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let

shellLinux/Unix

The let command is one of the most frequently used commands in Linux/Unix-like operating systems. let Perform arithmetic operations on shell variables

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Quick Reference

Command Name:

let

Category:

shell

Platform:

Linux/Unix

Basic Usage:

let [options] [arguments]

Common Use Cases

Syntax

let expression [expression ...]

Options

Operator	Description
`id++, id--`	Post-increment, post-decrement
`++id, --id`	Pre-increment, pre-decrement
`-, +`	Unary minus, plus
`!, ~`	Logical and bitwise negation
`**`	Exponentiation
`*, /, %`	Multiplication, division, remainder
`+, -`	Addition, subtraction
`<<, >>`	Left and right bitwise shifts
`<=, >=, <, >`	Comparison operators
`==, !=`	Equality and inequality
`&`	Bitwise AND
`^`	Bitwise XOR (exclusive OR)
`\|`	Bitwise OR
`&&`	Logical AND
`\|\|`	Logical OR
`expr ? expr : expr`	Conditional operator (ternary)
`=, *=, /=, %=, +=, -=, <<=, >>=, &=, ^=, \|=`	Assignment operators
`expr1 , expr2`	Comma operator (evaluates both, returns expr2)

Examples

How to Use These Examples

The examples below show common ways to use the let command. Try them in your terminal to see the results. You can copy any example by clicking on the code block.

# Basic Examples Basic

let a=5

Assign the value 5 to variable a.

let "a = a + 3"

Add 3 to the current value of a.

# Advanced Examples Advanced

let "a += 1" Increment variable a by 1. let "a = b * c" Multiply b by c and assign the result to a. let "result = (a + b) * c / d" Perform a complex arithmetic calculation. let "a = 2**8" Calculate 2 raised to the power of 8 (exponentiation). let "a = a << 2" Shift bits of a left by 2 (multiply by 4). let "a = a & 0xF" Perform a bitwise AND operation with 0xF (get last 4 bits). let "a = a > b ? a : b" Ternary operation: assign the maximum of a and b to a. let "a++" "b--" Increment a and decrement b in separate expressions.

Try It Yourself

Practice makes perfect! The best way to learn is by trying these examples on your own system with real files.

Understanding Syntax

Pay attention to the syntax coloring: commands, options, and file paths are highlighted differently.

Notes

The 'let' command is a built-in shell command in Bash and other compatible shells that evaluates arithmetic expressions and assigns the result to variables. It provides a way to perform integer arithmetic operations directly within the shell, without having to invoke external commands like 'expr' or 'bc'. Key features of the let command: 1. Integer Arithmetic: let supports all standard arithmetic operations including addition, subtraction, multiplication, division, modulo, and exponentiation, but only for integer values (no floating-point support). 2. Variable Assignment: It can assign the results of arithmetic expressions to shell variables, making it useful for counters, indexing, and other numeric operations in shell scripts. 3. Bitwise Operations: let supports bitwise operations (AND, OR, XOR, shifts), which are particularly useful for low-level operations and flag manipulation. 4. Comparison Operators: It includes comparison operators that return 0 (true) or 1 (false) in the shell's logic, useful for conditional tests. 5. Compound Expressions: Multiple expressions can be evaluated in a single let command, allowing for complex arithmetic operations. 6. Increment/Decrement: Support for C-style increment (++) and decrement (--) operators makes it convenient for loop counters. 7. Return Value: The let command returns a status of 0 (success) if the last expression evaluates to a non-zero value, and 1 (failure) otherwise, which can be used directly in conditional structures. Common use cases for the let command include: - Loop counters in shell scripts (incrementing/decrementing variables) - Calculating array indices - Simple mathematical calculations in scripts - Bit manipulation for flags and configuration values - Testing numeric conditions - Performing modular arithmetic It's worth noting that in modern Bash scripts, the arithmetic expansion syntax $(( ... )) is often preferred over let for clarity and flexibility. The expressions that can be used with let can also be used within $(( ... )), and the latter doesn't require quoting when using spaces or shell special characters. For example, instead of: ```bash let "a = a + 3" ``` Many would now write: ```bash a=$((a + 3)) ``` However, let remains useful in certain contexts, particularly for its ability to evaluate multiple expressions in a single command and its return value behavior, which can be used directly in conditionals.

Related Commands

These commands are frequently used alongside let or serve similar purposes:

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Use Cases

Learn By Doing

The best way to learn Linux commands is by practicing. Try out these examples in your terminal to build muscle memory and understand how the let command works in different scenarios.

See Examples Try Lab Exercises

$ let

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