Module clap::_tutorial

Available on crate feature unstable-doc only.

Documentation: Builder Tutorial

1. Quick Start
2. Configuring the Parser
3. Adding Arguments
   1. Positionals
   2. Options
   3. Flags
   4. Subcommands
   5. Defaults
4. Validation
   1. Enumerated values
   2. Validated values
   3. Argument Relations
   4. Custom Validation
5. Testing

See also

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

Quick Start

You can create an application with several arguments using usage strings.

use std::path::PathBuf;

use clap::{arg, command, value_parser, ArgAction, Command};

fn main() {
    let matches = command!() // requires `cargo` feature
        .arg(arg!([name] "Optional name to operate on"))
        .arg(
            arg!(
                -c --config <FILE> "Sets a custom config file"
            )
            // We don't have syntax yet for optional options, so manually calling `required`
            .required(false)
            .value_parser(value_parser!(PathBuf)),
        )
        .arg(
            arg!(
                -d --debug "Turn debugging information on"
            )
            .action(ArgAction::Count),
        )
        .subcommand(
            Command::new("test")
                .about("does testing things")
                .arg(arg!(-l --list "lists test values").action(ArgAction::SetTrue)),
        )
        .get_matches();

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

    if let Some(config_path) = matches.get_one::<PathBuf>("config") {
        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 matches
        .get_one::<u8>("debug")
        .expect("Count's are defaulted")
    {
        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
    if let Some(matches) = matches.subcommand_matches("test") {
        // "$ myapp test" was run
        if *matches.get_one::<bool>("list").expect("defaulted by clap") {
            // "$ myapp test -l" was run
            println!("Printing testing lists...");
        } else {
            println!("Not printing testing lists...");
        }
    }

    // Continued program logic goes here...
}

$ 01_quick --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    01_quick[EXE] [OPTIONS] [name] [SUBCOMMAND]

ARGS:
    <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 information
    -V, --version          Print version information

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

By default, the program does nothing:

$ 01_quick
Debug mode is off

But you can mix and match the various features

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

Configuring the Parser

You use Command to start building a parser.

use clap::{arg, Command};

fn main() {
    let matches = Command::new("MyApp")
        .version("1.0")
        .author("Kevin K. <kbknapp@gmail.com>")
        .about("Does awesome things")
        .arg(arg!(--two <VALUE>))
        .arg(arg!(--one <VALUE>))
        .get_matches();

    println!(
        "two: {:?}",
        matches.get_one::<String>("two").expect("required")
    );
    println!(
        "one: {:?}",
        matches.get_one::<String>("one").expect("required")
    );
}

$ 02_apps --help
MyApp 1.0
Kevin K. <kbknapp@gmail.com>
Does awesome things

USAGE:
    02_apps[EXE] --two <VALUE> --one <VALUE>

OPTIONS:
    -h, --help           Print help information
        --one <VALUE>    
        --two <VALUE>    
    -V, --version        Print version information

$ 02_apps --version
MyApp 1.0

You can use command!() to fill these fields in from your Cargo.toml file. This requires the cargo feature flag.

use clap::{arg, command};

fn main() {
    // requires `cargo` feature, reading name, version, author, and description from `Cargo.toml`
    let matches = command!()
        .arg(arg!(--two <VALUE>))
        .arg(arg!(--one <VALUE>))
        .get_matches();

    println!(
        "two: {:?}",
        matches.get_one::<String>("two").expect("required")
    );
    println!(
        "one: {:?}",
        matches.get_one::<String>("one").expect("required")
    );
}

$ 02_crate --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    02_crate[EXE] --two <VALUE> --one <VALUE>

OPTIONS:
    -h, --help           Print help information
        --one <VALUE>    
        --two <VALUE>    
    -V, --version        Print version information

$ 02_crate --version
clap [..]

