[Rust Guide]12.8. Writing Error Messages to Standard Error

This article explains how to modify a Rust command-line grep program to separate error messages from standard output by using the `eprintln!` macro instead of `println!`. The key advantage is that when normal output is redirected to a file using `cargo run > output.txt`, error messages will still appear on the screen rather than being written to the file. The change only requires modifying `main.rs` to use `eprintln!` for all error handling, leaving `lib.rs` unchanged.

12.8.0 Before We Begin Chapter 12 builds a sample project: a command-line program. The program is grep Global Regular Expression Print , a tool for global regular-expression searching and output. Its function is to search for specified text in a specified file . This project is divided into these steps: - Receiving command-line arguments - Reading files - Refactoring: improving modules and error handling - Using TDD test-driven development to develop library functionality - Using environment variables Writing error messages to standard error instead of standard output this article If you find this helpful, please like, bookmark, and follow. To keep learning along, follow this series. 12.8.1 Review Here is all the code written up to the previous article. lib.rs : use std::error::Error; use std::fs; pub struct Config { pub query: String, pub filename: String, pub case sensitive: bool, } impl Config { pub fn new args: & String - Result<Config, &'static str { if args.len < 3 { return Err "Not enough arguments" ; } let query = args 1 .clone ; let filename = args 2 .clone ; let case sensitive = std::env::var "IGNORE CASE" .is err ; Ok Config { query, filename, case sensitive, } } } pub fn run config: Config - Result< , Box<dyn Error { let contents = fs::read to string config.filename ?; let results = if config.case sensitive { search &config.query, &contents } else { search case insensitive &config.query, &contents }; for line in results { println "{}", line ; } Ok } pub fn search<'a query: &str, contents: &'a str - Vec<&'a str { let mut results = Vec::new ; for line in contents.lines { if line.contains query { results.push line ; } } results } pub fn search case insensitive<'a query: &str, contents: &'a str - Vec<&'a str { let mut results = Vec::new ; let query = query.to lowercase ; for line in contents.lines { if line.to lowercase .contains &query { results.push line ; } } results } cfg test mod tests { use super:: ; test fn case sensitive { let query = "duct"; let contents = "\ Rust: safe, fast, productive. Pick three. Duct tape."; assert eq vec "safe, fast, productive." , search query, contents ; } test fn case insensitive { let query = "rUsT"; let contents = "\ Rust: safe, fast, productive. Pick three. Trust me."; assert eq vec "Rust:", "Trust me." , search case insensitive query, contents ; } } main.rs : js use std::env; use std::process; use minigrep::Config; fn main { let args: Vec<String = env::args .collect ; let config = Config::new &args .unwrap or else |err| { println "Problem parsing arguments: {}", err ; process::exit 1 ; } ; if let Err e = minigrep::run config { println "Application error: {}", e ; process::exit 1 ; } } 12.8.2 Standard Output vs. Standard Error The current code prints all information, including error messages, to the terminal. Most terminals provide two output streams: standard output stdout and standard error . stderr General information should go to standard output, while error messages should go to standard error. The advantage of this separation is that normal output can be redirected into a file while error messages still appear on the screen. The println macro can only print to standard output. The eprintln macro can print to standard error. Using the current code, run this command in the terminal: cargo run output.txt This redirects output to output.txt , but the command does not include any arguments, so the program should error. Because the error messages are also written to standard output, they end up in output.txt . A better approach is to print error messages to standard error, which keeps standard output clean and separate from errors. 12.8.3 Modifying the Code Changing the code so that error messages go to standard error is quite simple. We only need to change all error printing from println to eprintln . Because all error handling is in main.rs , we only need a small change there, and lib.rs does not need to be modified at all: js use std::env; use std::process; use minigrep::Config; fn main { let args: Vec<String = env::args .collect ; let config = Config::new &args .unwrap or else |err| { eprintln "Problem parsing arguments: {}", err ; process::exit 1 ; } ; if let Err e = minigrep::run config { eprintln "Application error: {}", e ; process::exit 1 ; } } Now run the previous command again. It still has no arguments, so the program errors, but this time it will not put the error message into output.txt ; instead, it will print directly in the terminal: bash $ cargo run output.txt Problem parsing arguments: not enough arguments Then try a normal run with arguments: bash $ cargo run -- to poem.txt output.txt The output is redirected to output.txt . Open it: Are you nobody, too? How dreary to be somebody That is the result we want: errors are printed directly in the terminal, while normal output is redirected into the file.