Arithmetic in Bash — Integer Math and Calculations

# Basic operations
echo $(( 5 + 3 ))      # 8
echo $(( 10 - 4 ))     # 6
echo $(( 3 * 7 ))      # 21
echo $(( 17 / 5 ))     # 3   (integer division — truncates)
echo $(( 17 % 5 ))     # 2   (modulo — remainder)
echo $(( 2 ** 10 ))    # 1024 (exponentiation)

# Variables don't need $ inside (( )), but it works either way
x=10
y=3
echo $(( x * y ))      # 30
echo $(( $x + $y ))    # 13  (also fine)

# Assign with $(( ))
count=$(( 5 + 3 ))
echo "$count"           # 8

# In-place modification
count=$(( count + 1 ))  # increment
count=$(( count * 2 ))  # double

# Compound assignment operators — shorter
(( count++ ))           # increment by 1 (like C)
(( count-- ))           # decrement by 1
(( count += 5 ))        # add 5 in place
(( count *= 2 ))        # multiply in place
(( count **= 2 ))       # square in place

# (( )) as a statement (no $ needed when you don't capture output)
(( count = 10 ))        # sets count to 10

count=15

# Using (( )) in if
if (( count > 10 )); then
    echo "count is greater than 10"
fi

# Comparison operators inside (( ))
(( a == b ))    # equal
(( a != b ))    # not equal
(( a < b ))     # less than
(( a <= b ))    # less than or equal
(( a > b ))     # greater than
(( a >= b ))    # greater than or equal

# Basic decimal
echo "scale=2; 22 / 7" | bc      # 3.14

# Square root
echo "scale=4; sqrt(2)" | bc     # 1.4142

# Using variables in bc
radius=5
area=$(echo "scale=4; 3.14159 * $radius * $radius" | bc)
echo "Area: $area"               # Area: 78.5397

# bc with a here-string (cleaner)
result=$(bc <<< "scale=3; 10 / 3")
echo "$result"                   # 3.333

#!/usr/bin/env bash
# Count files in a directory and report

dir="${1:-.}"   # default: current directory
count=0

for f in "$dir"/*; do
    [[ -f "$f" ]] && (( count++ ))
done

echo "Found $count files in $dir"

# Show percentage if target is 100
target=100
pct=$(( count * 100 / target ))
echo "That's $pct% of target"