You can use Command methods to change the application level behavior of clap.

use clap::{arg, command, AppSettings, ArgAction};

fn main() {
    let matches = command!() // requires `cargo` feature
        .global_setting(AppSettings::DeriveDisplayOrder)
        .allow_negative_numbers(true)
        .arg(arg!(--two <VALUE>).action(ArgAction::Set))
        .arg(arg!(--one <VALUE>).action(ArgAction::Set))
        .get_matches();

    println!(
        "two: {:?}",
        matches.get_one::<String>("two").expect("required")
    );
    println!(
        "one: {:?}",
        matches.get_one::<String>("one").expect("required")
    );
}

$ 02_app_settings --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    02_app_settings[EXE] --two <VALUE> --one <VALUE>

OPTIONS:
        --two <VALUE>    
        --one <VALUE>    
    -h, --help           Print help information
    -V, --version        Print version information

$ 02_app_settings --one -1 --one -3 --two 10
two: "10"
one: "-3"

Adding Arguments

Positionals

You can have users specify values by their position on the command-line:

use clap::{arg, command};

fn main() {
    let matches = command!() // requires `cargo` feature
        .arg(arg!([NAME]))
        .get_matches();

    println!("NAME: {:?}", matches.get_one::<String>("NAME"));
}

$ 03_03_positional --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_03_positional[EXE] [NAME]

ARGS:
    <NAME>    

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 03_03_positional
NAME: None

$ 03_03_positional bob
NAME: Some("bob")

Options

You can name your arguments with a flag:

• Order doesn’t matter
• They can be optional
• Intent is clearer

use clap::{arg, command};

fn main() {
    let matches = command!() // requires `cargo` feature
        .arg(arg!(-n --name <NAME>).required(false))
        .get_matches();

    println!("name: {:?}", matches.get_one::<String>("name"));
}

$ 03_02_option --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_02_option[EXE] [OPTIONS]

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

$ 03_02_option
name: None

$ 03_02_option --name bob
name: Some("bob")

$ 03_02_option --name=bob
name: Some("bob")

$ 03_02_option -n bob
name: Some("bob")

$ 03_02_option -n=bob
name: Some("bob")

$ 03_02_option -nbob
name: Some("bob")

Flags

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

use clap::{command, Arg, ArgAction};

fn main() {
    let matches = command!() // requires `cargo` feature
        .arg(
            Arg::new("verbose")
                .short('v')
                .long("verbose")
                .action(ArgAction::SetTrue),
        )
        .get_matches();

    println!(
        "verbose: {:?}",
        *matches
            .get_one::<bool>("verbose")
            .expect("defaulted by clap")
    );
}

$ 03_01_flag_bool --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_01_flag_bool[EXE] [OPTIONS]

OPTIONS:
    -h, --help       Print help information
    -v, --verbose    
    -V, --version    Print version information

$ 03_01_flag_bool
verbose: false

$ 03_01_flag_bool --verbose
verbose: true

$ 03_01_flag_bool --verbose --verbose
verbose: true

Or counted.

use clap::{arg, command, ArgAction};

fn main() {
    let matches = command!() // requires `cargo` feature
        .arg(arg!(-v - -verbose).action(ArgAction::Count))
        .get_matches();

    println!(
        "verbose: {:?}",
        matches
            .get_one::<u8>("verbose")
            .expect("Count always defaulted")
    );
}

$ 03_01_flag_count --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_01_flag_count[EXE] [OPTIONS]

OPTIONS:
    -h, --help       Print help information
    -v, --verbose    
    -V, --version    Print version information

$ 03_01_flag_count
verbose: 0

$ 03_01_flag_count --verbose
verbose: 1

$ 03_01_flag_count --verbose --verbose
verbose: 2

Subcommands

Subcommands are defined as Commands that get added via Command::subcommand. Each instance of a Subcommand can have its own version, author(s), Args, and even its own subcommands.

use clap::{arg, command, Command};

fn main() {
    let matches = command!() // requires `cargo` feature
        .propagate_version(true)
        .subcommand_required(true)
        .arg_required_else_help(true)
        .subcommand(
            Command::new("add")
                .about("Adds files to myapp")
                .arg(arg!([NAME])),
        )
        .get_matches();

    match matches.subcommand() {
        Some(("add", sub_matches)) => println!(
            "'myapp add' was used, name is: {:?}",
            sub_matches.get_one::<String>("NAME")
        ),
        _ => unreachable!("Exhausted list of subcommands and subcommand_required prevents `None`"),
    }
}

$ 03_04_subcommands help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_04_subcommands[EXE] <SUBCOMMAND>

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

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

$ 03_04_subcommands help add
03_04_subcommands[EXE]-add [..]
Adds files to myapp

USAGE:
    03_04_subcommands[EXE] add [NAME]

ARGS:
    <NAME>    

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 03_04_subcommands add bob
'myapp add' was used, name is: Some("bob")

Because we set Command::arg_required_else_help:

$ 03_04_subcommands
? failed
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_04_subcommands[EXE] <SUBCOMMAND>

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

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

Because we set Command::propagate_version:

$ 03_04_subcommands --version
clap [..]

