How to Write Shell Completions for Rust CLIs

Use the clap_complete crate to generate shell completion scripts for your Rust CLI by defining a Command and running the generator for your target shell. Add clap and clap_complete to your Cargo.toml, define your command structure in main.rs, and run the generator to output the script.

use clap::{Command, Arg, value_parser};
use clap_complete::{generate, shells::Bash, Generator};
use std::io;

fn build_cli() -> Command {
    Command::new("my-cli")
        .version("0.1")
        .arg(
            Arg::new("input")
                .short('i')
                .long("input")
                .value_parser(value_parser!(String))
                .help("Sets the input file to use")
        )
}

fn main() {
    let mut cmd = build_cli();
    generate(Bash, &mut cmd, "my-cli", &mut io::stdout());
}

Run cargo run > my-cli.bash to generate the script, then source it in your shell.

Shell completions let your terminal automatically suggest commands and flags as you type, reducing errors and saving time. Think of it like a smart autocomplete feature for your command line that knows exactly what your program accepts. You use this when building a command-line tool so users don't have to memorize every flag.