Is Linux shell a CUI?
Strictly speaking, a "shell" is simply the interface between the
user and the rest of the operating system. In that definition not
all shells on Linux are command lines.
However, most day-to-day usage of the term "shell" does refer to
command line user interfaces.
Finally, "Linux shell" is not a thing. Linux is an operating
system, specifically a kernel, not a shell. Most Linux
distributions use BASH, but I personally prefer ZSH.