[rustc.git] / src / doc / book / src / ch12-00-an-io-project.md

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

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

Rust’s speed, safety, single binary output, and cross-platform support make it
an ideal language for creating command line tools, so for our project, we’ll
make our own version of the classic command line search tool `grep`
(**g**lobally search a **r**egular **e**xpression and **p**rint). In the
simplest use case, `grep` searches a specified file for a specified string. To
do so, `grep` takes as its arguments a filename and a string. Then it reads the
file, finds lines in that file that contain the string argument, and prints
those lines.

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

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

Our `grep` project will combine a number of concepts you’ve learned so far:

* Organizing code (using what you learned about modules in [Chapter 7][ch7]<!--
  ignore -->)
* Using vectors and strings (collections, [Chapter 8][ch8]<!-- ignore -->)
* Handling errors ([Chapter 9][ch9]<!-- ignore -->)
* Using traits and lifetimes where appropriate ([Chapter 10][ch10]<!-- ignore
  -->)
* Writing tests ([Chapter 11][ch11]<!-- ignore -->)

We’ll also briefly introduce closures, iterators, and trait objects, which
Chapters [13][ch13]<!-- ignore --> and [17][ch17]<!-- ignore --> will cover in
detail.

[ch7]: ch07-00-managing-growing-projects-with-packages-crates-and-modules.html
[ch8]: ch08-00-common-collections.html
[ch9]: ch09-00-error-handling.html
[ch10]: ch10-00-generics.html
[ch11]: ch11-00-testing.html
[ch13]: ch13-00-functional-features.html
[ch17]: ch17-00-oop.html
Commit	Line	Data
13cf67c4 XL	1	# An I/O Project: Building a Command Line Program
	2
	3	This chapter is a recap of the many skills you’ve learned so far and an
	4	exploration of a few more standard library features. We’ll build a command line
	5	tool that interacts with file and command line input/output to practice some of
	6	the Rust concepts you now have under your belt.
	7
	8	Rust’s speed, safety, single binary output, and cross-platform support make it
	9	an ideal language for creating command line tools, so for our project, we’ll
04454e1e FG	10	make our own version of the classic command line search tool `grep`
	11	(globally search a regular expression and print). In the
	12	simplest use case, `grep` searches a specified file for a specified string. To
	13	do so, `grep` takes as its arguments a filename and a string. Then it reads the
	14	file, finds lines in that file that contain the string argument, and prints
	15	those lines.
	16
	17	Along the way, we’ll show how to make our command line tool use the terminal
	18	features that many other command line tools use. We’ll read the value of an
13cf67c4	19	environment variable to allow the user to configure the behavior of our tool.
532ac7d7 XL	20	We’ll also print error messages to the standard error console stream (`stderr`)
	21	instead of standard output (`stdout`), so, for example, the user can redirect
	22	successful output to a file while still seeing error messages onscreen.
13cf67c4 XL	23
	24	One Rust community member, Andrew Gallant, has already created a fully
	25	featured, very fast version of `grep`, called `ripgrep`. By comparison, our
04454e1e FG	26	version will be fairly simple, but this chapter will give you some of the
04454e1e FG	27	background knowledge you need to understand a real-world project such as
13cf67c4 XL	28	`ripgrep`.
	29
	30	Our `grep` project will combine a number of concepts you’ve learned so far:
	31
48663c56 XL	32	* Organizing code (using what you learned about modules in [Chapter 7][ch7]<!--
	33	ignore -->)
	34	* Using vectors and strings (collections, [Chapter 8][ch8]<!-- ignore -->)
	35	* Handling errors ([Chapter 9][ch9]<!-- ignore -->)
	36	* Using traits and lifetimes where appropriate ([Chapter 10][ch10]<!-- ignore
	37	-->)
	38	* Writing tests ([Chapter 11][ch11]<!-- ignore -->)
13cf67c4 XL	39
13cf67c4 XL	40	We’ll also briefly introduce closures, iterators, and trait objects, which
48663c56 XL	41	Chapters [13][ch13]<!-- ignore --> and [17][ch17]<!-- ignore --> will cover in
	42	detail.
	43
	44	[ch7]: ch07-00-managing-growing-projects-with-packages-crates-and-modules.html
	45	[ch8]: ch08-00-common-collections.html
	46	[ch9]: ch09-00-error-handling.html
	47	[ch10]: ch10-00-generics.html
	48	[ch11]: ch11-00-testing.html
	49	[ch13]: ch13-00-functional-features.html
	50	[ch17]: ch17-00-oop.html