Reading and Writing a File in Rust
This article will review how you can create, open a file, read its contents and write into it in Rust.
As a software engineer or someone interested in technologies, read and write (aka I/O for Input and Output) operations are some essential concepts that you should know.
After reading this article, you will be able to create, open a file, read its content, and add content to it. So let get dive in.
But first, let's lay some vital groundwork by reviewing how characters are represented in Rust.
String and str (aka string slices)
I know this title (☝🏿)seems a bit confusing … in Rust, we have two types to represent strings :
str: string slice, the only string type of the core Rust programing language. This type is static and immutable, and we usually see it in its borrowed form:&str.String: is the second string type made available in Rust, thanks to the standard library. This type is more flexible, as it can be owned and is mutable.
Both string types are UTF-8 encoded (more on that here); String is used when you need to own or change your string and &str when you want to visualize a string.
The Rust standard library allows us to perform diverse file operations thanks to the awesome struct File and associated methods. We are going to leverage it in the rest of this article.
We are all set now, so let's jump right into it!!
How do you create a file?
To create a file, we are going to utilize create , a method of the struct File as shown in the below example:
The create method returns a Result , as we can be successful in the creation of the file, or we might have an error (disk entire, no privileges, etc., choose your poison 💀).
Resultis an enum also provided by the Rust standard library, allowing us to manage errors (more here).
How do you open a file?
To open a file, we are going to use the open method of the struct File , which also returns a Result .
Here open will try to find the file at the root level of our Rust project. If the file does not exist, it will be created.
I encourage you to run our last example with an existing and nonexisting file to see the outcomes.
How do you read the content of a file?
The simple way to read the content of a file is to open it and read its content as a String (now you see why we had this first section introducing the string types).
As you can see, we create a mutable String variable named contents , and we are updating it by passing the file’s content on line 7 using the method read_to_string(). I encourage you to check the method read , which allows you to retrieve the content of a file as a bytes vector(more on it here).
I prefer to read a content of a file using the struct BufReaderBufreader provides a buffering component while reading from a file (more on that here).
As you can see, we started our example by opening a file, as we learned earlier. Then on line 6, we are creating the reader variable; a BufReader on the file we just opened. We will then use this reader to extract the file's content (line 8).
How do you add some content to a file?
Adding content is not a simple task, as we might want to add some content to a new file, overdrive the content of an existing file or maybe add some content to an existing file.
What seemed trivial a few seconds ago now appears more complex … welcome (back) to the beauty of software development.
So let's start with the first two cases; The following example shows you how to add content to a newly created file or open an existing one and override its content.
In our example, if the file missy.txt does not exist, we will create it before adding our content. If the file exists, our new content will replace its content.
Yes, we are using create once again. create allow us to create files and open existing ones in write-only mode.
Using create on existing files does work, but if we want to be more precise, we should use the structOpenOptions to determine how a file is accessed.
In this example, we open an existing file and add some content at the beginning. If there is some content in the file, our new content will be added at the beginning replacing only the amount of data needed(be careful here; we set the append option to false on line 9).
In the last example below, we will add some content to a file by appending it at the end of any existing data. See here the append option to true on line 9.
Like the writing process, Rust has a BufWriter struct, which provides an optimal way to write into a stream.
To Conclude
I hope I could give you a solid base for performing some file I/O operations in Rust. This is just the beginning. I encourage you to explore the documentation of the struct File and then deep dive into BufReaderan BufWriter.
