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## Module _tutorial

Source
Available on crate feature `unstable-doc` only.
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### §Tutorial for the Derive API

See the side bar for the Table of Contents

### §Quick Start

You can create an application declaratively with a `struct` and some attributes.

First, ensure `clap` is available with the `derive` feature flag:

    $ cargo add clap --features derive

Here is a preview of the type of application you can make:

    use std::path::PathBuf;

    use clap::{Parser, Subcommand};

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        /// Optional name to operate on
        name: Option<String>,

        /// Sets a custom config file
        #[arg(short, long, value_name = "FILE")]
        config: Option<PathBuf>,

        /// Turn debugging information on
        #[arg(short, long, action = clap::ArgAction::Count)]
        debug: u8,

        #[command(subcommand)]
        command: Option<Commands>,
    }

    #[derive(Subcommand)]
    enum Commands {
        /// does testing things
        Test {
            /// lists test values
            #[arg(short, long)]
            list: bool,
        },
    }

    fn main() {
        let cli = Cli::parse();

        // You can check the value provided by positional arguments, or option arguments
        if let Some(name) = cli.name.as_deref() {
            println!("Value for name: {name}");
        }

        if let Some(config_path) = cli.config.as_deref() {
            println!("Value for config: {}", config_path.display());
        }

        // You can see how many times a particular flag or argument occurred
        // Note, only flags can have multiple occurrences
        match cli.debug {
            0 => println!("Debug mode is off"),
            1 => println!("Debug mode is kind of on"),
            2 => println!("Debug mode is on"),
            _ => println!("Don't be crazy"),
        }

        // You can check for the existence of subcommands, and if found use their
        // matches just as you would the top level cmd
        match &cli.command {
            Some(Commands::Test { list }) => {
                if *list {
                    println!("Printing testing lists...");
                } else {
                    println!("Not printing testing lists...");
                }
            }
            None => {}
        }

        // Continued program logic goes here...
    }

    $ 01_quick_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 01_quick_derive[EXE] [OPTIONS] [NAME] [COMMAND]

    Commands:
      test  does testing things
      help  Print this message or the help of the given subcommand(s)

    Arguments:
      [NAME]  Optional name to operate on

    Options:
      -c, --config <FILE>  Sets a custom config file
      -d, --debug...       Turn debugging information on
      -h, --help           Print help
      -V, --version        Print version

By default, the program does nothing:

    $ 01_quick_derive
    Debug mode is off

But you can mix and match the various features

    $ 01_quick_derive -dd test
    Debug mode is on
    Not printing testing lists...

See also

- FAQ: When should I use the builder vs derive APIs?
- The cookbook for more application-focused examples

### §Configuring the Parser

You use derive `Parser` to start building a parser.

    use clap::Parser;

    #[derive(Parser)]
    #[command(name = "MyApp")]
    #[command(version = "1.0")]
    #[command(about = "Does awesome things", long_about = None)]
    struct Cli {
        #[arg(long)]
        two: String,
        #[arg(long)]
        one: String,
    }

    fn main() {
        let cli = Cli::parse();

        println!("two: {:?}", cli.two);
        println!("one: {:?}", cli.one);
    }

    $ 02_apps_derive --help
    Does awesome things

    Usage: 02_apps_derive[EXE] --two <TWO> --one <ONE>

    Options:
          --two <TWO>
          --one <ONE>
      -h, --help       Print help
      -V, --version    Print version

    $ 02_apps_derive --version
    MyApp 1.0

You can use `#[command(version, about)]` attribute defaults on the struct to fill these fields in from your `Cargo.toml` file.

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)] // Read from `Cargo.toml`
    struct Cli {
        #[arg(long)]
        two: String,
        #[arg(long)]
        one: String,
    }

    fn main() {
        let cli = Cli::parse();

        println!("two: {:?}", cli.two);
        println!("one: {:?}", cli.one);
    }

    $ 02_crate_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 02_crate_derive[EXE] --two <TWO> --one <ONE>

    Options:
          --two <TWO>
          --one <ONE>
      -h, --help       Print help
      -V, --version    Print version

    $ 02_crate_derive --version
    clap [..]

You can use `#[command]` attributes on the struct to change the application level behavior of clap. Any `Command` builder function can be used as an attribute, like `Command::next_line_help`.

