[][src]Crate structopt

This crate defines the StructOpt trait and its custom derive.


If you want to disable all the clap features (colors, suggestions, ..) add default-features = false to the structopt dependency:

structopt = { version = "0.3", default-features = false }

Support for paw (the Command line argument paw-rser abstraction for main) is disabled by default, but can be enabled in the structopt dependency with the feature paw:

structopt = { version = "0.3", features = [ "paw" ] }
paw = "1.0"

How to derive(StructOpt)

First, let's look at the example:

use std::path::PathBuf;
use structopt::StructOpt;

#[derive(Debug, StructOpt)]
#[structopt(name = "example", about = "An example of StructOpt usage.")]
struct Opt {
    /// Activate debug mode
    // short and long flags (-d, --debug) will be deduced from the field's name
    #[structopt(short, long)]
    debug: bool,

    /// Set speed
    // we don't want to name it "speed", need to look smart
    #[structopt(short = "v", long = "velocity", default_value = "42")]
    speed: f64,

    /// Input file
    input: PathBuf,

    /// Output file, stdout if not present
    output: Option<PathBuf>,

    /// Where to write the output: to `stdout` or `file`
    out_type: String,

    /// File name: only required when `out` is set to `file`
    #[structopt(name = "FILE", required_if("out_type", "file"))]
    file_name: String,

fn main() {
    let opt = Opt::from_args();
    println!("{:?}", opt);

So derive(StructOpt) tells Rust to generate a command line parser, and the various structopt attributes are simply used for additional parameters.

First, define a struct, whatever its name. This structure corresponds to a clap::App, its fields correspond to clap::Arg (unless they're subcommands), and you can adjust these apps and args by #[structopt(...)] attributes.


#[structopt(...)] attributes fall into two categories:

  • structopt's own magical methods.

    They are used by structopt itself. They come mostly in attr = ["whatever"] form, but some attr(args...) also exist.

  • raw attributes.

    They represent explicit clap::Arg/App method calls. They are what used to be explicit #[structopt(raw(...))] attrs in pre-0.3 structopt

Every structopt attribute looks like comma-separated sequence of methods:

This example is not tested
    short, // method with no arguments - always magical
    long = "--long-option", // method with one argument
    required_if("out", "file"), // method with one and more args
    parse(from_os_str = path::to::parser) // some magical methods have their own syntax

#[structopt(...)] attributes can be placed on top of struct, enum, struct field or enum variant. Attributes on top of struct or enum represent clap::App method calls, field or variant attributes correspond to clap::Arg method calls.

In other words, the Opt struct from the example above will be turned into this (details omitted):

    .about("An example of StructOpt usage.")
    .help("Activate debug mode")
    .help("Set speed")
// and so on

Raw methods

They are the reason why structopt is so flexible.

Each and every method from clap::App and clap::Arg can be used directly - just #[structopt(method_name = single_arg)] or #[structopt(method_name(arg1, arg2))] and it just works. As long as method_name is not one of the magical methods - it's just a method call.

Note: the Arg::raw method is allowed only with true or false literals.

Magical methods

They are the reason why structopt is so easy to use and convenient in most cases. Many of them have defaults, some of them get used even if not mentioned.

Methods may be used on "top level" (on top of a struct, enum or enum variant) and/or on "field-level" (on top of a struct field or inside of an enum variant). Top level (non-magical) methods correspond to App::method calls, field-level methods are Arg::method calls.

This example is not tested
struct Foo {
    field: u32

enum Bar {
    Pineapple {
        chocolate: String

  • name: [name = "name"]

    • On top level: App::new("name").

      The binary name displayed in help messages. Defaults to the crate name given by Cargo.

    • On field-level: Arg::with_name("name").

      The name for the argument the field stands for, this name appears in help messages. Defaults to a name, deduced from a field, see also rename_all.

  • version: [version = "version"]

    Usable only on top level: App::version("version" or env!(CARGO_PKG_VERSION)).

    The version displayed in help messages. Defaults to the crate version given by Cargo. If CARGO_PKG_VERSION is not set no .version() calls will be generated unless requested.

  • no_version: no_version

    Usable only on top level. Prevents default App::version call, i.e when no version = "version" mentioned.

  • author: author [= "author"]

    Usable only on top level: App::author("author" or env!(CARGO_PKG_AUTHOR)).

    Author/maintainer of the binary, this name appears in help messages. Defaults to the crate author given by cargo, but only when author explicitly mentioned.

  • about: about [= "about"]

    Usable only on top level: App::about("about" or env!(CARGO_PKG_DESCRIPTION)).

    Short description of the binary, appears in help messages. Defaults to the crate description given by cargo, but only when about explicitly mentioned.

  • short: short [= "short-opt-name"]

    Usable only on field-level.

  • long: long [= "long-opt-name"]

    Usable only on field-level.

  • rename_all: [rename_all = "kebab"/"snake"/"screaming-snake"/"camel"/"pascal"/"verbatim"]

    Usable only on top level

  • parse: parse(type [= path::to::parser::fn])

    Usable only on field-level.

  • skip: skip = [expr]

    Usable only on field-level.

