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Question 1

What is pathname expansion?

Answer 1

If you use * in command argument,
it is expanded to all files in current directory,
similiary, x*, expands to all files starting with x

Question 2

What is arithmetic expansion?

Answer 2

It is expansion done on $( ( calculation ) ) expression also present in command parameters

Question 3

What is brace expansion?

Answer 3

Replace whole string from space to space with each element in brace separated by comma and append to output.
E.g {A,B}{1,2} produces A1 A2 B1 B1
but {A,B} {1,2} with space - produces A B 1 2