$ 03_04_subcommands add --version
03_04_subcommands[EXE]-add [..]

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.

use clap::{arg, command};

fn main() {
    let matches = command!() // requires `cargo` feature
        .arg(arg!([NAME]).default_value("alice"))
        .get_matches();

    println!(
        "NAME: {:?}",
        matches
            .get_one::<String>("NAME")
            .expect("default ensures there is always a value")
    );
}

$ 03_05_default_values --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_05_default_values[EXE] [NAME]

ARGS:
    <NAME>    [default: alice]

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 03_05_default_values
NAME: "alice"

$ 03_05_default_values bob
NAME: "bob"

Validation

Enumerated values

If you have arguments of specific values you want to test for, you can use the PossibleValuesParser or Arg::value_parser(["val1", ...]) for short.

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::{arg, command};

fn main() {
    let matches = command!() // requires `cargo` feature
        .arg(
            arg!(<MODE>)
                .help("What mode to run the program in")
                .value_parser(["fast", "slow"]),
        )
        .get_matches();

    // Note, it's safe to call unwrap() because the arg is required
    match matches
        .get_one::<String>("MODE")
        .expect("'MODE' is required and parsing will fail if its missing")
        .as_str()
    {
        "fast" => {
            println!("Hare");
        }
        "slow" => {
            println!("Tortoise");
        }
        _ => unreachable!(),
    }
}

$ 04_01_possible --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_01_possible[EXE] <MODE>

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

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 04_01_possible fast
Hare

$ 04_01_possible slow
Tortoise

$ 04_01_possible medium
? failed
error: "medium" isn't a valid value for '<MODE>'
	[possible values: fast, slow]

For more information try --help

When enabling the derive feature, you can use ValueEnum to take care of the boiler plate for you, giving the same results.

use clap::{arg, builder::PossibleValue, command, value_parser, ValueEnum};

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
enum Mode {
    Fast,
    Slow,
}

// Can also be derived] with feature flag `derive`
impl ValueEnum for Mode {
    fn value_variants<'a>() -> &'a [Self] {
        &[Mode::Fast, Mode::Slow]
    }

    fn to_possible_value<'a>(&self) -> Option<PossibleValue<'a>> {
        Some(match self {
            Mode::Fast => PossibleValue::new("fast"),
            Mode::Slow => PossibleValue::new("slow"),
        })
    }
}

impl std::fmt::Display for Mode {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.to_possible_value()
            .expect("no values are skipped")
            .get_name()
            .fmt(f)
    }
}

impl std::str::FromStr for Mode {
    type Err = String;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        for variant in Self::value_variants() {
            if variant.to_possible_value().unwrap().matches(s, false) {
                return Ok(*variant);
            }
        }
        Err(format!("Invalid variant: {}", s))
    }
}

fn main() {
    let matches = command!() // requires `cargo` feature
        .arg(
            arg!(<MODE>)
                .help("What mode to run the program in")
                .value_parser(value_parser!(Mode)),
        )
        .get_matches();

    // Note, it's safe to call unwrap() because the arg is required
    match matches
        .get_one::<Mode>("MODE")
        .expect("'MODE' is required and parsing will fail if its missing")
    {
        Mode::Fast => {
            println!("Hare");
        }
        Mode::Slow => {
            println!("Tortoise");
        }
    }
}

$ 04_01_enum --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_01_enum[EXE] <MODE>

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

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 04_01_enum fast
Hare

$ 04_01_enum slow
Tortoise

$ 04_01_enum medium
? failed
error: "medium" isn't a valid value for '<MODE>'
	[possible values: fast, slow]

For more information try --help

Validated values

More generally, you can validate and parse into any data type.

use clap::{arg, command, value_parser};

fn main() {
    let matches = command!() // requires `cargo` feature
        .arg(
            arg!(<PORT>)
                .help("Network port to use")
                .value_parser(value_parser!(u16).range(1..)),
        )
        .get_matches();

    // Note, it's safe to call unwrap() because the arg is required
    let port: u16 = *matches
        .get_one::<u16>("PORT")
        .expect("'PORT' is required and parsing will fail if its missing");
    println!("PORT = {}", port);
}

$ 04_02_parse --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_02_parse[EXE] <PORT>

