rust 52 lines · 10 steps

Building a CLI with clap's derive macros

clap turns annotated Rust structs and enums into a full command-line parser with subcommands, validation, and defaults.

Explained by highlit

clap Intermediate cli derive-macros argument-parsing subcommands validation

1use std::path::PathBuf;
2 
3use clap::{Parser, Subcommand, ValueEnum};
4 
5#[derive(Parser)]
6#[command(name = "imgtool", version, about = "Batch image processor")]
7struct Cli {
8    #[arg(short, long, global = true, action = clap::ArgAction::Count)]
9    verbose: u8,
10 
11    #[arg(long, value_name = "FILE", global = true)]
12    config: Option<PathBuf>,
13 
14    #[command(subcommand)]
15    command: Command,
16}
17 
18#[derive(Subcommand)]
19enum Command {
20    Convert {
21        #[arg(required = true)]
22        inputs: Vec<PathBuf>,
23 
24        #[arg(short, long)]
25        output_dir: PathBuf,
26 
27        #[arg(short, long, value_enum, default_value_t = Format::Png)]
28        format: Format,
29 
30        #[arg(short, long, value_parser = clap::value_parser!(u8).range(1..=100), default_value_t = 85)]
31        quality: u8,
32    },
33    Resize {
34        input: PathBuf,
35 
36        #[arg(short = 'W', long)]
37        width: Option<u32>,
38 
39        #[arg(short = 'H', long)]
40        height: Option<u32>,
41 
42        #[arg(long, conflicts_with_all = ["width", "height"])]
43        scale: Option<f32>,
44    },
45}
46 
47#[derive(Copy, Clone, ValueEnum)]
48enum Format {
49    Png,
50    Jpeg,
51    Webp,
52}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Deriving Parser and Subcommand lets clap generate parsing, help, and validation from your type definitions.
2Attribute options like global, conflicts_with_all, and value_parser encode CLI rules declaratively instead of hand-written checks.
3Modeling optional flags as Option and choices as ValueEnum makes invalid states hard to represent.

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