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    #[command(next_line_help = true)]
    struct Cli {
        #[arg(long)]
        two: String,
        #[arg(long)]
        one: String,
    }

    fn main() {
        let cli = Cli::parse();

        println!("two: {:?}", cli.two);
        println!("one: {:?}", cli.one);
    }

    $ 02_app_settings_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 02_app_settings_derive[EXE] --two <TWO> --one <ONE>

    Options:
          --two <TWO>

          --one <ONE>

      -h, --help
              Print help
      -V, --version
              Print version

### §Adding Arguments

1. Positionals
2. Options
3. Flags
4. Optional
5. Defaults
6. Subcommands

Arguments are inferred from the fields of your struct.

#### §Positionals

By default, struct fields define positional arguments:

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        name: String,
    }

    fn main() {
        let cli = Cli::parse();

        println!("name: {:?}", cli.name);
    }

    $ 03_03_positional_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 03_03_positional_derive[EXE] <NAME>

    Arguments:
      <NAME>

    Options:
      -h, --help     Print help
      -V, --version  Print version

    $ 03_03_positional_derive
    ? 2
    error: the following required arguments were not provided:
      <NAME>

    Usage: 03_03_positional_derive[EXE] <NAME>

    For more information, try '--help'.

    $ 03_03_positional_derive bob
    name: "bob"

Note that the default `ArgAction` is `Set`. To accept multiple values, override the action with `Append` via `Vec`:

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        name: Vec<String>,
    }

    fn main() {
        let cli = Cli::parse();

        println!("name: {:?}", cli.name);
    }

    $ 03_03_positional_mult_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 03_03_positional_mult_derive[EXE] [NAME]...

    Arguments:
      [NAME]...

    Options:
      -h, --help     Print help
      -V, --version  Print version

    $ 03_03_positional_mult_derive
    name: []

    $ 03_03_positional_mult_derive bob
    name: ["bob"]

    $ 03_03_positional_mult_derive bob john
    name: ["bob", "john"]

#### §Options

You can name your arguments with a flag:

- Intent of the value is clearer
- Order doesn’t matter

To specify the flags for an argument, you can use `#[arg(short = 'n')]` and/or `#[arg(long = "name")]` attributes on a field. When no value is given (e.g. `#[arg(short)]`), the flag is inferred from the field’s name.

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        #[arg(short, long)]
        name: String,
    }

    fn main() {
        let cli = Cli::parse();

        println!("name: {:?}", cli.name);
    }

    $ 03_02_option_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 03_02_option_derive[EXE] --name <NAME>

    Options:
      -n, --name <NAME>
      -h, --help         Print help
      -V, --version      Print version

    $ 03_02_option_derive
    ? 2
    error: the following required arguments were not provided:
      --name <NAME>

    Usage: 03_02_option_derive[EXE] --name <NAME>

    For more information, try '--help'.

    $ 03_02_option_derive --name bob
    name: "bob"

    $ 03_02_option_derive --name=bob
    name: "bob"

    $ 03_02_option_derive -n bob
    name: "bob"

    $ 03_02_option_derive -n=bob
    name: "bob"

    $ 03_02_option_derive -nbob
    name: "bob"

Note that the default `ArgAction` is `Set`. To accept multiple occurrences, override the action with `Append` via `Vec`:

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        #[arg(short, long)]
        name: Vec<String>,
    }

    fn main() {
        let cli = Cli::parse();

        println!("name: {:?}", cli.name);
    }

    $ 03_02_option_mult_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 03_02_option_mult_derive[EXE] [OPTIONS]

    Options:
      -n, --name <NAME>
      -h, --help         Print help
      -V, --version      Print version

    $ 03_02_option_mult_derive
    name: []

    $ 03_02_option_mult_derive --name bob
    name: ["bob"]

    $ 03_02_option_mult_derive --name bob --name john
    name: ["bob", "john"]

    $ 03_02_option_mult_derive --name bob --name=john -n tom -n=chris -nsteve
    name: ["bob", "john", "tom", "chris", "steve"]

#### §Flags

Flags can also be switches that can be on/off:

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        #[arg(short, long)]
        verbose: bool,
    }

    fn main() {
        let cli = Cli::parse();

        println!("verbose: {:?}", cli.verbose);
    }

    $ 03_01_flag_bool_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 03_01_flag_bool_derive[EXE] [OPTIONS]

    Options:
      -v, --verbose
      -h, --help     Print help
      -V, --version  Print version

    $ 03_01_flag_bool_derive
    verbose: false

    $ 03_01_flag_bool_derive --verbose
    verbose: true

    $ 03_01_flag_bool_derive --verbose --verbose
    ? failed
    error: the argument '--verbose' cannot be used multiple times

    Usage: 03_01_flag_bool_derive[EXE] [OPTIONS]

    For more information, try '--help'.

