book/nostarch/chapter12.md

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[TOC]

# 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 file path 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 that, 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 (Chapter 7)
* Using vectors and strings (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
Chapter 13 and Chapter 17 will cover in detail.

## Accepting Command Line Arguments

Let’s create a new project with, as always, `cargo new`. We’ll call our project
`minigrep` to distinguish it from the `grep` tool that you might already have
on your system.

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

The first task is to make `minigrep` accept its two command line arguments: the
file path and a string to search for. That is, we want to be able to run our
program with `cargo run`, two hyphens to indicate the following arguments are
for our program rather than for `cargo`, a string to search for, and a path to
a file to search in, like so:

```
$ cargo run -- searchstring example-filename.txt
```

Right now, the program generated by `cargo new` cannot process arguments we
give it. Some existing libraries on *https://crates.io* can help with writing a
program that accepts command line arguments, but because you’re just learning
this concept, let’s implement this capability ourselves.

### Reading the Argument Values

To enable `minigrep` to read the values of command line arguments we pass to
it, we’ll need the `std::env::args` function provided in Rust’s standard
library. This function returns an iterator of the command line arguments passed
to `minigrep`. We’ll cover iterators fully in Chapter 13. For now, you only
need to know two details about iterators: iterators produce a series of values,
and we can call the `collect` method on an iterator to turn it into a
collection, such as a vector, that contains all the elements the iterator
produces.

The code in Listing 12-1 allows your `minigrep` program to read any command
line arguments passed to it, and then collect the values into a vector.

Filename: src/main.rs

```
use std::env;

fn main() {
    let args: Vec<String> = env::args().collect();
    dbg!(args);
}
```

Listing 12-1: Collecting the command line arguments into a vector and printing
them

First we bring the `std::env` module into scope with a `use` statement so we
can use its `args` function. Notice that the `std::env::args` function is
nested in two levels of modules. As we discussed in Chapter 7, in cases where
the desired function is nested in more than one module, we’ve chosen to bring
the parent module into scope rather than the function. By doing so, we can
easily use other functions from `std::env`. It’s also less ambiguous than
adding `use std::env::args` and then calling the function with just `args`,
because `args` might easily be mistaken for a function that’s defined in the
current module.

> ### The args Function and Invalid Unicode
>
> Note that `std::env::args` will panic if any argument contains invalid
Unicode. If your program needs to accept arguments containing invalid Unicode,
use `std::env::args_os` instead. That function returns an iterator that
produces `OsString` values instead of `String` values. We’ve chosen to use
`std::env::args` here for simplicity because `OsString` values differ per
platform and are more complex to work with than `String` values.

On the first line of `main`, we call `env::args`, and we immediately use
`collect` to turn the iterator into a vector containing all the values produced
by the iterator. We can use the `collect` function to create many kinds of
collections, so we explicitly annotate the type of `args` to specify that we
want a vector of strings. Although you very rarely need to annotate types in
Rust, `collect` is one function you do often need to annotate because Rust
isn’t able to infer the kind of collection you want.

Finally, we print the vector using the debug macro. Let’s try running the code
first with no arguments and then with two arguments:

```
$ cargo run
--snip--
[src/main.rs:5] args = [
    "target/debug/minigrep",
]
$ cargo run -- needle haystack
--snip--
[src/main.rs:5] args = [
    "target/debug/minigrep",
    "needle",
    "haystack",
]
```

Notice that the first value in the vector is `"target/debug/minigrep"`, which
is the name of our binary. This matches the behavior of the arguments list in
C, letting programs use the name by which they were invoked in their execution.
It’s often convenient to have access to the program name in case you want to
print it in messages or change the behavior of the program based on what
command line alias was used to invoke the program. But for the purposes of this
chapter, we’ll ignore it and save only the two arguments we need.

### Saving the Argument Values in Variables

The program is currently able to access the values specified as command line
arguments. Now we need to save the values of the two arguments in variables so
we can use the values throughout the rest of the program. We do that in Listing
12-2.