  • flatten: flatten

    Usable only on field-level.

  • subcommand: subcommand

    Usable only on field-level.

Type magic

One of major things that makes structopt so awesome is it's type magic. Do you want optional positional argument? Use Option<T>! Or perhaps optional argument that optionally takes value ([--opt=[val]])? Use Option<Option<T>>!

Here is the table of types and clap methods they correspond to:

TypeEffectAdded method call to clap::Arg
booltrue if the flag is present.takes_value(false).multiple(false)
Option<T: FromStr>optional positional argument or option.takes_value(true).multiple(false)
Option<Option<T: FromStr>>optional option with optional value.takes_value(true).multiple(false).min_values(0).max_values(1)
Vec<T: FromStr>list of options or the other positional arguments.takes_value(true).multiple(true)
Option<Vec<T: FromStr>optional list of options.takes_values(true).multiple(true).min_values(0)
T: FromStrrequired option or positional argument.takes_value(true).multiple(false).required(!has_default)

The FromStr trait is used to convert the argument to the given type, and the Arg::validator method is set to a method using to_string() (FromStr::Err must implement std::fmt::Display). If you would like to use a custom string parser other than FromStr, see the same titled section below.

Thus, the speed argument is generated as:

    .help("Set speed")

Specifying argument types

There are three types of arguments that can be supplied to each (sub-)command:

  • short (e.g. -h),
  • long (e.g. --help)
  • and positional.

Like clap, structopt defaults to creating positional arguments.

If you want to generate a long argument you can specify either long = $NAME, or just long to get a long flag generated using the field name. The generated casing style can be modified using the rename_all attribute. See the rename_all example for more.

For short arguments, short will use the first letter of the field name by default, but just like the long option it's also possible to use a custom letter through short = $LETTER.

If an argument is renamed using name = $NAME any following call to short or long will use the new name.

Attention: If these arguments are used without an explicit name the resulting flag is going to be renamed using kebab-case if the rename_all attribute was not specified previously. The same is true for subcommands with implicit naming through the related data structure.

use structopt::StructOpt;

#[structopt(rename_all = "kebab-case")]
struct Opt {
    /// This option can be specified with something like `--foo-option
    /// value` or `--foo-option=value`
    foo_option: String,

    /// This option can be specified with something like `-b value` (but
    /// not `--bar-option value`).
    bar_option: String,

    /// This option can be specified either `--baz value` or `-z value`.
    #[structopt(short = "z", long = "baz")]
    baz_option: String,

    /// This option can be specified either by `--custom value` or
    /// `-c value`.
    #[structopt(name = "custom", long, short)]
    custom_option: String,

    /// This option is positional, meaning it is the first unadorned string
    /// you provide (multiple others could follow).
    my_positional: String,

    /// This option is skipped and will be filled with the default value
    /// for its type (in this case 0).
    skipped: u32,


Help messages

Help messages for the whole binary or individual arguments can be specified using the about attribute on the struct and the help attribute on the field, as we've already seen. For convenience, they can also be specified using doc comments. For example:

/// The help message that will be displayed when passing `--help`.
struct Foo {
  /// The description for the arg that will be displayed when passing `--help`.
  bar: String

If it is necessary or wanted to provide a more complex help message then the previous used ones, it could still be a good idea to distinguish between the actual help message a short summary. In this case about and help should only contain the short and concise form while the two additional arguments long_about and long_help can be used to store a descriptive and more in depth message.

If both - the short and the long version of the argument - are present, the user can later chose between the short summary (-h) and the long descriptive version (--help) of the help message. Also in case of subcommands the short help message will automatically be used for the command description inside the parents help message and the long version as command description if help is requested on the actual subcommand.

This feature can also be used with doc comments instead of arguments through proper comment formatting. To be activated it requires, that the first line of the comment is separated from the rest of the comment through an empty line. In this case the first line is used as summary and the whole comment represents the long descriptive message.

/// The help message that will be displayed when passing `--help`.
struct Foo {
  /// Only this summary is visible when passing `-h`.
  /// But the whole comment will be displayed when passing `--help`.
  /// This could be quite useful to provide further hints are usage
  /// examples.
  bar: String

Environment Variable Fallback

It is possible to specify an environment variable fallback option for an arguments so that its value is taken from the specified environment variable if not given through the command-line:

struct Foo {
  #[structopt(short, long, env = "PARAMETER_VALUE")]
  param: String

By default, values from the environment are shown in the help output (i.e. when invoking --help):

$ cargo run -- --help
  -p, --param <param>     [env: PARAMETER_VALUE=env_value]

In some cases this may be undesirable, for example when being used for passing credentials or secret tokens. In those cases you can use hide_env_values to avoid having strucopt emit the actual secret values:

struct Foo {
  #[structopt(long = "secret", env = "SECRET_VALUE", hide_env_values = true)]
  param: String

Skipping fields

Sometimes you may want to add a field to your Opt struct that is not a command line option and clap should know nothing about it. You can ask structopt to skip the field entirely via #[structopt(skip = value)] (value must implement Into<FieldType>) or #[structopt(skip)] if you want assign the field with Default::default() (obviously, the field's type must implement Default).