Note that the default `ArgAction` for a `bool` field is `SetTrue`. To accept multiple flags, override the action with `Count`:

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        #[arg(short, long, action = clap::ArgAction::Count)]
        verbose: u8,
    }

    fn main() {
        let cli = Cli::parse();

        println!("verbose: {:?}", cli.verbose);
    }

    $ 03_01_flag_count_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 03_01_flag_count_derive[EXE] [OPTIONS]

    Options:
      -v, --verbose...
      -h, --help        Print help
      -V, --version     Print version

    $ 03_01_flag_count_derive
    verbose: 0

    $ 03_01_flag_count_derive --verbose
    verbose: 1

    $ 03_01_flag_count_derive --verbose --verbose
    verbose: 2

This also shows that any`Arg` method may be used as an attribute.

#### §Optional

By default, arguments are assumed to be `required`. To make an argument optional, wrap the field’s type in `Option`:

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        name: Option<String>,
    }

    fn main() {
        let cli = Cli::parse();

        println!("name: {:?}", cli.name);
    }

    $ 03_06_optional_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 03_06_optional_derive[EXE] [NAME]

    Arguments:
      [NAME]

    Options:
      -h, --help     Print help
      -V, --version  Print version

    $ 03_06_optional_derive
    name: None

    $ 03_06_optional_derive bob
    name: Some("bob")

#### §Defaults

We’ve previously showed that arguments can be `required` or optional. When optional, you work with a `Option` and can `unwrap_or`. Alternatively, you can set `#[arg(default_value_t)]`.

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        #[arg(default_value_t = 2020)]
        port: u16,
    }

    fn main() {
        let cli = Cli::parse();

        println!("port: {:?}", cli.port);
    }

    $ 03_05_default_values_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 03_05_default_values_derive[EXE] [PORT]

    Arguments:
      [PORT]  [default: 2020]

    Options:
      -h, --help     Print help
      -V, --version  Print version

    $ 03_05_default_values_derive
    port: 2020

    $ 03_05_default_values_derive 22
    port: 22

#### §Subcommands

Subcommands are derived with `#[derive(Subcommand)]` and be added via `#[command(subcommand)]` attribute on the field using that type. Each instance of a Subcommand can have its own version, author(s), Args, and even its own subcommands.

    use clap::{Parser, Subcommand};

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    #[command(propagate_version = true)]
    struct Cli {
        #[command(subcommand)]
        command: Commands,
    }

    #[derive(Subcommand)]
    enum Commands {
        /// Adds files to myapp
        Add { name: Option<String> },
    }

    fn main() {
        let cli = Cli::parse();

        // You can check for the existence of subcommands, and if found use their
        // matches just as you would the top level cmd
        match &cli.command {
            Commands::Add { name } => {
                println!("'myapp add' was used, name is: {name:?}");
            }
        }
    }

We used a struct-variant to define the `add` subcommand. Alternatively, you can use a struct for your subcommand’s arguments:

    use clap::{Args, Parser, Subcommand};

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    #[command(propagate_version = true)]
    struct Cli {
        #[command(subcommand)]
        command: Commands,
    }

    #[derive(Subcommand)]
    enum Commands {
        /// Adds files to myapp
        Add(AddArgs),
    }

    #[derive(Args)]
    struct AddArgs {
        name: Option<String>,
    }

    fn main() {
        let cli = Cli::parse();

        // You can check for the existence of subcommands, and if found use their
        // matches just as you would the top level cmd
        match &cli.command {
            Commands::Add(name) => {
                println!("'myapp add' was used, name is: {:?}", name.name);
            }
        }
    }

    $ 03_04_subcommands_derive help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 03_04_subcommands_derive[EXE] <COMMAND>

    Commands:
      add   Adds files to myapp
      help  Print this message or the help of the given subcommand(s)

    Options:
      -h, --help     Print help
      -V, --version  Print version

    $ 03_04_subcommands_derive help add
    Adds files to myapp

    Usage: 03_04_subcommands_derive[EXE] add [NAME]

    Arguments:
      [NAME]

    Options:
      -h, --help     Print help
      -V, --version  Print version

    $ 03_04_subcommands_derive add bob
    'myapp add' was used, name is: Some("bob")

When specifying commands with `command: Commands`, they are required. By default, a missing subcommand will show help rather than error.

