Rust
Error and exit
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,
}
}
Exercise 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.
Show example 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
}
Test yourself
-
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
Show answer
exit terminates immediately and the drops of values on the stack don't run — A normal return lets cleanups like flushing buffers and closing files happen.
-
Error stream
Err(e) => {
____!("hata: {e}");
ExitCode::from(1)
}
Show answer
Err(e) => {
eprintln!("hata: {e}");
ExitCode::from(1)
}
-
Concept and meaning
- ExitCode::SUCCESS
- 0
- ExitCode::from(2)
- Incorrect usage
- eprintln!
- stderr
- println!
- stdout
Next lesson: Error trait
