Bash set subtraction

Question

How to subtract a set from another in Bash?

This is similar to: Is there a "set" data structure in bash? but different as it asks how to perform the subtra

Answer 1

uniq -u (manpage) is often the simplest tool for list subtraction:

Usage

uniq [OPTION]... [INPUT [OUTPUT]] 
[...]
-u, --unique
    only print unique lines

Example: list files found in directory a but not in b

$ ls a
file1  file2  file3
$ ls b
file1  file3

$ echo "$(ls a ; ls b)" | sort | uniq -u
file2