double-parenthesis

((expression))$((expression))

(( )) is the shell's arithmetic evaluation construct. It evaluates mathematical expressions and returns success (0) if the result is non-zero, failure (1) if zero.Inside ((...)), variables don't need the $ prefix and whitespace is ignored. Standard C-style operators work as expected.$((...)) performs arithmetic expansion, substituting the result. ((...)) evaluates without substitution (for side effects like assignment).

$ # Evaluation (for conditionals/assignments)
(( x = 5 ))
(( x > 3 )) && echo "yes"

# Expansion (to get the value)
echo "Result: $(( 5 + 3 ))"

Arithmetic: +, -, *, /, % (modulo), ** (exponent)Assignment: =, +=, -=, *=, /=, %=Comparison: ==, !=, <, >, <=, >=Logical: && (and), || (or), ! (not)Bitwise: &, |, ^, ~, <<, >>Increment/Decrement: ++, --Ternary: condition ? true : false

((...)) only handles integers. For floating-point, use bc or awk.Division is integer division: $(( 5 / 2 )) gives 2, not 2.5.Return value logic is inverted from typical boolean: (( 0 )) returns failure (exit 1), (( 1 )) returns success (exit 0).This is a bash/zsh feature, not POSIX. Use $((...)) for better portability.

bash(1), bc(1), expr(1), let(1)