parenthesis

Run commands in a subshell

$ (cd [/tmp] && [command])

$ array=([one] [two] [three])

$ { [command1]; [command2]; }

$ function_name() { [commands]; }

$ (( count++ ))

( commands )
((arithmetic))

Parentheses ( ) in shell have several distinct uses depending on context:
Subshell execution: Commands in (...) run in a child shell. Environment changes (cd, variable assignments) don't affect the parent shell.
Array literals: In bash/zsh, array=(a b c) creates an array.
Function definition: name() { ... } defines a function (parentheses are part of syntax, not grouping).
Command substitution: $(...) captures command output.
Arithmetic: ((...)) performs arithmetic evaluation, $((...)) expands to the result.

Subshells have overhead from process creation. For simple grouping without isolation, use braces { ...; } instead.
{ } grouping requires a space after { and semicolon before }; parentheses don't.
Subshell exit status is visible to parent, but variable changes are not.
Nested parentheses may need careful quoting to avoid syntax issues.

bash(1), sh(1), test(1)