How to write a String to file?

I am very new to Rust, and have the question, how to write a string to a file in Rust. There are plenty of tutorials and documentations out there, how to write an &str type to file, but no tutorial how to write a String type to files. I wrote this code by myself and it compiles, but I always get an "Bad file descriptor (os error 9)". I’m on Linux(Manjaro).
I’m very thankfully for every help I will get.

//Import
use std::fs::File;
use std::io::*;


//Mainfunction
fn main() {
    //Programm startet hier | program starts here
    println!("Program started...");
    
    // Lesen des Files, createn des Buffers | reading file createing buffer 
    let mut testfile = File::open("/home/julian/.rust_test/test_0.txt").unwrap();
    let mut teststring = String::from("lol");
    
    //Vom File auf den Buffer schreiben | writing from file to buffer
    testfile.read_to_string(&mut teststring).unwrap();
    
    //Buffer ausgeben | print to buffer
    println!("teststring: {}", teststring);
    
    // Neue Variable deklarieren | declare new variable
    let msg = String::from("Writetest tralalalal.");
    
    // msg an ursprünglichen String anhängen | append msg to string
    teststring.push_str(&msg);
    println!("teststring: {}", teststring);
    
    // Neuen String nach File schreiben | write new sting to file
    let eg = testfile.write_all(&teststring.as_bytes());
    match eg {
    Ok(()) => println!("OK"),
    Err(e) => println!("{}",e)

    }    
    println!("Fertig")
}

>Solution :

Your issue is that the file you opened is opened in read-only mode.

As @Herohtar correctly pointed out, from the documentation of File::open():

Attempts to open a file in read-only mode.

What you are trying to do requires read & write mode. There is no pre-made function for that, so you need to build your own using OpenOptions:

//Import
use std::fs::OpenOptions;
use std::io::*;

//Mainfunction
fn main() {
    //Programm startet hier | program starts here
    println!("Program started...");

    // Lesen des Files, createn des Buffers | reading file createing buffer
    let mut testfile = OpenOptions::new()
        .read(true)
        .write(true)
        .open("test_0.txt")
        .unwrap();
    let mut teststring = String::from("lol");

    //Vom File auf den Buffer schreiben | writing from file to buffer
    testfile.read_to_string(&mut teststring).unwrap();

    //Buffer ausgeben | print to buffer
    println!("teststring: {}", teststring);

    // Neue Variable deklarieren | declare new variable
    let msg = String::from("Writetest tralalalal.");

    // msg an ursprünglichen String anhängen | append msg to string
    teststring.push_str(&msg);
    println!("teststring: {}", teststring);

    // Neuen String nach File schreiben | write new sting to file
    let eg = testfile.write_all(teststring.as_bytes());
    match eg {
        Ok(()) => println!("OK"),
        Err(e) => println!("{:?}", e),
    }
    println!("Fertig")
}

The rest of your code is pretty much fine.

The only nitpick I have is that testfile.write_all(&teststring.as_bytes()) doesn’t make much sense, because as_bytes() already returns a reference, so I removed the & from it.