How To Use Let Command.

In this tutorial, we will go over the let command syntax, options, and usage examples.

Bash let is a built-in command in Linux systems used for evaluating arithmetic expressions. Unlike other arithmetic evaluation and expansion commands, let is a simple command with its own environment. The let command also allows for arithmetic expansion.

Bash let Statement Syntax

The let command uses the following basic syntax:

let [expression]

In the syntax above, expression is an arithmetic expression you want the let command to evaluate. let only works with expressions that contain integers. The command does not support floating-point numerals.

The let command allows users to evaluate more than one expression simultaneously. In this case, the syntax is:

let [expression 1] [expression 2] … [expression 3]

let Arithmetic Operators

The Bash let command is able to evaluate expressions that contain the arithmetic operators from the table below. The entries are listed in the order of decreasing precedence, with the operators of the same precedence listed in the same row:

Bash let Examples

The let command lets you assign values to variables and perform arithmetic, bitwise, and logical operations. Read the sections below for examples.

Assigning a Value to a Variable

Use the let command to assign a value to a variable with:

let "[variable] = [value]"

For instance, assign the value of 5 to the variable var1 and print out the value as the output:

let "var1 = 5"; echo $var1

Likewise, after setting a variable value, the let command lets you assign the same value to another variable:

let "var1 = 5" "var2=var1"; echo $var2

Performing Basic Arithmetic Operations

After setting variable values, use the let command to perform basic arithmetic operations. For example, set var1 as 6 and var2 as 2 and perform addition, subtraction, multiplication, division, and modulus:

let "var1 = 6" "var2 = 2" "var3=var1+var2"; echo $var3
let "var1 = 6" "var2 = 2" "var3=var1-var2"; echo $var3
let "var1 = 6" "var2 = 2" "var3=var1*var2"; echo $var3
let "var1 = 6" "var2 = 2" "var3=var1/var2"; echo $var3
let "var1 = 6" "var2 = 2" "var3=var1%var2"; echo $var3

In this example, the let command sets the value of var3 as the result of the arithmetic operation and prints out the result.

Performing Exponentiation

The let command requires two integer values to perform exponentiation. The first value (base) is raised to the power of the second value (power):

let "[base] ** [power]"

For example, to raise the base value of 3 to the power of 2 and print the result, use this command:

let "var1 = 3 ** 2"; echo $var1

Performing Unary Operations

Use a unary minus with the let command to change an expression from positive to negative and vice versa:

let "var1 = 2 + 3" "var1=-var1"; echo $var1

The unary plus multiplies the expression by 1 and returns an unchanged value:

let "var1 = 2 + 3" "var1=+var1"; echo $var1

Changing the Variable Value Incrementally

Using the post-increment or post-decrement operators interprets the variable value and then increases or decreases it by 1:

let "var1 = 10" "var2=var1++"; echo $var1 $var2

In the example above, the variable var2 gets the value of var1 (10) before it is increased by 1. The same happens when performing a post-decrement:

let "var1 = 10" "var2=var1--"; echo $var1 $var2

Performing a pre-increment increases the value of var1 by 1 (11), then assigns the new value to var2:

let "var1 = 10" "var2=++var1"; echo $var1 $var2

Performing a pre-decrement does the same. However, this time it decreases the value of var1 by 1 (9):

let "var1 = 10" "var2=--var1"; echo $var1 $var2

Performing Bit Shifts

Performing bit shifts using the let command shifts the order of the digits in a binary number to the left or right. For instance, performing a left bit shift of the number 8 twice multiplies it by two twice:

let "var1 = 8 << 2"; echo $var1

Conversely, performing a right bit shift twice divides the value by 2 twice:

let "var1 = 8 >> 2"; echo $var1

Performing Bitwise Operations

A bitwise negation reverses each digit of a binary number. This operation changes 1 into 0 and vice versa:

let "var1 = 5" "var2=~var1"; echo $var2

In the example above, the let command converts the value of 5 to a 32-bit signed integer (00000000000000000000000000000101) and reverses the digits. This operation results in -6 (11111111111111111111111111111010).

A bitwise AND operator converts a pair of values into 32-bit signed integers and compares the digits in order. The action returns 1 if both digits are 1, or else it returns 0:

let "var1 = 5" "var2 = 10" "var3 = var1&var2"; echo $var3

The bitwise OR operator compares the corresponding digits of two values and returns 1. But, at least one of the bits must be 1:

let "var1 = 5" "var2 = 10" "var3 = var1|var2"; echo $var3

The bitwise XOR operator compares the corresponding digits of two values and returns 0 if the digits match. Otherwise, the operator returns 1:

let "var1 = 5" "var2 = 10" "var3 = var1^var2"; echo $var3

Conclusion

After reading this tutorial, you should know how to use the Bash let command to evaluate arithmetic expressions in the Bash shell.