bash aritmethic - ghdrako/doc_snipets GitHub Wiki

The shell allows arithmetic expressions to be evaluated, as one of the

• shell expansions or
• by using the (( compound command,
• the let builtin, or
• the -i option to the declare builtin.

shell expansions - double parentheses syntax

A more modern and convenient way to perform arithmetic operations is using the double parentheses syntax. Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is:

$(( expression ))

#add integers a to b and assign to c:
a=5
b=7
c=$((a+b))

# Perform addition
result=$((5 + 3))
echo $result
# Output: 8

# Perform multiplication
result=$((10 * 2))
echo $result
# Output: 20

# Perform arithmetic operation with precedence
result=$((1 + 2 * 3))
echo $result
# Output: 7


# Initialize value
value=0

# Increment value by 1
((value++))
echo $value
# Output: 1

# Decrement value by 1
((value--))
echo $value
# Output: 0

Aritmetics

$ a=5; a+=2; echo "$a"; unset a
52
$ a=5; let a+=2; echo "$a"; unset a
7
$ declare -i a=5; a+=2; echo "$a"; unset a
7
$ a=5+2; echo "$a"; unset a
5+2
$ declare -i a=5+2; echo "$a"; unset a
7

Most experienced shell scripters prefer to use explicit arithmetic commands (with ((...)) or let) when they want to perform arithmetic. And they don't use declare -i. The same is explicit declaration of an array using declare -a. It is sufficient to write array=(...) insteed. The exception to this is the associative array, which must be declared explicitly: declare -A myarray.

declare -i MYVAR #declare variable as integer
declare -i SEE
X=9
Y=3
SEE=X+Y            # only this one will be arithmetic
SAW=X+Y            # this is just a literal string
SUM=$X+$Y          # this is string concatenation
echo "SEE = $SEE"
echo "SAW = $SAW"
echo "SUM = $SUM"

step=$(( step + 1 )) # $((step++)) will return a numeric value — Which the shell will then take as the name of a command to be executed. If step++ evaluated to 3 then the shell would look for a command named 3.
median_loc=$(( len / 2 ))
dist=$(( dist * 4 ))
if (( OTHERVAL * 10 <= VAL / 5 )) ; then ...

EXPR

The expr command evaluates expressions and prints the result. Basic arithmetic using expr

# Perform addition
result=$(expr 5 + 3)
echo $result
# Output: 8

# Perform multiplication (escape the asterisk)
result=$(expr 10 \* 2)
echo $result
# Output: 20

expr 5 + 4
expr "5 + 4"
expr 5+4
expr 5 \* $1
expr 11 % 2
a=$( expr 10 - 3 )
echo $a # 7

let

let evaluates the expression in a manner similar to the double parentheses syntax

# Perform addition
let result=5+3
echo $result
# Output: 8

# Perform multiplication
let result=10*2
echo $result
# Output: 20

# Initialize value
value=0

# Increment value by 1
let value++
echo $value
# Output: 1

# Decrement value by 1
let value--
echo $value
# Output: 0

Floating-point aritmetics

In Bash, expressions like $((1.5 + 2.3)) will throw an error because Bash does not support floating-point arithmetic natively. Instead, you must rely on external tools like bc and awk.

BC

The shell natively handles only integer arithmetic. For floating-point operations, bc (Basic Calculator)

# Perform division with two decimal places
result=$(echo "scale=2; 5 / 3" | bc)
echo $result
# Output: 1.66

By setting the scale parameter, we can specify the number of decimal places.

echo "12+5" | bc
echo "10^2" | bc
x=`echo "12+5" | bc`
echo $x
#Output:17

pi=`echo "h=10;4*a(1)" | bc -l`
echo $pi

echo "e(3)" | bc -l #exponential^value as output
echo "ibase=2;1111" | bc -l # Convert Decimal to Octal
echo "ibase=2;1111" | bc -l # Convert Binary to Decimal
echo "ibase=2;obase=8;10" | bc -l # Convert Binary to Octal

awk

num1=1.5
num2=2.3
result=$(echo "$num1 $num2" | awk '{print $1 + $2}')
echo "Result using awk: $result"