Command line counter with plain text file storage in Rust

read_to_string trim_end parse create writeln! directories BaseDirs home_dir

As part of the Counter example this program will show a series of ever increasing numbers every time we run it.

$ cargo run
0
$ cargo run
1
$ cargo run
2

We use a file called "counter.txt"

We use the fact that an if-statement will return the last expression of the if or the else part. If the file exists, we read the content using std::fs::read_to_string, remove the trailing newline with trim_end and then convert it to an i32 number using the parse method.

In-between we had to call unwrap twice to pick the result and panic! if there was an error. As this is not some important application I thought I'll leave error handling out.

If the "counter.txt" file does not exist, the else-part returns 0.

Then we increment the counter.

Save it to the disk and print it.

examples/counter-text-file/src/main.rs

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

fn main() {
    let path = std::path::Path::new("counter.txt");

    let counter: u32 = if path.exists() {
        std::fs::read_to_string(path)
            .unwrap()
            .trim_end()
            .parse()
            .unwrap()
    } else {
        0
    };

    let counter = counter + 1;

    let mut fh = File::create(path).unwrap();
    writeln!(&mut fh, "{}", counter).unwrap();

    println!("{counter}");
}

One of the big issues of this program, besides not handling errors, is that the "counter.txt" file is relative to the current working directory of when the code is executed. This is fine as long as we run it with cargo run, but if we create a binary and distribute it, it would be probably better if the "counter.txt" file was in a fixed place on the computer, or at least fixed per user.

Home directory

In order to solve this we need a way to find the home directory of the current user. The home_dir in the standard library is deprecated, but I found the directories crate to be useful and quite popular.

In order to use it we need to to add it as a dependency.

First we create a new crate:

cargo new counter-text-file-in-homedir
cd counter-text-file-in-homedir

Then we add the crate:

cargo add directories

This will add a line to the Cargo.toml file listing this dependency.

examples/counter-text-file-in-homedir/Cargo.toml

[package]
name = "counter-text-file-in-homedir"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
directories = "5.0.1"

Then we replace the code declaring the path with these two lines:

let bd = directories::BaseDirs::new().unwrap();
let path = bd.home_dir().join("counter.txt");

In this way "counter.txt" will be in the home directory of the current user.

We could use cargo run, but we would like to make sure that this code works regardless where the executable is located and what is our current working directory. So we compile the code in release mode:

cargo build --release

This will create the file target/release/counter-text-file-in-homedir. We can move it to any folder. (e.g. I have a folder called ~/work where I have all of the projects I work on. So:

mv target/release/counter-text-file-in-homedir ~/work/

Then I can run ~/work/counter-text-file-in-homedir and the counter will work. The counter.txt file will be located in the home directory: ~/counter.txt.

Full example

examples/counter-text-file-in-homedir/src/main.rs

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

fn main() {
    let bd = directories::BaseDirs::new().unwrap();
    let path = bd.home_dir().join("counter.txt");

    let counter: u32 = if path.exists() {
        std::fs::read_to_string(&path)
            .unwrap()
            .trim_end()
            .parse()
            .unwrap()
    } else {
        0
    };

    let counter = counter + 1;

    let mut fh = File::create(path).unwrap();
    writeln!(&mut fh, "{}", counter).unwrap();

    println!("{counter}");
}

Related Pages

Command line multi-counter with storage in JSON file

Author

Gabor Szabo (szabgab)

Gabor Szabo, the author of the Rust Maven web site maintains several Open source projects in Rust and while he still feels he has tons of new things to learn about Rust he already offers training courses in Rust and still teaches Python, Perl, git, GitHub, GitLab, CI, and testing.

Gabor Szabo