# An I/O Project: Building a Command Line Program

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. To do so, `grep` takes a filename and a string as its arguments, then reads the file and finds lines in that file that contain the string argument. It’ll then print out those lines.

Along the way, we’ll show how to make our command line tool use features of the terminal that many command line tools use. We’ll read the value of an environment variable in order to allow the user to configure the behavior of our tool. We’ll print to the standard error console stream (`stderr`) instead of standard output (`stdout`) so that, for example, the user can choose to redirect successful output to a file while still seeing error messages on the screen.

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, but 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 `minigrep` to distinguish from the `grep` tool that you may already have on your system:

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