Bash Operators

Arithmetic Expansion with $(( ))

Bash performs integer math inside $(( )), called arithmetic expansion. Variables inside it do not need a leading $, though using one is allowed. The result of the expression can be printed directly, stored in another variable, or used anywhere a value is expected. Bash arithmetic only handles integers -- there is no built-in floating point support.

#!/bin/bash
a=10
b=3

echo "Sum: $((a + b))"
echo "Remainder: $((a % b))"
echo "Power: $((a ** 2))"

((a++))
echo "a after increment: $a"

Common Arithmetic Operators

• + and - -- addition and subtraction
• * and / -- multiplication and integer division
• % -- remainder (modulo)
• ** -- exponentiation
• ++ and -- -- increment and decrement

Comparison Operators: Numeric vs String

Numeric and string values use different comparison operators inside [ ] or [[ ]]. Numeric tests use two-letter mnemonics like -eq and -lt, while string tests reuse familiar symbols like = and !=. Mixing them up -- for example using == to compare numbers -- can silently produce wrong results because it performs a string comparison instead of a numeric one.

#!/bin/bash
score=85
passing=60

if [ "$score" -ge "$passing" ]; then
  echo "You passed with $score points"
else
  echo "You did not pass"
fi

String comparisons should be done inside [[ ]] when possible, since it avoids word-splitting surprises and supports pattern matching with ==. Inside plain [ ], always quote the variables being compared so an empty value does not break the test into invalid syntax.

#!/bin/bash
role="admin"

if [[ "$role" == "admin" ]]; then
  echo "Full access granted"
elif [[ -z "$role" ]]; then
  echo "No role set"
else
  echo "Limited access"
fi