let

A simple calculator for performing arithmetic expressions.

Synopsis

let arg [arg ...]

Main Purpose

Execute one or more arithmetic expressions.

Parameters

arg: Arithmetic expression

Return Value

Returns 1 if the result of the last expression evaluated by let is 0; otherwise returns 0. Returns 1 and an error message if an expression executed by let involves division by zero.

Operator Precedence (Descending Order)

Operator	Description
`id++, id--`	Variable post-increment, post-decrement
`++id, --id`	Variable pre-increment, pre-decrement
`-, +`	Unary minus, unary plus
`!, ~`	Logical negation, bitwise negation
`**`	Exponentiation
`*, /, %`	Multiplication, division, modulo
`+, -`	Addition, subtraction
`<<, >>`	Bitwise left shift, right shift
`<=, >=, <, >`	Comparison
`==, !=`	Equality, inequality
`&`	Bitwise AND
`^`	Bitwise XOR
`	`
`&&`	Logical AND
`
`expr ? expr : expr`	Conditional (ternary) operator
`=, *=, /=, %=, +=, -=,` `<<=, >>=, &=, ^=,	=`

Examples

# Attempting to execute an arithmetic expression directly in the terminal (like Python IDLE).
3+4
bash: 3+4: command not found...
# Trying another way.
3 + 4
bash: 3: command not found...
# It seems it doesn't work directly.

# Using the let command for assignment.
let a=3**4
echo ${a}
# Output: 81
# ((...)) is equivalent to the let command.
((a=3**4))

# let is commonly used for variable assignment, while the external command expr can directly return the value of an expression.
let 3+4
# No output (7 is not displayed).
# This outputs 7; note the spaces.
expr 3 + 4

# Conditional expressions.
if ((8>4)); then
  echo '8 is greater than 4.'
else
  echo 'error'
fi
# Note the spaces in the [[]] form.
if [[ 12 -le 10 ]]; then
  echo 'error'
else
  echo '12 is greater than 10.'
fi

# Arithmetic operations can be performed by setting the integer attribute with the declare command.
# The local command works similarly.

# Integer attribute not specified; the output is the string 'a+b'.
declare a=3 b=4 c
c=a+b
echo ${c}
# However, you can assign it like this:
c=$((a+b))
echo ${c}
# Output: 7

# With the integer attribute set, you can perform addition directly.
declare -i a=3 b=4 c
c=a+b
echo ${c}
# Same as above.
declare -i a
a=2*3
echo ${a}
# Output: 6

Notes

This command is a Bash built-in; use the help command for more information.
In addition to let, other commands for arithmetic calculation include external commands like expr and bc.