this command (as root):
```
exit
```
```
chmod g+s /usr/local/share/Music
```
```
ls -ld /usr/local/share/Music
```
When we talked about permissions we did not mention the special permission s. When we give this permission to the group of a directory, the files that are created on this directory will belong to the same group as the directory.

Let's try this by creating another test file as user bill:
```
su -l bill
```
```
> /usr/local/share/Music/test_1
```
```
ls -al /usr/local/share/Music
```
Notice that the second file belongs to the group music.
```
exit
whoami

exit
whoami
```

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