structopt-derive 0.1.6

Parse command line argument by defining a struct, derive crate.

How to derive(StructOpt)

First, let's look at an example:

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

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.

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). For functions that do not take a &str as argument, the attribute can be called function_name_raw, e. g. aliases_raw = "&[\"alias\"]".

The type of the field gives the kind of argument:

Type Effect Added method call to clap::Arg
bool true if the flag is present .takes_value(false).multiple(false)
u64 number of times the flag is used .takes_value(false).multiple(true)
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: FromStr required 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")

Help messages

Help messages for the whole binary or individual arguments can be specified using the about attribute on the struct/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


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<String>
    #[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", required = false, 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")]
        magicality: u64,
        #[structopt(short = "c")]
        color: String
    #[structopt(name = "finish")]
    Finish {
        #[structopt(short = "t")]
        time: u32,
        #[structopt(subcommand)]  // Note that we mark a field as a subcommand
        type: FinishType

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 {

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 four kinds custom string parsers:

Kind Signature Default
from_str fn(&str) -> T ::std::convert::From::from
try_from_str fn(&str) -> Result<T, E> ::std::str::FromStr::from_str
from_os_str fn(&OsStr) -> T ::std::convert::From::from
try_from_os_str fn(&OsStr) -> Result<T, OsString> (no default function)

When supplying a custom string parser, bool and u64 will not be treated specially:

Type Effect Added method call to clap::Arg
Option<T> optional argument .takes_value(true).multiple(false)
Vec<T> list of arguments .takes_value(true).multiple(true)
T required 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.