Build Your Own Markdown Parser:Part 2 - Reading File from Command Line

In this post, we will implement a command line interface that’ll accept a markdown file as a command line argument, read its content and print the entire content of the file in the terminal.

After doing this we will be well on our way to start implementing parsing basic texts, headers, lists and so on.

I’ll explain all the code in detail and in the end, I’ll give you the entire code so that you can try without any issues.

Required Modules

There are two modules that we need to use:

• std::env - we need env module for interacting with the environment the program is currently running in. Now, we use it here to access the command-line arguments passed to our program.
• std::fs - we need fs for file operations like reading from and writing to files.

use std::env;
use std::fs;

Getting the markdown file from command line

After getting the required modules, we can start working on getting the markdown file from the command line argument. From now on, we will write every line of code inside the main function.

let args: Vec<String> = env::args().collect();

Let’s understand what this line does.

• env::args() - this function returns an iterator of the arguments provided in the command line to the program.
• collect() - then we are are using the collect() method to consume the iterator and collect all the items from that iterator into a collection.
• Vec<String> - To store all the items from the iterator we are using the Vector collection that stores elements of type String. We gave the variable name of this Vec<String> as args. So, this args is holding all the items from that command line iterator.

Now, let’s handle the case where user either knowingly or unknowingly skips giving the markdown file in the command line argument. In this case, we need to ask the user to provide a markdown file.

if args.len() < 2 {
		println!("Please provide a markdown file");
		return;
}

Here, we are saying if the number of commands is less than 2, then print a statement asking user to provide a markdown file and then return from the program.

[!Note] Number of Arguments In command line, the first argument is the program’s name. We need another argument that’ll represent the filename. Hence, we need the length of the args to be at least 2.

Reading the file and print its content

Now lets store the filename that we got from the command line argument.

let filename = &args[1];

We are creating a variable filename and storing the filename from the 2nd command line argument.

Now, lets start reading the raw content of the file and print the entire content as it is.

println!("Attempting to read file: {}", filename);
let content = fs::read_to_string(filename).expect("Something went wrong reading the file");
println!("File content:\n{}", content);

• First, we are just printing a logging statement to know that we are attempting to read the file.
• fs::read_to_string(filename) - this function comes from the fs module that reads the entire content of the file specified by the filename.
• Then, we are storing the entire content in the content variable and finally printing out the entire content on the console.

Before explaining expect() method, lets understand a specific data type in rust.

Result in Rust

In Rust, the way we handle operations that might fail is by using Result type. It’s an enum with two variants:

enum Result<T, E> {
    Ok(T),    // Success - contains the value of type T
    Err(E),   // Error - contains the error of type E
}

Now, lets get back to the fs::read_to_string() function. If you’ll look in the docs, it returns a Result type and for a success case it returns a String type and for failure case it returns a std::io::Error type.

Now, lets connect this with the expect() method.

This expect() method is a shortcut that says:

• if fs::read_to_string() works fine and returns Ok(string), then return the string value.
• if fs::read_to_string() doesn’t work, then crash the program with the custom error message (“Something went wrong reading the file”).

Testing the Program

To test, lets create a test.md file in the root of the project and put some random markdown text.

# this is a header
- this is a list
this is a normal text

Now, open your terminal from your project directory and type the command:

cargo run -- test.md

It’ll compile and execute the code and give you the following output.

This output shows that we can successfully get the markdown file’s content and print their raw data. In the next post we will start to learn about parsing.

Complete Source Code

use std::env;
use std::fs;

fn main() {
    let args: Vec<String> = env::args().collect();

    if args.len() < 2 {
        println!("Please provide a markdown file");
        return;
    }

    let filename = &args[1];

    println!("Attempting to read file: {}", filename);

    let content = fs::read_to_string(filename).expect("Something went wrong reading the file");

    println!("File content:\n{}", content);
}

Conclusion

In this post, we understood how to interact with the command line and also read the markdown file from command line argument. We also understood basic error handling using Result where we know things might go wrong and we made sure that our program can handle that.

Now, that we understood reading and printing all the markdown file content. From the next post, we are slowly going to implement our parsing algorithm step by step and cover basic text, headers, list and other things one by one. I’m really excited for the next post, I hope you are excited too. See you soon.