Arithmetic

Bash does integer arithmetic inside $(( ... )).

a=17
b=5
echo $(( a + b ))
echo $(( a - b ))
echo $(( a * b ))
echo $(( a / b ))
echo $(( a % b ))
echo $(( a ** 2 ))
echo $(( (a + b) * 2 ))

22
12
85
3
2
289
44

• Inside $(( )), variables don't need $.
• Division is integer division — 17 / 5 is 3, with the remainder from %.
• ** raises to a power.

Changing a variable

count=0
(( count++ ))
(( count += 10 ))
echo "$count"

11

(( ... )) on its own (without $) evaluates an expression — handy for updating counters.

Decimals

Bash can't do decimal arithmetic. Use awk, which is available everywhere:

awk 'BEGIN { printf "%.2f\n", 10 / 3 }'
price=19.99
awk -v p="$price" 'BEGIN { printf "%.2f\n", p * 1.2 }'

3.33
23.99

awk -v p="$price" passes a shell variable into awk. On many systems, the bc calculator also works: echo "scale=2; 10 / 3" | bc.

Common mistakes

• echo 7 / 2 just prints the text 7 / 2. Arithmetic needs $(( )).
• Expecting $(( 7 / 2 )) to be 3.5.
• Numbers with leading zeros are read as octal: $(( 08 + 1 )) is an error. Strip leading zeros from dates like 08, or write $(( 10#08 + 1 )).

Practice

Given minutes=135, print it as hours and minutes: 2h 15m.

minutes=135
echo "$(( minutes / 60 ))h $(( minutes % 60 ))m"

2h 15m