    $ 03_04_subcommands_derive
    ? failed
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 03_04_subcommands_derive[EXE] <COMMAND>

    Commands:
      add   Adds files to myapp
      help  Print this message or the help of the given subcommand(s)

    Options:
      -h, --help     Print help
      -V, --version  Print version

To make a subcommand optional, wrap it in an `Option` (e.g. `command: Option<Commands>`).

Since we specified `#[command(propagate_version = true)]`, the `--version` flag is available in all subcommands:

    $ 03_04_subcommands_derive --version
    clap [..]

    $ 03_04_subcommands_derive add --version
    clap-add [..]

### §Validation

1. Enumerated values
2. Validated values
3. Argument Relations
4. Custom Validation

An appropriate default parser/validator will be selected for the field’s type. See `value_parser!` for more details.

#### §Enumerated values

For example, if you have arguments of specific values you want to test for, you can derive `ValueEnum` (any `PossibleValue` builder function can be used as a `#[value]` attribute on enum variants).

This allows you specify the valid values for that argument. If the user does not use one of those specific values, they will receive a graceful exit with error message informing them of the mistake, and what the possible valid values are

    use clap::{Parser, ValueEnum};

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        /// What mode to run the program in
        #[arg(value_enum)]
        mode: Mode,
    }

    #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, ValueEnum)]
    enum Mode {
        /// Run swiftly
        Fast,
        /// Crawl slowly but steadily
        ///
        /// This paragraph is ignored because there is no long help text for possible values.
        Slow,
    }

    fn main() {
        let cli = Cli::parse();

        match cli.mode {
            Mode::Fast => {
                println!("Hare");
            }
            Mode::Slow => {
                println!("Tortoise");
            }
        }
    }

    $ 04_01_enum_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 04_01_enum_derive[EXE] <MODE>

    Arguments:
      <MODE>
              What mode to run the program in

              Possible values:
              - fast: Run swiftly
              - slow: Crawl slowly but steadily

    Options:
      -h, --help
              Print help (see a summary with '-h')

      -V, --version
              Print version

    $ 04_01_enum_derive -h
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 04_01_enum_derive[EXE] <MODE>

    Arguments:
      <MODE>  What mode to run the program in [possible values: fast, slow]

    Options:
      -h, --help     Print help (see more with '--help')
      -V, --version  Print version

    $ 04_01_enum_derive fast
    Hare

    $ 04_01_enum_derive slow
    Tortoise

    $ 04_01_enum_derive medium
    ? failed
    error: invalid value 'medium' for '<MODE>'
      [possible values: fast, slow]

    For more information, try '--help'.

#### §Validated values

More generally, you can validate and parse into any data type with `Arg::value_parser`.

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        /// Network port to use
        #[arg(value_parser = clap::value_parser!(u16).range(1..))]
        port: u16,
    }

    fn main() {
        let cli = Cli::parse();

        println!("PORT = {}", cli.port);
    }

    $ 04_02_parse_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 04_02_parse_derive[EXE] <PORT>

    Arguments:
      <PORT>  Network port to use

    Options:
      -h, --help     Print help
      -V, --version  Print version

    $ 04_02_parse_derive 22
    PORT = 22

    $ 04_02_parse_derive foobar
    ? failed
    error: invalid value 'foobar' for '<PORT>': invalid digit found in string

    For more information, try '--help'.

    $ 04_02_parse_derive 0
    ? failed
    error: invalid value '0' for '<PORT>': 0 is not in 1..=65535

    For more information, try '--help'.

A custom parser can be used to improve the error messages or provide additional validation:

    use std::ops::RangeInclusive;

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        /// Network port to use
        #[arg(value_parser = port_in_range)]
        port: u16,
    }

    fn main() {
        let cli = Cli::parse();

        println!("PORT = {}", cli.port);
    }

    const PORT_RANGE: RangeInclusive<usize> = 1..=65535;

    fn port_in_range(s: &str) -> Result<u16, String> {
        let port: usize = s
            .parse()
            .map_err(|_| format!("`{s}` isn't a port number"))?;
        if PORT_RANGE.contains(&port) {
            Ok(port as u16)
        } else {
            Err(format!(
                "port not in range {}-{}",
                PORT_RANGE.start(),
                PORT_RANGE.end()
            ))
        }
    }

    $ 04_02_validate_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 04_02_validate_derive[EXE] <PORT>

    Arguments:
      <PORT>  Network port to use

    Options:
      -h, --help     Print help
      -V, --version  Print version

    $ 04_02_validate_derive 22
    PORT = 22

    $ 04_02_validate_derive foobar
    ? failed
    error: invalid value 'foobar' for '<PORT>': `foobar` isn't a port number

    For more information, try '--help'.

