Shell by Example: Arithmetic POSIX 

#!/bin/sh

Shell scripting supports arithmetic operations through several mechanisms. This example covers the main ways to do math in shell scripts.

POSIX arithmetic expansion with $(( )):

 

echo "Basic arithmetic with \$(( )):"
echo "  5 + 3 = $((5 + 3))"
echo "  10 - 4 = $((10 - 4))"
echo "  6 * 7 = $((6 * 7))"
echo "  20 / 4 = $((20 / 4))"
echo "  17 % 5 = $((17 % 5))" # Modulo (remainder)

Variables in arithmetic (no $ needed inside):

 

x=10
y=3
echo "With variables (x=$x, y=$y):"
echo "  x + y = $((x + y))"
echo "  x * y = $((x * y))"
echo "  x / y = $((x / y))" # Integer division
echo "  x % y = $((x % y))"

Compound assignment:

 

count=0
count=$((count + 1))
echo "After increment: count = $count"
Bash

Bash provides shorter increment/decrement:

 

((count++))
((count--))
((count += 5))
((count *= 2))

Comparison operators (return 1 for true, 0 for false):

 

a=5
b=10
echo "Comparisons (a=$a, b=$b):"
echo "  a < b: $((a < b))"
echo "  a > b: $((a > b))"
echo "  a == b: $((a == b))"
echo "  a != b: $((a != b))"
echo "  a <= b: $((a <= b))"
echo "  a >= b: $((a >= b))"

Logical operators:

 

echo "Logical operators:"
echo "  1 && 1 = $((1 && 1))"
echo "  1 && 0 = $((1 && 0))"
echo "  1 || 0 = $((1 || 0))"
echo "  !0 = $((!0))"
echo "  !1 = $((!1))"

Bitwise operators:

 

echo "Bitwise operators:"
echo "  5 & 3 = $((5 & 3))"     # AND
echo "  5 | 3 = $((5 | 3))"     # OR
echo "  5 ^ 3 = $((5 ^ 3))"     # XOR
echo "  ~5 = $((~5))"           # NOT
echo "  4 << 2 = $((4 << 2))"   # Left shift
echo "  16 >> 2 = $((16 >> 2))" # Right shift

Ternary operator:

 

max=$((a > b ? a : b))
echo "Max of $a and $b: $max"

Parentheses for grouping:

 

echo "Order of operations:"
echo "  2 + 3 * 4 = $((2 + 3 * 4))"
echo "  (2 + 3) * 4 = $(((2 + 3) * 4))"

Different bases:

 

echo "Different bases:"
echo "  Hex 0xFF = $((0xFF))"
echo "  Octal 077 = $((077))"
echo "  Binary 2#1010 = $((2#1010))" # Bash-specific

Using expr (older method, POSIX):

 

echo "Using expr:"
echo "  5 + 3 = $(expr 5 + 3)"
echo "  10 - 4 = $(expr 10 - 4)"
echo "  6 \* 7 = $(expr 6 \* 7)" # Note: * must be escaped
echo "  20 / 4 = $(expr 20 / 4)"

expr for string length and matching:

 

str="hello"
echo "  Length of '$str': $(expr length "$str")"

Floating-point arithmetic requires external tools:

 

echo "Floating-point with bc:"
echo "  5.5 + 3.2 = $(echo "5.5 + 3.2" | bc)"
echo "  10 / 3 = $(echo "scale=2; 10 / 3" | bc)"
echo "  sqrt(2) = $(echo "scale=4; sqrt(2)" | bc)"

Using awk for floating-point:

 

echo "Floating-point with awk:"
echo "  5.5 * 3.2 = $(awk 'BEGIN {print 5.5 * 3.2}')"
echo "  22 / 7 = $(awk 'BEGIN {printf "%.4f", 22/7}')"
Bash

Bash’s let command:

 

let "result = 5 + 3"
echo "let result: $result"

let "x++"
let "y = x * 2"

Bash’s (( )) for arithmetic in conditions:

 

if ((x > 5)); then
    echo "x is greater than 5"
fi

Declare integer variables:

 

declare -i num
num="5 + 3"  # Automatically evaluated
echo "Declared int: $num"

Random number generation:

 

echo "Random numbers:"
echo "  \$RANDOM: $RANDOM"
echo "  1-100: $((RANDOM % 100 + 1))"

Practical examples:

Calculate percentage:

 
total=250
part=75
percent=$((part * 100 / total))
echo "Percentage: $part of $total = $percent%"

Loop counter:

 
sum=0
i=1
while [ "$i" -le 10 ]; do
  sum=$((sum + i))
  i=$((i + 1))
done
echo "Sum of 1-10: $sum"

Temperature conversion:

 
celsius=25
fahrenheit=$(echo "scale=1; $celsius * 9/5 + 32" | bc)
echo "Temperature: ${celsius}C = ${fahrenheit}F"

Powers (bash only has ** operator, use bc for POSIX):

 
base=2
exp=10
power=$(echo "$base ^ $exp" | bc)
echo "Power: $base^$exp = $power"

echo "Arithmetic examples complete"

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by Jose Diaz-Gonzalez | inspired by Go by Example | source