https://stackoverflow.com/questions/12765340/difference-between-parentheses-and-brackets-in-bash-conditionals/12765593#12765593

bash-brackets-conditionals.md

The tests you had listed :

• Single Parenthesis - ( ... ) is creating a subshell
• Double Parenthesis - (( ... )) is for arithmetic operation
• Single Square Bracket - [ ... ] is the syntax for the POSIX test
• Double Square Brackets - [[ ... ]] is the syntax for bash conditional expressions (similar to test but more powerful)