    $ 04_02_validate_derive 0
    ? failed
    error: invalid value '0' for '<PORT>': port not in range 1-65535

    For more information, try '--help'.

See `Arg::value_parser` for more details.

#### §Argument Relations

You can declare dependencies or conflicts between `Arg`s or even `ArgGroup`s.

`ArgGroup`s make it easier to declare relations instead of having to list each individually, or when you want a rule to apply “any but not all” arguments.

Perhaps the most common use of `ArgGroup`s is to require one and only one argument to be present out of a given set. Imagine that you had multiple arguments, and you want one of them to be required, but making all of them required isn’t feasible because perhaps they conflict with each other.

`ArgGroup`s are automatically created for a `struct` with its `ArgGroup::id` being the struct’s name.

    use clap::{Args, Parser};

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        #[command(flatten)]
        vers: Vers,

        /// some regular input
        #[arg(group = "input")]
        input_file: Option<String>,

        /// some special input argument
        #[arg(long, group = "input")]
        spec_in: Option<String>,

        #[arg(short, requires = "input")]
        config: Option<String>,
    }

    #[derive(Args)]
    #[group(required = true, multiple = false)]
    struct Vers {
        /// set version manually
        #[arg(long, value_name = "VER")]
        set_ver: Option<String>,

        /// auto inc major
        #[arg(long)]
        major: bool,

        /// auto inc minor
        #[arg(long)]
        minor: bool,

        /// auto inc patch
        #[arg(long)]
        patch: bool,
    }

    fn main() {
        let cli = Cli::parse();

        // Let's assume the old version 1.2.3
        let mut major = 1;
        let mut minor = 2;
        let mut patch = 3;

        // See if --set_ver was used to set the version manually
        let vers = &cli.vers;
        let version = if let Some(ver) = vers.set_ver.as_deref() {
            ver.to_owned()
        } else {
            // Increment the one requested (in a real program, we'd reset the lower numbers)
            let (maj, min, pat) = (vers.major, vers.minor, vers.patch);
            match (maj, min, pat) {
                (true, _, _) => major += 1,
                (_, true, _) => minor += 1,
                (_, _, true) => patch += 1,
                _ => unreachable!(),
            };
            format!("{major}.{minor}.{patch}")
        };

        println!("Version: {version}");

        // Check for usage of -c
        if let Some(config) = cli.config.as_deref() {
            let input = cli
                .input_file
                .as_deref()
                .unwrap_or_else(|| cli.spec_in.as_deref().unwrap());
            println!("Doing work using input {input} and config {config}");
        }
    }

    $ 04_03_relations_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 04_03_relations_derive[EXE] [OPTIONS] <--set-ver <VER>|--major|--minor|--patch> [INPUT_FILE]

    Arguments:
      [INPUT_FILE]  some regular input

    Options:
          --set-ver <VER>      set version manually
          --major              auto inc major
          --minor              auto inc minor
          --patch              auto inc patch
          --spec-in <SPEC_IN>  some special input argument
      -c <CONFIG>
      -h, --help               Print help
      -V, --version            Print version

    $ 04_03_relations_derive
    ? failed
    error: the following required arguments were not provided:
      <--set-ver <VER>|--major|--minor|--patch>

    Usage: 04_03_relations_derive[EXE] <--set-ver <VER>|--major|--minor|--patch> [INPUT_FILE]

    For more information, try '--help'.

    $ 04_03_relations_derive --major
    Version: 2.2.3

    $ 04_03_relations_derive --major --minor
    ? failed
    error: the argument '--major' cannot be used with '--minor'

    Usage: 04_03_relations_derive[EXE] <--set-ver <VER>|--major|--minor|--patch> [INPUT_FILE]

    For more information, try '--help'.

    $ 04_03_relations_derive --major -c config.toml
    ? failed
    error: the following required arguments were not provided:
      <INPUT_FILE|--spec-in <SPEC_IN>>

    Usage: 04_03_relations_derive[EXE] -c <CONFIG> <--set-ver <VER>|--major|--minor|--patch> <INPUT_FILE|--spec-in <SPEC_IN>>

    For more information, try '--help'.

