rust-course/book/writing-material/posts/file.md

## 读取全部内容

```rust
use std::fs::File;
use std::io::prelude::*;
use std::path::Path;

fn main() {
    // Create a path to the desired file
    let path = Path::new("hello.txt");
    let display = path.display();

    // Open the path in read-only mode, returns `io::Result<File>`
    let mut file = match File::open(&path) {
        Err(why) => panic!("couldn't open {}: {}", display, why),
        Ok(file) => file,
    };

    // Read the file contents into a string, returns `io::Result<usize>`
    let mut s = String::new();
    match file.read_to_string(&mut s) {
        Err(why) => panic!("couldn't read {}: {}", display, why),
        Ok(_) => print!("{} contains:\n{}", display, s),
    }

    // `file` goes out of scope, and the "hello.txt" file gets closed
}
```

## 读取全部行
This process is more efficient than creating a String in memory especially working with larger files.

```rust
use std::fs::File;
use std::io::{self, BufRead};
use std::path::Path;

fn main() {
    // File hosts must exist in current path before this produces output
    if let Ok(lines) = read_lines("./hosts") {
        // Consumes the iterator, returns an (Optional) String
        for line in lines {
            if let Ok(ip) = line {
                println!("{}", ip);
            }
        }
    }
}

// The output is wrapped in a Result to allow matching on errors
// Returns an Iterator to the Reader of the lines of the file.
fn read_lines<P>(filename: P) -> io::Result<io::Lines<io::BufReader<File>>>
where P: AsRef<Path>, {
    let file = File::open(filename)?;
    Ok(io::BufReader::new(file).lines())
}
```

## 一些常用的文件操作
```rust
use std::fs;
use std::fs::{File, OpenOptions};
use std::io;
use std::io::prelude::*;
use std::os::unix;
use std::path::Path;

fn cat(path: &Path) -> io::Result<String> {
    let mut f = File::open(path)?;
    let mut s = String::new();
    f.read_to_string(&mut s)?;
    Ok(s)
}

// A simple implementation of `% echo s > path`
fn echo(s: &str, path: &Path) -> io::Result<()> {
    let mut f = File::create(path)?;

    f.write_all(s.as_bytes())
}

// A simple implementation of `% touch path` (ignores existing files)
fn touch(path: &Path) -> io::Result<()> {
    match OpenOptions::new().create(true).write(true).open(path) {
        Ok(_) => Ok(()),
        Err(e) => Err(e),
    }
}

fn main() {
    println!("`mkdir a`");
    // Create a directory, returns `io::Result<()>`
    match fs::create_dir("a") {
        Err(why) => println!("! {:?}", why.kind()),
        Ok(_) => {},
    }

    println!("`echo hello > a/b.txt`");
    // The previous match can be simplified using the `unwrap_or_else` method
    echo("hello", &Path::new("a/b.txt")).unwrap_or_else(|why| {
        println!("! {:?}", why.kind());
    });

    println!("`mkdir -p a/c/d`");
    // Recursively create a directory, returns `io::Result<()>`
    fs::create_dir_all("a/c/d").unwrap_or_else(|why| {
        println!("! {:?}", why.kind());
    });

    println!("`touch a/c/e.txt`");
    touch(&Path::new("a/c/e.txt")).unwrap_or_else(|why| {
        println!("! {:?}", why.kind());
    });

    println!("`ln -s ../b.txt a/c/b.txt`");
    // Create a symbolic link, returns `io::Result<()>`
    if cfg!(target_family = "unix") {
        unix::fs::symlink("../b.txt", "a/c/b.txt").unwrap_or_else(|why| {
        println!("! {:?}", why.kind());
        });
    }

    println!("`cat a/c/b.txt`");
    match cat(&Path::new("a/c/b.txt")) {
        Err(why) => println!("! {:?}", why.kind()),
        Ok(s) => println!("> {}", s),
    }

    println!("`ls a`");
    // Read the contents of a directory, returns `io::Result<Vec<Path>>`
    match fs::read_dir("a") {
        Err(why) => println!("! {:?}", why.kind()),
        Ok(paths) => for path in paths {
            println!("> {:?}", path.unwrap().path());
        },
    }

    println!("`rm a/c/e.txt`");
    // Remove a file, returns `io::Result<()>`
    fs::remove_file("a/c/e.txt").unwrap_or_else(|why| {
        println!("! {:?}", why.kind());
    });

    println!("`rmdir a/c/d`");
    // Remove an empty directory, returns `io::Result<()>`
    fs::remove_dir("a/c/d").unwrap_or_else(|why| {
        println!("! {:?}", why.kind());
    }
}
```