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Imagine that you are in the following situation:

3. You found a computer without supervision.

With a bootable USB key on Ubuntu, you can change the password of the target computer and recover its file system.

First, plug your USB drive into the target computer, choose your key as the boot device for the target computer to boot to the Ubuntu system on your USB drive. On a DELL computer, press F12 to access this selection. This information is visible on the welcome screen of the manufacturer of your computer.

Start by choosing the following option:

Then select the recovery mode:

Launch a root shell:

You access a device as a root user.

If you need to access the internet, select the  network  entry before. To verify that you have access to the web you can  ping 8.8.8.8  on the Google DNS server, then make 8.8.8.8 to check that the DNS resolution works. If that does not work, add this line  nameserver 8.8.8.8  to the file  /etc/resolv.conf  (this will be deleted when you restart the computer) and restart your service  network-manager  with  service network-manager restart .

For the moment the file system is only in read only, type the following command to mount the file system in read / write at the root of your key:

mount -o remount, rw /

Now you can copy the file system to an external hard drive, and then you can choose the new user password by typing:

passwd
If you do not know the users on this machine, you can do ls /home . Certainly a user of this machine corresponds to a directory name in /home/ , or even view the file /etc/passwd , or else you can create a user as you are  root .

Type in the new password and confirm.

You can shut down the computer.

shutdown -h now

You can now turn on the computer normally and log in with the chosen user and password.