Qu'est-ce que le noyau et la coquille

A shell is basically an interface present between the kernel and the user.
A kernel is the very core of a typical OS. 
Meaning. A shell is a CLI (command-line interpreter). 
A kernel is a type of low-level program that 
has its interfacing with the hardware on top of which 
all the applications run (disks, RAM, CPU, etc.).
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