Rust File Handling

In computer programming, file handling is a way to deal with data in a file. File handling enables us to open, read, write, create and update files on the local system.

File handling is commonly performed by many applications including databases, web servers. It is an important example of how I/O (Input/Output) operations work.

File handling is also generally known as File I/O.

File Struct in Rust

In Rust, the std::fs::File struct represents a file. It allows us to perform read/write operations on a file.

The file I/O is performed through the std::fs module which provides functions for working with the file system.

All methods in the File struct return a variant of the std:io::Result or simply the Result enum.

Let's look at the basics of file I/O in Rust with these operations:

• Opening a file
• Reading from a file
• Writing to a file
• Removing a file
• Appending to a file

Opening a File in Rust

To open a file in Rust, we use the File::open() method. This method takes a file path as an argument and returns a File object. If the file does not exist, it returns an error (Err).

Let's look at an example.

use std::fs::File;

fn main() {
    // Open a file in read only mode in the local file system
    let data_result = File::open("data.txt");

    // Reading a file returns a Result enum
    // Result can be a file or an error
    let data_file = match data_result {
        Ok(file) => file,
        Err(error) => panic!("Problem opening the data file: {:?}", error),
    };

    println!("Data file: {:?}", data_file);
}

Output

Data file: File { fd: 3, path: "/code/rust-practice/data.txt", read: true, write: false }

Here, we import the module std::fs::File on the top of the program to use the file I/O functions.

To open a file, we call File::open("data.txt") which reads the data.txt file in the local file system.

The open() function returns a Result enum which will return the File object or an Err.

Then, we pattern match the data_result variable and panic! if there is an error with opening the file. If opening the file doesn't error, we output the File object.

Reading a File in Rust

To read a file in Rust, we use the read_to_string() method of the std::io:Read trait. This method reads all bytes until end of file (EOF) and copies it to a mutable string.

Here's an example.

use std::fs::File;
use std::io::Read;

fn main() {
    // Read a file in the local file system
    let mut data_file = File::open("data.txt").unwrap();

    // Create an empty mutable string
    let mut file_content = String::new();

    // Copy contents of file to a mutable string
    data_file.read_to_string(&mut file_content).unwrap();

    println!("File content: {:?}", file_content);
}

Output

File content: "The quick brown fox jumps over the lazy dog.\n"

Here, we import two modules: std::fs::File and std::io::Read for reading a file. We first open the file data.txt with File::open("data.txt") method call and bind its result to a variable data_file.

Once we open the file, we use the read_to_string() method which takes an empty mutable string file_content as an argument and copies the content of the file data.txt to file_content.

Writing to a File in Rust

To write to a file in Rust, we can use the write() method from the std::io:Write trait. This method writes contents to a file.

Let's look at an example.

use std::fs::File;
use std::io::Write;

fn main() {
    // Create a file
    let mut data_file = File::create("data.txt").expect("creation failed");

    // Write contents to the file
    data_file.write("Hello, World!".as_bytes()).expect("write failed");

    println!("Created a file data.txt");
}

Output

Created a file data.txt

Here, we import std::fs::File and std::io::Write modules for writing to a file. We first create a file data.txt with the File::create("data.txt") method and bind it to a mutable variable data_file.

After we create a file, we write to the file using the write() method with the content "Hello, World!".

Removing a File in Rust

To remove or delete a file in Rust, we can use the remove_file() method from the std::fs module.

For example,

use std::fs;

fn main() {
    // Remove a file
    fs::remove_file("data.txt").expect("could not remove file");
    
    println!("Removed file data.txt");
}

Output

Removed file data.txt

Here, we import the std::fs module for deleting a file. We use the remove_file() method to delete the file data.txt.

If the operation does not proceed, we return a custom message: could not remove file in case of an error.

If the file data.txt is not found or cannot be removed, we encounter an error.

thread 'main' panicked at 'could not remove file: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/main.rs:5:33

Appending to a File in Rust

To append to a file in Rust, we should open the file in append mode. We can use the append() method in std::fs::OpenOptions which opens a file for appending.

Then, we can use the write() method in std::io::Write trait to write data to the file.

Let's look at an example.

use std::fs::OpenOptions;
use std::io::Write;

fn main() {
    // Open a file with append option
    let mut data_file = OpenOptions::new()
        .append(true)
        .open("data.txt")
        .expect("cannot open file");

    // Write to a file
    data_file
        .write("I am learning Rust!".as_bytes())
        .expect("write failed");

    println!("Appended content to a file");
}

Output

Appended to a file

Here, we import the std::fs::OpenOptions and std::io::Write modules for appending to a file.

The OpenOptions::new() and the append(true) method opens the file data.txt for appending.

Next, we use the write() method from the File object to write additional content "I am learning Rust!" to the file.

To deal with the errors, we chain the expect() method with a custom error message.