Let’s say you are installing Debian Linux from a regular ISO Image CD. You see various menus like these. Now some time while installing you would like to know what is going behind the scenes i.e. you want to see the log of the entire process that is going on when you install Linux.  You can simply do this by hitting:
Ctrl-Alt-F4
key.You will see a text console with lots of messages scrolling specially if you were in the middle of downloading software packages from the Internet.

To go back to your original graphical installer screen press:

Ctrl-Alt-F5

and you will see your regular graphical installer screen.

Additionally you can also execute some commands just as if you were executing from a regularly installed Linux by pressing:

Ctrl-Alt-F2
or
Ctrl-Alt-F3

Basically what you see when you press above combination of keys is a busybox shell where you can execute common GNU/Linux commands like:

mount, mv, cp, lspci, etc

So in nutshell here are is the summary of different console screens (ttys) during Debian Linux installation:
Ctrl-Alt-F2 or Ctrl-Alt-F3 - Busybox shell to execute commands
Ctrl-Alt-F4 - Log screen of the installation process
Ctrl-Alt-F5 - Installation screen which you see by default

Happy behind the scenes.

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