Command-Line Rust

Chapter 3. On the Catwalk

When you are alone / You are the cat, you are the phone / You are an animal

They Might Be Giants, “Don’t Let’s Start” (1986)

In this chapter, the challenge is to write a clone of cat, which is so named because it can concatenate many files into one file. That is, given files a, b, and c, you could execute cat a b c > all to stream all the lines from these three files and redirect them into a file called all. The program will accept a couple of different options to prefix each line with the line number.

You’ll learn how to do the following:

• Use testing-first development

• Test for the existence of a file

• Create a random string for a filename that does not exist

• Read regular files or STDIN (pronounced standard in)

• Use eprintln! to print to STDERR and format! to format a string

• Write a test that provides input on STDIN

• Define mutually exclusive arguments

• Use the enumerate method of an iterator

How cat Works

I’ll start by showing how cat works so that you know what is expected of the challenge. The BSD version of cat does not print the usage for the -h|--help flags, so I must use man cat to read the manual page. For such a limited program, it has a surprising number of options, but the challenge program will implement only a subset of these:

CAT(1) BSD General Commands Manual CAT(1) NAME cat -- concatenate and print files SYNOPSIS cat [-benstuv] [file ...] DESCRIPTION The cat utility reads files sequentially, writing them to the standard ...