Understanding Bash Brackets: Parentheses, Brackets, and Brace Expansions

This article explains the different types of brackets used in Bash scripting, including parentheses (), double parentheses (( )), square brackets [], double square brackets [[ ]], and curly braces {}.

Single parentheses create a subshell for a command group; variables defined inside are not available outside. They also support command substitution and array initialization (e.g., array=(a b c d)).

Double parentheses perform arithmetic evaluation and C‑style expressions, allowing integer calculations, variable increment ( ((a++))), and complex expressions such as ((i=0; i<5; i++)).

Single square brackets are the test command for file and string comparisons, using operators like -eq, -gt, and logical -a / -o. Double square brackets are Bash keywords offering extended pattern matching, regex ( =~), and safer logical operators.

Curly braces are used for brace expansion (e.g., touch {a,b}.txt or touch {a..d}.txt) and for grouping commands without creating a subshell ( { cmd1; cmd2; }). They also introduce parameter substitution forms such as ${var:-default}, ${var:=default}, ${var:+value}, and ${var:?error}, as well as pattern removal ( ${var%pattern}, ${var%%pattern}, ${var#pattern}, ${var##pattern}) and substring extraction ( ${var:2}, ${var:1:4}).

Examples demonstrate conditional statements using different brackets, loops, and variable manipulations, illustrating how each bracket type affects scope, evaluation, and syntax in Bash scripts.