rust read a file line by line (BufReader::lines and read_to_string)

To read a file line by line in Rust, open it with File::open, wrap it in BufReader, and iterate over .lines(): each item is an io::Result<String> without the line ending. For small files, std::fs::read_to_string(path)? reads everything into one String, and .lines() on that string iterates without further I/O errors.

Both approaches

printf 'alpha\r\nbeta\n\ngamma' > input.txt
cat > lines.rs <<'EOF'
use std::fs::{self, File};
use std::io::{self, BufRead, BufReader};

fn main() -> io::Result<()> {
    // Streaming: constant memory, works for large files
    let reader = BufReader::new(File::open("input.txt")?);
    for (number, line) in reader.lines().enumerate() {
        let line = line?;
        println!("{:>2}: {:?}", number + 1, line);
    }

    // Whole file in memory: simplest for small files
    let text = fs::read_to_string("input.txt")?;
    let non_empty: Vec<&str> = text.lines().filter(|l| !l.is_empty()).collect();
    println!("non-empty lines: {:?}", non_empty);

    // A missing file is an io::Error, propagated by ?
    match File::open("missing.txt") {
        Ok(_) => println!("unexpected"),
        Err(e) => println!("error kind: {:?}", e.kind()),
    }
    Ok(())
}
EOF
rustc --edition 2024 lines.rs && ./lines

 1: "alpha"
 2: "beta"
 3: ""
 4: "gamma"
non-empty lines: ["alpha", "beta", "gamma"]
error kind: NotFound

Both methods strip \n and \r\n, so Windows line endings need no extra handling; the last line is returned even without a final newline, and an empty line comes back as an empty string.

Choosing a method

Pitfalls

• ? needs a Result-returning function. Using ? inside fn main() without -> io::Result<()> (or another Result) fails to compile with E0277. Declare the return type as in the example.
• Unwrapping each line. for line in reader.lines() yields Results; printing line directly prints Ok("..."). Use let line = line?; or handle the error.
• Borrowed lines. Lines from read_to_string(...).lines() borrow from the String; keep the String alive while you use them, or convert with .to_string(). See String vs &str.
• Numbers in lines must be parsed: see parse a string to an int.

Confirm it worked

Print the line count and compare it with wc -l (which counts newline characters, so it is one lower when the file has no trailing newline, as in the example).

FAQ