Error and exit
- Rust course
- Intermediate
- Lesson 64/115
- 6 min
Return a proper exit code on error.
Errors and exit
A good CLI writes error messages to stderr and returns a non-zero exit code, so scripts can detect failure. main can return an ExitCode:
use std::process::ExitCode;
fn main() -> ExitCode {
let args: Vec<String> = std::env::args().collect();
match calistir(&args) {
Ok(mesaj) => {
println!("{mesaj}");
ExitCode::SUCCESS
}
Err(e) => {
eprintln!("hata: {e}");
ExitCode::from(1)
}
}
}stdout and stderr
println! writes to standard output, eprintln! to standard error. When the user redirects output to a file (not liste > notlar.txt), error messages don't mix into the file and stay on screen. Exit code convention: 0 success, 1 general error, 2 incorrect usage.
fn cikis_kodu(r: &Result<String, String>) -> u8 {
match r {
Ok(_) => 0,
Err(_) => 1,
}
}Practice Exit codes
Write fn cikis_kodu(r: &Result<String, String>) -> u8. Let main return ExitCode: print the codes for Ok("tamam") and Err("hata") separated by a space, then return ExitCode::SUCCESS.
Starter code
fn main() {
// buraya yaz
}Sample solution
use std::process::ExitCode;
fn cikis_kodu(r: &Result<String, String>) -> u8 {
match r {
Ok(_) => 0,
Err(_) => 1,
}
}
fn main() -> ExitCode {
let a: Result<String, String> = Ok(String::from("tamam"));
let b: Result<String, String> = Err(String::from("hata"));
println!("{} {}", cikis_kodu(&a), cikis_kodu(&b));
ExitCode::SUCCESS
}Expected output
0 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Why is returning <code>ExitCode</code> from <code>main</code> preferred over <code>std::process::exit(1)</code>?
- exit doesn't compile
- ExitCode is smaller
- No difference
- exit terminates immediately and the drops of values on the stack don't run Correct answer
Explanation: A normal return lets cleanups like flushing buffers and closing files happen.
2 Error stream
Err(e) => {
____!("hata: {e}");
ExitCode::from(1)
}Err(e) => {
eprintln!("hata: {e}");
ExitCode::from(1)
}Explanation: eprintln! writes the message to standard error.
3 Concept and meaning
| Term | Meaning |
|---|---|
ExitCode::SUCCESS |
0 |
ExitCode::from(2) |
Incorrect usage |
eprintln! |
stderr |
println! |
stdout |