Parameter Attributes Reference

This page lists every attribute you can set on individual parameters via param(ident, key = value, ...) blocks inside the #[command] attribute. The parameter keyword is accepted as a synonym for param.

Display attributes

Key	Type	Default	Description
`help`	string	Empty	Short description shown in argument listings
`details`	string	Empty	Long description shown only in `--help` output
`name`	string	Rust parameter name	Override the CLI flag name
`short`	char	None	Single-character alias
`placeholder`	string	Parameter name	Custom help token rendered in angle brackets
`hide`	bool	`false`	Omit this parameter from all help listings

Value resolution attributes

Key	Type	Default	Description
`env`	string	None	Environment variable consulted when the argument is not provided
`default`	string	None	Default value used when neither CLI argument nor environment variable is present

Validation attributes

Key	Type	Default	Description
`choices`	array of strings	None	Set of permitted string values
`conflicts_with`	array of strings	None	Names of parameters that cannot appear alongside this one
`requires`	array of strings	None	Names of parameters that must also be present

Completion attributes

Key	Type	Default	Description
`value_hint`	string	None	Hint for shell completion engines (reserved for future use)

Attribute types

String attributes

param(host, help = "Target host", name = "target", placeholder = "HOST", env = "DEPLOY_HOST", default = "localhost")

Character attributes

param(port, short = 'p')

Boolean attributes

param(secret, hide = true)

Array attributes

param(level, choices = ["debug", "info", "error"])
param(verbose, conflicts_with = ["quiet"])
param(output, requires = ["format"])

Resolution order

For any parameter with env and/or default, values are resolved in this order:

Command-line argument provided by the user
Environment variable specified by env
Default value specified by default
Error if the parameter is required and no value was found

Examples

Full parameter configuration

#[command(
    param(
        host,
        help = "Target host",
        details = "The hostname or IP address of the deployment target.",
        name = "target",
        short = 't',
        placeholder = "HOST",
        env = "DEPLOY_HOST",
        default = "localhost",
    ),
)]
fn deploy(host: Pos<String>) { ... }

Validation with choices

#[command(
    param(level, short = 'l', choices = ["debug", "info", "warn", "error"]),
)]
fn log(level: Named<String>) { ... }

Mutual exclusion

#[command(
    param(verbose, short = 'v', conflicts_with = ["quiet"]),
    param(quiet, short = 'q', conflicts_with = ["verbose"]),
)]
fn build(verbose: bool, quiet: bool) { ... }

Prerequisites

#[command(
    param(output, short = 'o', requires = ["format"]),
    param(format, short = 'f'),
)]
fn build(output: Named<Option<String>>, format: Named<Option<String>>) { ... }

Parameter name resolution

By default the CLI flag name comes from the Rust parameter name. Underscores are preserved in the default name:

Rust name	Default CLI name
`host`	`--host`
`dry_run`	`--dry_run`
`env`	`--env`

Use the name attribute to override this:

param(dry_run, name = "dry-run")

This registers --dry-run instead of --dry_run.

Duplicate short flag validation

Each short character can only be assigned to one parameter within a command. If two parameters share the same short, the macro produces a compile-time error:

error: duplicate short flag '-v' used by multiple parameters

This applies across both named options and flags. A short character used by a named option cannot also be used by a flag in the same command.