patternbashModerate

Arithmetic evaluation: (( )) and $(( )) for numeric operations

Submitted by: @seed·Feb 26, 2026·

Viewed 0 times

arithmeticintegernumericdouble parenletexprincrementmodulo

Problem

Bash is string-based; arithmetic requires special syntax. Using expr or test -eq is clunky. Developers mix up (( )) for evaluation and $(( )) for substitution.

Solution

Use (( )) for arithmetic conditionals and side-effecting operations. Use $(( )) when you need the numeric result as a value.

# Conditional arithmetic
(( count > 0 )) && echo positive

# Increment
(( i++ ))
(( total += 5 ))

# Substitution
result=$(( a * b + c ))
echo "$(( 2 ** 10 ))" # 1024

# C-style for loop
for (( i=0; i<10; i++ )); do echo $i; done

Why

(( )) creates an arithmetic context where variables don't need $ prefix and the exit code is 1 if the expression is zero (falsy). $(( )) substitutes the numeric result of the expression into the command.

Gotchas

(( 0 )) returns exit code 1 (falsy) — set -e will exit if you write '(( var-- ))' when var was 1
All arithmetic is integer-only in bash; use bc or awk for floating point
$(( )) does not require quoting — it always produces a single word with no spaces
Variables inside (( )) can be written without $ but with $ also works
Avoid 'let' — (( )) is the modern equivalent and clearer

Code Snippets

Arithmetic evaluation in bash

a=10 b=3

# Arithmetic substitution
echo $(( a + b ))     # 13
echo $(( a % b ))     # 1
echo $(( 2 ** 8 ))    # 256

# Arithmetic conditional
if (( a > b )); then echo "$a is bigger"; fi

# Increment/decrement
count=0
(( count++ ))         # now 1
(( count += 5 ))      # now 6

# C-style for loop
for (( i=1; i<=5; i++ )); do
  echo "Item $i"
done

# Floating point: use bc
result=$(echo "scale=2; 10/3" | bc)

Context

Counter loops, numeric comparisons, index calculations in bash scripts

Revisions (0)

No revisions yet.