bash-hackers-wiki/docs/commands/builtin/let.md

# The let builtin command

## Synopsis

    let arg [arg ...]

## Description

The `let` builtin command evaluates each supplied word from left to
right as an [arithmetic expression](../../syntax/arith_expr.md) and returns an
exit code according to the truth value of the rightmost expression.

-   0 (TRUE) when `arg` evaluated to not 0 (arithmetic "true")
-   1 (FALSE) when `arg` evaluated to 0 (arithmetic "false")

For this return code mapping, please see [this
section](../../syntax/arith_expr.md#arithmetic_expressions_and_return_codes).
They work in the same way as `((`.

## Examples

`let` is very similar to [((](../../syntax/ccmd/arithmetic_eval.md) - the only
difference being `let` is a builtin (simple command), and `((` is a
compound command. The arguments to `let` are therefore subject to all
the same expansions and substitutions as any other simple command -
requiring proper quoting and escaping - whereas the contents of `((`
aren't subject to [word-splitting](../../syntax/expansion/wordsplit.md) or
[pathname expansion](../../syntax/expansion/globs.md) (almost never desirable
for arithmetic). For this reason, **the [arithmetic compound
command](../../syntax/ccmd/arithmetic_eval.md) should generally be preferred
over `let`**.

    $ let 'b = a' "(a += 3) + $((a = 1)), b++"
    $ echo "$a - $b - $?"
    4 - 2 - 0

Is equivalent to the [arithmetic evaluation compound
command](../../syntax/ccmd/arithmetic_eval.md):

    $ (( b = a, (a += 3) + $((a = 1)), b++ ))
    $ echo "$a - $b - $?"
    4 - 2 - 0

!!! INFO ""
    Remember that inside arithmetic evaluation contexts, all
    other expansions are processed as usual (from left-to-right), and the
    resulting text is evaluated as an arithmetic expression. Arithmetic
    already has a way to control precedence using parentheses, so it's very
    rare to need to nest arithmetic expansions within one another. It's
    used above only to illustrate how this precedence works.

Unlike `((`, being a simple command `let` has its own environment. In
Bash, built-ins that can set variables process any arithmetic under
their own environment, which makes the variable effectively "local" to
the builtin unless the variable is also set or modified by the builtin.
This differs in other shells, such as ksh93, where environment
assignments to regular builtins are always local even if the variable is
modified by the builtin.

     ~ $ ( y=1+1 let x=y; declare -p x y )
    declare -- x="2"
    bash: declare: y: not found

     ~ $ ( y=1+1 let x=y++; declare -p x y )
    declare -- x="2"
    declare -- y="3"

This can be useful in certain situations where a temporary variable is
needed.

## Portability considerations

-   the `let` command is not specified by POSIX(r). The portable
    alternative is: `[ "$(( <EXPRESSION> ))" -ne 0 ]`. To make portable
    scripts simpler and cleaner, `let` can be defined as: `# POSIX
    let() {
        IFS=, command eval test '$(($*))' -ne 0
    }
    ` Aside from differences in supported arithmetic features, this
    should be identical to the Bash/Ksh `let`.
-   It seems to be a common misunderstanding that `let` has some legacy
    purpose. Both `let` and [[^1]](../../syntax/ccmd/arithmetic_eval.md) were
    ksh88 features and almost identical in terms of portability as
    everything that inherited one also tended to get the other. Don't
    choose `let` over `((` expecting it to work in more places.
-   [expr(1)](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/expr.html#tag_20_42)
    is a command one is likely to come across sooner or later. While it
    is more "standard" than `let`, the above should always be
    preferred. Both [arithmetic expansion](../../syntax/arith_expr.md)s and the
    `[` test operator are specified by POSIX(r) and satisfy almost all
    of expr's use-cases. Unlike `let`, `expr` cannot assign directly to
    bash variables but instead returns a result on stdout. `expr` takes
    each operator it recognizes as a separate word and then concatenates
    them into a single expression that's evaluated according to it's
    own rules (which differ from shell arithmetic). `let` parses each
    word it recieves on its own and evaluates it as an expression
    without generating any output other than a return code.
-   For unknown reasons, `let` is one of the Bash commands with special
    parsing for arguments formatted like compound array assignments.
    See: [eval](../../commands/builtin/eval.md#portability_considerations) for
    details. There are no known practical uses for this. Parentheses are
    treated as grouping operators and compound assignment is not
    possible by arithmetic expressions.

## See also

-   Internal: [arithmetic expansion](../../syntax/expansion/arith.md)
-   Internal: [arithmetic expressions](../../syntax/arith_expr.md)
-   Internal: [arithmetic evaluation compound
    command](../../syntax/ccmd/arithmetic_eval.md)

[^1]: ...