Crate structopt

source ·
Expand description

This crate defines the StructOpt trait and its custom derrive.


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

structopt = { version = "0.2", default-features = false }

§How to derive(StructOpt)

First, let’s look at an example:

extern crate structopt;

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

#[derive(Debug, StructOpt)]
#[structopt(name = "example", about = "An example of StructOpt usage.")]
struct Opt {
    /// Activate debug mode
    #[structopt(short = "d", long = "debug")]
    debug: bool,
    /// Set speed
    #[structopt(short = "s", long = "speed", default_value = "42")]
    speed: f64,
    /// Input file
    input: PathBuf,
    /// Output file, stdout if not present
    output: Option<PathBuf>,

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 will correspond to a clap::App. Every method of clap::App in the form of fn function_name(self, &str) can be use through attributes placed on the struct. In our example above, the about attribute will become an .about("An example of StructOpt usage.") call on the generated clap::App. There are a few attributes that will default if not specified:

  • name: The binary name displayed in help messages. Defaults to the crate name given by Cargo.
  • version: Defaults to the crate version given by Cargo.
  • author: Defaults to the crate author name given by Cargo.
  • about: Defaults to the crate description given by Cargo.

Methods from clap::App that don’t take an &str can be called by wrapping them in raw(), e.g. to activate colored help text:

extern crate structopt;

use structopt::StructOpt;

#[derive(StructOpt, Debug)]
#[structopt(raw(setting = "structopt::clap::AppSettings::ColoredHelp"))]
struct Opt {
    #[structopt(short = "s")]
    speed: bool,
    #[structopt(short = "d")]
    debug: bool,

Then, each field of the struct not marked as a subcommand corresponds to a clap::Arg. As with the struct attributes, every method of clap::Arg in the form of fn function_name(self, &str) can be used through specifying it as an attribute. The name attribute can be used to customize the Arg::with_name() call (defaults to the field name in kebab-case). For functions that do not take a &str as argument, the attribute can be wrapped in raw(), e. g. raw(aliases = r#"&["alias"]"#, next_line_help = "true").

The type of the field gives the kind of argument:

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)
Vec<T: FromStr>list of options or the other positional arguments.takes_value(true).multiple(true)
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.

extern crate structopt;

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,

§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:

#[structopt(name = "foo")]
/// The help message that will be displayed when passing `--help`.
struct Foo {
  #[structopt(short = "b")]
  /// 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.

#[structopt(name = "foo")]
/// The help message that will be displayed when passing `--help`.
struct Foo {
  #[structopt(short = "b")]
  /// 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


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(name = "git", about = "the stupid content tracker")]
enum Git {
    #[structopt(name = "add")]
    Add {
        #[structopt(short = "i")]
        interactive: bool,
        #[structopt(short = "p")]
        patch: bool,
        files: Vec<PathBuf>
    #[structopt(name = "fetch")]
    Fetch {
        #[structopt(long = "dry-run")]
        dry_run: bool,
        #[structopt(long = "all")]
        all: bool,
        repository: Option<String>
    #[structopt(name = "commit")]
    Commit {
        #[structopt(short = "m")]
        message: Option<String>,
        #[structopt(short = "a")]
        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:

#[structopt(name = "make-cookie")]
struct MakeCookie {
    #[structopt(name = "supervisor", default_value = "Puck", long = "supervisor")]
    supervising_faerie: String,
    #[structopt(name = "tree")]
    /// 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 {
    #[structopt(name = "pound")]
    /// Pound acorns into flour for cookie dough.
    Pound {
        acorns: u32
    #[structopt(name = "sparkle")]
    /// Add magical sparkles -- the secret ingredient!
    Sparkle {
        #[structopt(short = "m", parse(from_occurrences))]
        magicality: u64,
        #[structopt(short = "c")]
        color: String
    #[structopt(name = "finish")]

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

// subsubcommand!
enum FinishType {
    #[structopt(name = "glaze")]
    Glaze {
        applications: u32
    #[structopt(name = "powder")]
    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:

#[structopt(name = "foo")]
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 {
    #[structopt(short = "v", help = "switch on verbosity")]
    verbose: bool,
    daemon_opts: DaemonOpts,

struct DaemonOpts {
    #[structopt(short = "u", help = "daemon user")]
    user: String,
    #[structopt(short = "g", help = "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 = "n", parse(try_from_str = "parse_hex"))]
    number: u32,
    #[structopt(short = "o", 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

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.).

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.


  • Re-export of clap


  • A struct that is converted from command line arguments.