    $ 04_03_relations_derive --major -c config.toml --spec-in input.txt
    Version: 2.2.3
    Doing work using input input.txt and config config.toml

#### §Custom Validation

As a last resort, you can create custom errors with the basics of clap’s formatting.

    use clap::error::ErrorKind;
    use clap::{CommandFactory, Parser};

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        /// set version manually
        #[arg(long, value_name = "VER")]
        set_ver: Option<String>,

        /// auto inc major
        #[arg(long)]
        major: bool,

        /// auto inc minor
        #[arg(long)]
        minor: bool,

        /// auto inc patch
        #[arg(long)]
        patch: bool,

        /// some regular input
        input_file: Option<String>,

        /// some special input argument
        #[arg(long)]
        spec_in: Option<String>,

        #[arg(short)]
        config: Option<String>,
    }

    fn main() {
        let cli = Cli::parse();

        // Let's assume the old version 1.2.3
        let mut major = 1;
        let mut minor = 2;
        let mut patch = 3;

        // See if --set-ver was used to set the version manually
        let version = if let Some(ver) = cli.set_ver.as_deref() {
            if cli.major || cli.minor || cli.patch {
                let mut cmd = Cli::command();
                cmd.error(
                    ErrorKind::ArgumentConflict,
                    "Can't do relative and absolute version change",
                )
                .exit();
            }
            ver.to_owned()
        } else {
            // Increment the one requested (in a real program, we'd reset the lower numbers)
            let (maj, min, pat) = (cli.major, cli.minor, cli.patch);
            match (maj, min, pat) {
                (true, false, false) => major += 1,
                (false, true, false) => minor += 1,
                (false, false, true) => patch += 1,
                _ => {
                    let mut cmd = Cli::command();
                    cmd.error(
                        ErrorKind::ArgumentConflict,
                        "Can only modify one version field",
                    )
                    .exit();
                }
            };
            format!("{major}.{minor}.{patch}")
        };

        println!("Version: {version}");

        // Check for usage of -c
        if let Some(config) = cli.config.as_deref() {
            let input = cli
                .input_file
                .as_deref()
                // 'or' is preferred to 'or_else' here since `Option::as_deref` is 'const'
                .or(cli.spec_in.as_deref())
                .unwrap_or_else(|| {
                    let mut cmd = Cli::command();
                    cmd.error(
                        ErrorKind::MissingRequiredArgument,
                        "INPUT_FILE or --spec-in is required when using --config",
                    )
                    .exit()
                });
            println!("Doing work using input {input} and config {config}");
        }
    }

    $ 04_04_custom_derive --help
    A simple to use, efficient, and full-featured Command Line Argument Parser

    Usage: 04_04_custom_derive[EXE] [OPTIONS] [INPUT_FILE]

    Arguments:
      [INPUT_FILE]  some regular input

    Options:
          --set-ver <VER>      set version manually
          --major              auto inc major
          --minor              auto inc minor
          --patch              auto inc patch
          --spec-in <SPEC_IN>  some special input argument
      -c <CONFIG>
      -h, --help               Print help
      -V, --version            Print version

    $ 04_04_custom_derive
    ? failed
    error: Can only modify one version field

    Usage: clap [OPTIONS] [INPUT_FILE]

    For more information, try '--help'.

    $ 04_04_custom_derive --major
    Version: 2.2.3

    $ 04_04_custom_derive --major --minor
    ? failed
    error: Can only modify one version field

    Usage: clap [OPTIONS] [INPUT_FILE]

    For more information, try '--help'.

    $ 04_04_custom_derive --major -c config.toml
    ? failed
    Version: 2.2.3
    error: INPUT_FILE or --spec-in is required when using --config

    Usage: clap [OPTIONS] [INPUT_FILE]

    For more information, try '--help'.

    $ 04_04_custom_derive --major -c config.toml --spec-in input.txt
    Version: 2.2.3
    Doing work using input input.txt and config config.toml

### §Testing

clap reports most development errors as `debug_assert!`s. Rather than checking every subcommand, you should have a test that calls `Command::debug_assert`:

    use clap::Parser;

    #[derive(Parser)]
    #[command(version, about, long_about = None)]
    struct Cli {
        /// Network port to use
        port: u16,
    }

    fn main() {
        let cli = Cli::parse();

        println!("PORT = {}", cli.port);
    }

    #[test]
    fn verify_cli() {
        use clap::CommandFactory;
        Cli::command().debug_assert();
    }

### §Next Steps

- Cookbook for application-focused examples
- Explore more features in the Derive reference
  - See also `Command`, `Arg`, `ArgGroup`, and `PossibleValue` builder functions which can be used as attributes

For support, see Discussions