Linux is a [multi user](https://en.wikipedia.org/wiki/Multi-user_software) operating system.
For an end user's perspective this means we can have multiple accounts on one machine.
For example, one for each member of your household or company.
User can log in, out or switch accounts when seated at the computer or do multiple simultaneous logins via `ssh` or remote desktop software.
From a more internal point of view, Linux uses different accounts to do perfrom different tasks on the system.
These accounts are often called [system accounts](https://unix.stackexchange.com/questions/80277/whats-the-difference-between-a-normal-user-and-a-system-user).
We can list all know accounts with the following command.
We can also **remove** a user via the graphical interface.
### command line
This why you're here no?
To learn how to manager a Linux system like a pro.
So let's look into how to add and remove users via the command line.
As it's an *administrative* task we'll need to prove we have to right to do so.
This implies the use of `sudo`.
```
waldek@debian:~$ sudo add
addgroup addpart add-shell adduser
waldek@debian:~$ apropos adduser
adduser.conf (5) - configuration file for adduser(8) and addgroup(8) .
adduser (8) - add a user or group to the system
waldek@debian:~$ sudo adduser
[sudo] password for waldek:
adduser: Only one or two names allowed.
waldek@debian:~$
```
The `adduser` program takes one or two arguments.
The behaviour changes quite dramatically depending on the number of arguments.
When you give *one* argument, the program will add a user to the system.
When using *two* arguments the program will add a user to a group.
```
waldek@debian:~$ sudo adduser ilies
Adding user `ilies' ...
Adding new group `ilies' (1001) ...
Adding new user `ilies' (1001) with group `ilies' ...
Creating home directory `/home/ilies' ...
Copying files from `/etc/skel' ...
New password:
Retype new password:
passwd: password updated successfully
Changing the user information for ilies
Enter the new value, or press ENTER for the default
Full Name []:
Room Number []:
Work Phone []:
Home Phone []:
Other []:
Is the information correct? [Y/n]
waldek@debian:~$ tail -1 /etc/passwd
ilies:x:1001:1001:,,,:/home/ilies:/bin/bash
waldek@debian:~$ su ilies
Password:
ilies@debian:/home/waldek$
```
If we want to add `ilies` to the group of administrators we can use the same program but with **two** arguments, first the user, then the group in question.
```
ilies@debian:~$ sudo apt update
[sudo] password for ilies:
ilies is not in the sudoers file. This incident will be reported.
1 package can be upgraded. Run 'apt list --upgradable' to see it.
ilies@debian:/home/waldek$
```
`ilies` is now a user account with full administrator rights!
An important detail you should know is that a user's groups are only loaded upon login.
This has as a side effect that when you add a user to a group, they need to log out and back in order for the changes to take effect.
There are [tricks](https://superuser.com/questions/272061/reload-a-linux-users-group-assignments-without-logging-out) to bypass the behaviour though but I must say I always log out and back in myself.
Removing a user is done with the `deluser` program.
Unix like systems differ greatly from a Windows system because they are not just *multi tasking* but also *multi user*.
Users and groups imply [permissions](https://en.wikipedia.org/wiki/File-system_permissions#Notation_of_traditional_Unix_permissions), meaning **who** is allowed to do **what** on a system.
Permissions are tied to a **file system**, either real or virtual, but can't exist outside of this context.
When we list the files in a directory, we can add the `-l` flag to see more information.
In the example below I create a file called `helloworld` and show it's details with `ls -l`.
```
waldek@debian:~$ touch helloworld
waldek@debian:~$ ls -l
total 0
-rw-r--r-- 1 waldek waldek 0 May 16 10:32 helloworld
waldek@debian:~$
```
Next up I create a directory to store my pictures in.
```
waldek@debian:~$ mkdir photos
waldek@debian:~$ ls -l
total 4
-rw-r--r-- 1 waldek waldek 0 May 16 10:32 helloworld
drwxr-xr-x 2 waldek waldek 4096 May 16 10:33 photos
waldek@debian:~$
```
The file and directory above points us to suite a bit of information.
Let's break it down.
The table below slices each individual element apart.
The first column indicates the *type* of file.
This can be a directory, link, block device, character device, among others.
Have a look [here](https://en.wikipedia.org/wiki/Unix_file_types) for an elaborate explanation of all possible types in Linux.
| type | owner | group | others | nr links | owner | group | size | mod date | name |