ARGS:
    <PORT>    Network port to use

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 04_02_parse 22
PORT = 22

$ 04_02_parse 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::{arg, command};

fn main() {
    let matches = command!() // requires `cargo` feature
        .arg(
            arg!(<PORT>)
                .help("Network port to use")
                .value_parser(port_in_range),
        )
        .get_matches();

    // Note, it's safe to call unwrap() because the arg is required
    let port: u16 = *matches
        .get_one::<u16>("PORT")
        .expect("'PORT' is required and parsing will fail if its missing");
    println!("PORT = {}", 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!("`{}` isn't a port number", s))?;
    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 --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_02_validate[EXE] <PORT>

ARGS:
    <PORT>    Network port to use

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 04_02_validate 22
PORT = 22

$ 04_02_validate foobar
? failed
error: Invalid value "foobar" for '<PORT>': `foobar` isn't a port number

For more information try --help

$ 04_02_validate 0
? failed
error: Invalid value "0" for '<PORT>': Port not in range 1-65535

For more information try --help

Argument Relations

You can declare dependencies or conflicts between Args or even ArgGroups.

ArgGroups 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 ArgGroups 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.

use std::path::PathBuf;

use clap::{arg, command, value_parser, ArgAction, ArgGroup};

fn main() {
    // Create application like normal
    let matches = command!() // requires `cargo` feature
        // Add the version arguments
        .arg(arg!(--"set-ver" <VER> "set version manually").required(false))
        .arg(arg!(--major         "auto inc major").action(ArgAction::SetTrue))
        .arg(arg!(--minor         "auto inc minor").action(ArgAction::SetTrue))
        .arg(arg!(--patch         "auto inc patch").action(ArgAction::SetTrue))
        // Create a group, make it required, and add the above arguments
        .group(
            ArgGroup::new("vers")
                .required(true)
                .args(&["set-ver", "major", "minor", "patch"]),
        )
        // Arguments can also be added to a group individually, these two arguments
        // are part of the "input" group which is not required
        .arg(
            arg!([INPUT_FILE] "some regular input")
                .value_parser(value_parser!(PathBuf))
                .group("input"),
        )
        .arg(
            arg!(--"spec-in" <SPEC_IN> "some special input argument")
                .required(false)
                .value_parser(value_parser!(PathBuf))
                .group("input"),
        )
        // Now let's assume we have a -c [config] argument which requires one of
        // (but **not** both) the "input" arguments
        .arg(
            arg!(config: -c <CONFIG>)
                .required(false)
                .value_parser(value_parser!(PathBuf))
                .requires("input"),
        )
        .get_matches();

    // 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) = matches.get_one::<String>("set-ver") {
        ver.to_owned()
    } else {
        // Increment the one requested (in a real program, we'd reset the lower numbers)
        let (maj, min, pat) = (
            *matches.get_one::<bool>("major").expect("defaulted by clap"),
            *matches.get_one::<bool>("minor").expect("defaulted by clap"),
            *matches.get_one::<bool>("patch").expect("defaulted by clap"),
        );
        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 matches.contains_id("config") {
        let input = matches
            .get_one::<PathBuf>("INPUT_FILE")
            .unwrap_or_else(|| matches.get_one::<PathBuf>("spec-in").unwrap())
            .display();
        println!(
            "Doing work using input {} and config {}",
            input,
            matches.get_one::<PathBuf>("config").unwrap().display()
        );
    }
}

$ 04_03_relations --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_03_relations[EXE] [OPTIONS] <--set-ver <VER>|--major|--minor|--patch> [INPUT_FILE]

ARGS:
    <INPUT_FILE>    some regular input

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

$ 04_03_relations
? failed
error: The following required arguments were not provided:
    <--set-ver <VER>|--major|--minor|--patch>

USAGE:
    04_03_relations[EXE] [OPTIONS] <--set-ver <VER>|--major|--minor|--patch> [INPUT_FILE]

For more information try --help

$ 04_03_relations --major
Version: 2.2.3

$ 04_03_relations --major --minor
? failed
error: The argument '--major' cannot be used with '--minor'

USAGE:
    04_03_relations[EXE] <--set-ver <VER>|--major|--minor|--patch>

For more information try --help

$ 04_03_relations --major -c config.toml
? failed
error: The following required arguments were not provided:
    <INPUT_FILE|--spec-in <SPEC_IN>>

USAGE:
    04_03_relations[EXE] -c <CONFIG> <--set-ver <VER>|--major|--minor|--patch> <INPUT_FILE|--spec-in <SPEC_IN>>