Filename: src/main.rs

```
use std::env;

fn main() {
    let args: Vec<String> = env::args().collect();

    let query = &args[1];
    let file_path = &args[2];

    println!("Searching for {}", query);
    println!("In file {}", file_path);
}
```

Listing 12-2: Creating variables to hold the query argument and file path
argument

As we saw when we printed the vector, the program’s name takes up the first
value in the vector at `args[0]`, so we’re starting arguments at index 1. The
first argument `minigrep` takes is the string we’re searching for, so we put a
reference to the first argument in the variable `query`. The second argument
will be the file path, so we put a reference to the second argument in the
variable `file_path`.

We temporarily print the values of these variables to prove that the code is
working as we intend. Let’s run this program again with the arguments `test`
and `sample.txt`:

```
$ cargo run -- test sample.txt
   Compiling minigrep v0.1.0 (file:///projects/minigrep)
    Finished dev [unoptimized + debuginfo] target(s) in 0.0s
     Running `target/debug/minigrep test sample.txt`
Searching for test
In file sample.txt
```

Great, the program is working! The values of the arguments we need are being
saved into the right variables. Later we’ll add some error handling to deal
with certain potential erroneous situations, such as when the user provides no
arguments; for now, we’ll ignore that situation and work on adding file-reading
capabilities instead.

## Reading a File

Now we’ll add functionality to read the file specified in the `file_path`
argument. First we need a sample file to test it with: we’ll use a file with a
small amount of text over multiple lines with some repeated words. Listing 12-3
has an Emily Dickinson poem that will work well! Create a file called
*poem.txt* at the root level of your project, and enter the poem “I’m Nobody!
Who are you?”

Filename: poem.txt

```
I'm nobody! Who are you?
Are you nobody, too?
Then there's a pair of us - don't tell!
They'd banish us, you know.

How dreary to be somebody!
How public, like a frog
To tell your name the livelong day
To an admiring bog!
```

Listing 12-3: A poem by Emily Dickinson makes a good test case.

With the text in place, edit *src/main.rs* and add code to read the file, as
shown in Listing 12-4.

Filename: src/main.rs

```
use std::env;
1 use std::fs;

fn main() {
    --snip--
    println!("In file {}", file_path);

  2 let contents = fs::read_to_string(file_path)
        .expect("Should have been able to read the file");

  3 println!("With text:\n{contents}");
}
```

Listing 12-4: Reading the contents of the file specified by the second argument

First we bring in a relevant part of the standard library with a `use`
statement: we need `std::fs` to handle files [1].

In `main`, the new statement `fs::read_to_string` takes the `file_path`, opens
that file, and returns an `std::io::Result<String>` of the file’s contents [2].

After that, we again add a temporary `println!` statement that prints the value
of `contents` after the file is read, so we can check that the program is
working so far [3].

Let’s run this code with any string as the first command line argument (because
we haven’t implemented the searching part yet) and the *poem.txt* file as the
second argument:

```
$ cargo run -- the poem.txt
   Compiling minigrep v0.1.0 (file:///projects/minigrep)
    Finished dev [unoptimized + debuginfo] target(s) in 0.0s
     Running `target/debug/minigrep the poem.txt`
Searching for the
In file poem.txt
With text:
I'm nobody! Who are you?
Are you nobody, too?
Then there's a pair of us - don't tell!
They'd banish us, you know.

How dreary to be somebody!
How public, like a frog
To tell your name the livelong day
To an admiring bog!
```

Great! The code read and then printed the contents of the file. But the code
has a few flaws. At the moment, the `main` function has multiple
responsibilities: generally, functions are clearer and easier to maintain if
each function is responsible for only one idea. The other problem is that we’re
not handling errors as well as we could. The program is still small, so these
flaws aren’t a big problem, but as the program grows, it will be harder to fix
them cleanly. It’s a good practice to begin refactoring early on when
developing a program because it’s much easier to refactor smaller amounts of
code. We’ll do that next.

## Refactoring to Improve Modularity and Error Handling

To improve our program, we’ll fix four problems that have to do with the
program’s structure and how it’s handling potential errors. First, our `main`
function now performs two tasks: it parses arguments and reads files. As our
program grows, the number of separate tasks the `main` function handles will
increase. As a function gains responsibilities, it becomes more difficult to
reason about, harder to test, and harder to change without breaking one of its
parts. It’s best to separate functionality so each function is responsible for
one task.

