Error Output to stderr

Proper CLI tools distinguish between regular output (stdout) and error messages (stderr) for better shell integration.

Current Problem

Our error messages currently go to stdout:

$ cargo run > output.txt
Problem parsing arguments: not enough arguments

The error appears in output.txt instead of the terminal, making debugging difficult when output is redirected.

Solution: eprintln! Macro

use std::env;
use std::error::Error;
use std::fs;
use std::process;

use minigrep::{search, search_case_insensitive};

fn main() {
    let args: Vec<String> = env::args().collect();

    let config = Config::build(&args).unwrap_or_else(|err| {
        eprintln!("Problem parsing arguments: {err}");
        process::exit(1);
    });

    if let Err(e) = run(config) {
        eprintln!("Application error: {e}");
        process::exit(1);
    }
}

pub struct Config {
    pub query: String,
    pub file_path: String,
    pub ignore_case: bool,
}

impl Config {
    fn build(args: &[String]) -> Result<Config, &'static str> {
        if args.len() < 3 {
            return Err("not enough arguments");
        }

        let query = args[1].clone();
        let file_path = args[2].clone();

        let ignore_case = env::var("IGNORE_CASE").is_ok();

        Ok(Config {
            query,
            file_path,
            ignore_case,
        })
    }
}

fn run(config: Config) -> Result<(), Box<dyn Error>> {
    let contents = fs::read_to_string(config.file_path)?;

    let results = if config.ignore_case {
        search_case_insensitive(&config.query, &contents)
    } else {
        search(&config.query, &contents)
    };

    for line in results {
        println!("{line}");
    }

    Ok(())
}

Testing the Fix

Error case with redirection:

$ cargo run > output.txt
Problem parsing arguments: not enough arguments

Now the error appears on terminal while output.txt remains empty.

Success case with redirection:

$ cargo run -- to poem.txt > output.txt

Terminal shows nothing, output.txt contains results:

Are you nobody, too?
How dreary to be somebody!

Summary

This implementation demonstrates:

  • Code organization: Clean separation between main.rs and lib.rs
  • Error handling: Proper Result types and error propagation
  • Testing: TDD approach with isolated, testable functions
  • CLI patterns: Command line args, environment variables, proper output streams
  • Rust features: Ownership, lifetimes, traits, and error handling

The resulting CLI tool follows Unix conventions and demonstrates production-ready patterns for Rust applications.