For more information try --help

$ 04_03_relations --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 std::path::PathBuf;

use clap::{arg, command, value_parser, ArgAction, ErrorKind};

fn main() {
    // Create application like normal
    let mut cmd = command!() // requires `cargo` feature
        // Add the version arguments
        .arg(arg!(--"set-ver" <VER> "set version manually").required(false))
        .arg(arg!(--major         "auto inc major").action(ArgAction::SetTrue))
        .arg(arg!(--minor         "auto inc minor").action(ArgAction::SetTrue))
        .arg(arg!(--patch         "auto inc patch").action(ArgAction::SetTrue))
        // Arguments can also be added to a group individually, these two arguments
        // are part of the "input" group which is not required
        .arg(arg!([INPUT_FILE] "some regular input").value_parser(value_parser!(PathBuf)))
        .arg(
            arg!(--"spec-in" <SPEC_IN> "some special input argument")
                .required(false)
                .value_parser(value_parser!(PathBuf)),
        )
        // Now let's assume we have a -c [config] argument which requires one of
        // (but **not** both) the "input" arguments
        .arg(
            arg!(config: -c <CONFIG>)
                .required(false)
                .value_parser(value_parser!(PathBuf)),
        );
    let matches = cmd.get_matches_mut();

    // 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) = matches.get_one::<String>("set-ver") {
        if *matches.get_one::<bool>("major").expect("defaulted by clap")
            || *matches.get_one::<bool>("minor").expect("defaulted by clap")
            || *matches.get_one::<bool>("patch").expect("defaulted by clap")
        {
            cmd.error(
                ErrorKind::ArgumentConflict,
                "Can't do relative and absolute version change",
            )
            .exit();
        }
        ver.to_string()
    } else {
        // Increment the one requested (in a real program, we'd reset the lower numbers)
        let (maj, min, pat) = (
            *matches.get_one::<bool>("major").expect("defaulted by clap"),
            *matches.get_one::<bool>("minor").expect("defaulted by clap"),
            *matches.get_one::<bool>("patch").expect("defaulted by clap"),
        );
        match (maj, min, pat) {
            (true, false, false) => major += 1,
            (false, true, false) => minor += 1,
            (false, false, true) => patch += 1,
            _ => {
                cmd.error(
                    ErrorKind::ArgumentConflict,
                    "Can only modify one version field",
                )
                .exit();
            }
        };
        format!("{}.{}.{}", major, minor, patch)
    };

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

    // Check for usage of -c
    if matches.contains_id("config") {
        let input = matches
            .get_one::<PathBuf>("INPUT_FILE")
            .or_else(|| matches.get_one::<PathBuf>("spec-in"))
            .unwrap_or_else(|| {
                cmd.error(
                    ErrorKind::MissingRequiredArgument,
                    "INPUT_FILE or --spec-in is required when using --config",
                )
                .exit()
            })
            .display();
        println!(
            "Doing work using input {} and config {}",
            input,
            matches.get_one::<PathBuf>("config").unwrap().display()
        );
    }
}

$ 04_04_custom --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_04_custom[EXE] [OPTIONS] [INPUT_FILE]

ARGS:
    <INPUT_FILE>    some regular input

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

$ 04_04_custom
? failed
error: Can only modify one version field

USAGE:
    04_04_custom[EXE] [OPTIONS] [INPUT_FILE]

For more information try --help

$ 04_04_custom --major
Version: 2.2.3

$ 04_04_custom --major --minor
? failed
error: Can only modify one version field

USAGE:
    04_04_custom[EXE] [OPTIONS] [INPUT_FILE]

For more information try --help

$ 04_04_custom --major -c config.toml
? failed
Version: 2.2.3
error: INPUT_FILE or --spec-in is required when using --config

USAGE:
    04_04_custom[EXE] [OPTIONS] [INPUT_FILE]

For more information try --help

$ 04_04_custom --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::{arg, command, value_parser};

fn main() {
    let matches = cmd().get_matches();

    // Note, it's safe to call unwrap() because the arg is required
    let port: usize = *matches
        .get_one::<usize>("PORT")
        .expect("'PORT' is required and parsing will fail if its missing");
    println!("PORT = {}", port);
}

fn cmd() -> clap::Command<'static> {
    command!() // requires `cargo` feature
        .arg(
            arg!(<PORT>)
                .help("Network port to use")
                .value_parser(value_parser!(usize)),
        )
}

#[test]
fn verify_cmd() {
    cmd().debug_assert();
}