let

Simple calculator that performs arithmetic expressions.

Summary

let arg [arg ...]

The main purpose

• Execute one or more arithmetic expressions.

Parameters

arg: arithmetic expression

return value

Returns 1 when the last expression executed by let evaluates to 0, otherwise returns 0. When the divisor of the expression executed by let is 0, 1 is returned and an error is reported.

Operator precedence decreasing table

Operator	Description
id++, id--	increment after variable, decrement after variable
++id, --id	Variable pre-increment, variable pre-decrement
-, +	positive sign, negative sign
!, ~	Logical No, bitwise negation
**	Power operation
*, /, %	````Multiplication, division, remainder```
+, -	Addition, subtraction
<<, >>	Bitwise shift left or right
<=, >=, <, >	Compare
==, !=	equal to, not equal to
&	Bitwise AND
^	Bitwise XOR
|	Bitwise OR
&&	Logical AND
||	Logical OR
expr ? expr : expr	Conditional operator (ternary operator)
=, *=, /=, %=, +=, -=, <<=, >>=, &=, ^=, |=	Assignment

example

# Try executing arithmetic expressions directly in the terminal (like in python's IDLE).
3+4
bash: 3+4: command not found...
# Another way.
3+4
bash: 3: command not found...
# It seems not possible.

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

# let is often used for variable assignment, while the external command expr can directly return the value of an expression.
let 3+4
#7 is not displayed.
# After execution, 7 is displayed, pay attention to the spaces.
expr 3 + 4

# Conditional expression.
if ((8>4)); then
   echo '8 is greater than 4.'
else
   echo 'error'
fi
# Pay attention to the spaces.
if [[ 12 -le 10 ]]; then
   echo 'error'
else
   echo '12 is greater than 10.'
fi

# You can perform arithmetic operations by setting integer attributes through the declare command.
# The local command is similar to this.

# If the integer attribute is not specified, the output is the string 'a+b'.
declare a=3 b=4 c
c=a+b
echo ${c}
# However, you can assign values in the following ways.
c=$((a+b))
echo ${c}
# show 7

# You can add it directly after setting the integer attribute.
declare -i a=3 b=4 c
c=a+b
echo ${c}
# Same as above.
declare -i a
a=2*3
echo ${a}
# Display 6.

Notice

1. This command is a built-in bash command. For related help information, please see the help command.

2. In addition to let, the commands to perform arithmetic calculations also include external commands expr, bc, etc.