An I/O Project Building a Small Grep

This chapter is both a recap of the many skills you've learned so far and an exploration of a few more standard library features. We're going to build a command-line tool that interacts with file and command line input/output to practice some of the Rust you now have under your belt.

Rust's speed, safety, 'single binary' output, and cross-platform support make it a good language for creating command line tools, so for our project we'll make our own version of the classic command line tool grep. Grep is an acronym for "Globally search a Regular Expression and Print." In the simplest use case, grep searches a specified file for a specified string using the following steps:

• Take as arguments a filename and a string.
• Read the file.
• Find lines in the file that contain the string argument.
• Print out those lines.

We'll also show how to use environment variables and print to standard error instead of standard out; these techniques are commonly used in command line tools.

One Rust community member, Andrew Gallant, has already created a fully-featured, very fast version of grep, called ripgrep. By comparison, our version of grep will be fairly simple, this chapter will give you some of the background knowledge to help you understand a real-world project like ripgrep.

This project will bring together a number of concepts you've learned so far:

• Organizing code (using what we learned in modules, Chapter 7)
• Using vectors and strings (collections, Chapter 8)
• Handling errors (Chapter 9)
• Using traits and lifetimes where appropriate (Chapter 10)
• Writing tests (Chapter 11)

We'll also briefly introduce closures, iterators, and trait objects, which Chapters 13 and 17 will cover in detail.

Let's create a new project with, as always, cargo new. We're calling our project greprs to distinguish from the grep tool that you may already have on your system:

$ cargo new --bin greprs
     Created binary (application) `greprs` project
$ cd greprs