You don't need to reinstall your whole operating system just because you forgot your Linux login. It's actually pretty easy ...

When I need to work with Linux user accounts, I tend to default to the command line. Sure, there are GUIs for this purpose, but I find the command line to be more efficient and effective at this task.

J'ai donc dupliquer mes fichiers common du répertoire /etc/pam.d sur un système client/serveur virtuel, et donc refais la commande passwd avec un login utilisateur sans problème: ...

Linux is a multi-user operating system. What does that mean? Like all good operating systems, you can have multiple user accounts on one machine. You can also have more than one user logged in at once ...