pub struct Opt {
    #[structopt(long, short)]
    number: u32,

    // these fields are to be assigned with Default::default()

    k: String,
    v: Vec<u32>,

    // these fields get set explicitly

    #[structopt(skip = vec![1, 2, 3])]
    k2: Vec<u32>,
    #[structopt(skip = "cake")] // &str implements Into<String>
    v2: String,


Some applications, especially large ones, split their functionality through the use of "subcommands". Each of these act somewhat like a separate command, but is part of the larger group. One example is git, which has subcommands such as add, commit, and clone, to mention just a few.

clap has this functionality, and structopt supports it through enums:

#[structopt(about = "the stupid content tracker")]
enum Git {
    Add {
        interactive: bool,
        patch: bool,
        files: Vec<PathBuf>
    Fetch {
        dry_run: bool,
        all: bool,
        repository: Option<String>
    Commit {
        message: Option<String>,
        all: bool

Using derive(StructOpt) on an enum instead of a struct will produce a clap::App that only takes subcommands. So git add, git fetch, and git commit would be commands allowed for the above example.

structopt also provides support for applications where certain flags need to apply to all subcommands, as well as nested subcommands:

struct MakeCookie {
    #[structopt(name = "supervisor", default_value = "Puck", long = "supervisor")]
    supervising_faerie: String,
    /// The faerie tree this cookie is being made in.
    tree: Option<String>,
    #[structopt(subcommand)]  // Note that we mark a field as a subcommand
    cmd: Command

enum Command {
    /// Pound acorns into flour for cookie dough.
    Pound {
        acorns: u32
    /// Add magical sparkles -- the secret ingredient!
    Sparkle {
        #[structopt(short, parse(from_occurrences))]
        magicality: u64,
        color: String

// Subcommand can also be externalized by using a 1-uple enum variant
struct Finish {
    time: u32,
    #[structopt(subcommand)]  // Note that we mark a field as a subcommand
    finish_type: FinishType

// subsubcommand!
enum FinishType {
    Glaze {
        applications: u32
    Powder {
        flavor: String,
        dips: u32

Marking a field with structopt(subcommand) will add the subcommands of the designated enum to the current clap::App. The designated enum must also be derived StructOpt. So the above example would take the following commands:

  • make-cookie pound 50
  • make-cookie sparkle -mmm --color "green"
  • make-cookie finish 130 glaze 3

Optional subcommands

A nested subcommand can be marked optional:

struct Foo {
    file: String,
    cmd: Option<Command>

enum Command {


It can sometimes be useful to group related arguments in a substruct, while keeping the command-line interface flat. In these cases you can mark a field as flatten and give it another type that derives StructOpt:

struct Cmdline {
    /// switch on verbosity
    verbose: bool,
    daemon_opts: DaemonOpts,

struct DaemonOpts {
    /// daemon user
    user: String,
    /// daemon group
    group: String,

In this example, the derived Cmdline parser will support the options -v, -u and -g.

This feature also makes it possible to define a StructOpt struct in a library, parse the corresponding arguments in the main argument parser, and pass off this struct to a handler provided by that library.

Custom string parsers

If the field type does not have a FromStr implementation, or you would like to provide a custom parsing scheme other than FromStr, you may provide a custom string parser using parse(...) like this:

use std::num::ParseIntError;
use std::path::PathBuf;

fn parse_hex(src: &str) -> Result<u32, ParseIntError> {
    u32::from_str_radix(src, 16)

struct HexReader {
    #[structopt(short, parse(try_from_str = parse_hex))]
    number: u32,
    #[structopt(short, parse(from_os_str))]
    output: PathBuf,

There are five kinds of custom parsers:

from_strfn(&str) -> T::std::convert::From::from
try_from_strfn(&str) -> Result<T, E>::std::str::FromStr::from_str
from_os_strfn(&OsStr) -> T::std::convert::From::from
try_from_os_strfn(&OsStr) -> Result<T, OsString>(no default function)
from_occurrencesfn(u64) -> Tvalue as T
from_flagfn(bool) -> T::std::convert::From::from

The from_occurrences parser is special. Using parse(from_occurrences) results in the number of flags occurrences being stored in the relevant field or being passed to the supplied function. In other words, it converts something like -vvv to 3. This is equivalent to .takes_value(false).multiple(true). Note that the default parser can only be used with fields of integer types (u8, usize, i64, etc.).

The from_flag parser is also special. Using parse(from_flag) or parse(from_flag = some_func) will result in the field being treated as a flag even if it does not have type bool.

When supplying a custom string parser, bool will not be treated specially:

TypeEffectAdded method call to clap::Arg
Option<T>optional argument.takes_value(true).multiple(false)
Vec<T>list of arguments.takes_value(true).multiple(true)
Trequired argument.takes_value(true).multiple(false).required(!has_default)

In the try_from_* variants, the function will run twice on valid input: once to validate, and once to parse. Hence, make sure the function is side-effect-free.


pub use clap;



A struct that is converted from command line arguments.