As I understand the windows or linux os is an interface which relates the user with the shell(Dos,bash...etc) which in turn communicate with the computer HW-through other layers of lower level- to perform the operations..is that right ?

then whats under windows is DOS ? I mean if I bought a computer with no OS I will found DOS ? and what about cmd.exe, is it DOS or some other shell ?

I know that GNU Linux works with a bash shell right ? so if installed Linux on a computer the bash shell will replace the DOS ?

sorry for my long topic I just want tounderstand this issue simply

thank you