Chapter 2. Test for Echo

By the time you get this note / We’ll no longer be alive / We’ll have all gone up in smoke / There’ll be no way to reply

They Might Be Giants, “By the Time You Get This” (2018)

In Chapter 1, you wrote three programs—hello, true, and false—that take no arguments and always produce the same output. In this chapter, I’ll show you how to use arguments from the command line to change the behavior of the program at runtime. The challenge program you’ll write is a clone of echo, which will print its arguments on the command line, optionally terminated with a newline.

In this chapter, you’ll learn how to do the following:

  • Process command-line arguments with the clap crate

  • Use Rust types like strings, vectors, slices, and the unit type

  • Use expressions like match, if, and return

  • Use Option to represent an optional value

  • Handle errors using the Result variants of Ok and Err

  • Understand the difference between stack and heap memory

  • Test for text that is printed to STDOUT and STDERR

How echo Works

In each chapter, you will be writing a Rust version of an existing command-line tool, so I will begin each chapter by describing how the tool works so that you understand what you’ll be creating. The features I describe are also the substance of the test suite I provide. For this challenge, you will create a Rust version of the echo program, which is blissfully simple. To start, echo will print its arguments to STDOUT:

$ echo Hello

I’m using the ...

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