let

Evaluate a simple arithmetic expression

$ let "[result = a + b]"

$ let "[x++]"

$ let "[result = (x > 10) ? x : 0]"

$ let --help

let expression [expression ...]

expression
    An arithmetic expression to be evaluated. Multiple expressions can be provided, separated by spaces.

The let command in Linux is a built-in shell command that evaluates arithmetic expressions. It's primarily used within shell scripts to perform calculations and assign the results to variables. let is a legacy command and is generally superseded by more modern and flexible arithmetic evaluation constructs within the shell (such as $((...))). It simplifies basic arithmetic operations, variable assignments and some bitwise operations directly within the shell environment without needing external tools like bc or expr.

The let command treats each argument as an arithmetic expression to be evaluated. Multiple expressions can be provided as separate arguments or joined with spaces. The command performs integer arithmetic and supports common operators like addition, subtraction, multiplication, division, and modulo. The result of the last evaluated expression is set as the exit status; a non-zero result corresponds to a zero exit status, and a zero result corresponds to a non-zero exit status. Variable names can be used directly in expressions, and their values are automatically substituted. Variables do not need to be prefixed with $.

let only supports integer arithmetic. It does not handle floating-point numbers. The exit status can be confusing. It is based on the arithmetic result not success or failure.

The preferred alternative to let is using the $((...)) arithmetic expansion. This provides greater flexibility and is generally easier to read. Example: result=$(( a + b * 2 )).

The exit status of let is 0 if the last expression evaluates to a non-zero value, and 1 if the last expression evaluates to zero. This is the opposite of most other commands which return zero for success.

let "x = 5 + 3": Assigns the value 8 to the variable x.
let "count++": Increments the value of the variable count by 1.
let "result = a * b": Multiplies the values of variables a and b and assigns the result to the variable result.

let is a relatively old command, dating back to early versions of the Bourne shell and subsequently adopted by other shells like bash. It provided a convenient way to perform arithmetic operations within shell scripts before more modern constructs like $((...)) became widely used. While still functional, it's considered less preferred than the more flexible and readable alternatives.

expr(1), bc(1)