This issue also ties into the second problem: although `query` and `file_path`
are configuration variables to our program, variables like `contents` are used
to perform the program’s logic. The longer `main` becomes, the more variables
we’ll need to bring into scope; the more variables we have in scope, the harder
it will be to keep track of the purpose of each. It’s best to group the
configuration variables into one structure to make their purpose clear.

The third problem is that we’ve used `expect` to print an error message when
reading the file fails, but the error message just prints `Should have been
able to read the file`. Reading a file can fail in a number of ways: for
example, the file could be missing, or we might not have permission to open it.
Right now, regardless of the situation, we’d print the same error message for
everything, which wouldn’t give the user any information!

Fourth, we use `expect` repeatedly to handle different errors, and if the user
runs our program without specifying enough arguments, they’ll get an `index out
of bounds` error from Rust that doesn’t clearly explain the problem. It would
be best if all the error-handling code were in one place so future maintainers
had only one place to consult the code if the error-handling logic needed to
change. Having all the error-handling code in one place will also ensure that
we’re printing messages that will be meaningful to our end users.

Let’s address these four problems by refactoring our project.

### Separation of Concerns for Binary Projects

The organizational problem of allocating responsibility for multiple tasks to
the `main` function is common to many binary projects. As a result, the Rust
community has developed guidelines for splitting the separate concerns of a
binary program when `main` starts getting large. This process has the following
steps:

* Split your program into a *main.rs* file and a *lib.rs* file and move your
program’s logic to *lib.rs*.
* As long as your command line parsing logic is small, it can remain in
*main.rs*.
* When the command line parsing logic starts getting complicated, extract it
from *main.rs* and move it to *lib.rs*.

The responsibilities that remain in the `main` function after this process
should be limited to the following:

* Calling the command line parsing logic with the argument values
* Setting up any other configuration
* Calling a `run` function in *lib.rs*
* Handling the error if `run` returns an error

This pattern is about separating concerns: *main.rs* handles running the
program and *lib.rs* handles all the logic of the task at hand. Because you
can’t test the `main` function directly, this structure lets you test all of
your program’s logic by moving it into functions in *lib.rs*. The code that
remains in *main.rs* will be small enough to verify its correctness by reading
it. Let’s rework our program by following this process.

#### Extracting the Argument Parser

We’ll extract the functionality for parsing arguments into a function that
`main` will call to prepare for moving the command line parsing logic to
src/lib.rs*. Listing 12-5 shows the new start of `main` that calls a new
function `parse_config`, which we’ll define in *src/main.rs* for the moment.

Filename: src/main.rs

```
fn main() {
    let args: Vec<String> = env::args().collect();

    let (query, file_path) = parse_config(&args);

    --snip--
}

fn parse_config(args: &[String]) -> (&str, &str) {
    let query = &args[1];
    let file_path = &args[2];

    (query, file_path)
}
```

Listing 12-5: Extracting a `parse_config` function from `main`

We’re still collecting the command line arguments into a vector, but instead of
assigning the argument value at index 1 to the variable `query` and the
argument value at index 2 to the variable `file_path` within the `main`
function, we pass the whole vector to the `parse_config` function. The
`parse_config` function then holds the logic that determines which argument
goes in which variable and passes the values back to `main`. We still create
the `query` and `file_path` variables in `main`, but `main` no longer has the
responsibility of determining how the command line arguments and variables
correspond.

This rework may seem like overkill for our small program, but we’re refactoring
in small, incremental steps. After making this change, run the program again to
verify that the argument parsing still works. It’s good to check your progress
often, to help identify the cause of problems when they occur.

#### Grouping Configuration Values

We can take another small step to improve the `parse_config` function further.
At the moment, we’re returning a tuple, but then we immediately break that
tuple into individual parts again. This is a sign that perhaps we don’t have
the right abstraction yet.

Another indicator that shows there’s room for improvement is the `config` part
of `parse_config`, which implies that the two values we return are related and
are both part of one configuration value. We’re not currently conveying this
meaning in the structure of the data other than by grouping the two values into
a tuple; we’ll instead put the two values into one struct and give each of the
struct fields a meaningful name. Doing so will make it easier for future
maintainers of this code to understand how the different values relate to each
other and what their purpose is.

Listing 12-6 shows the improvements to the `parse_config` function.

Filename: src/main.rs

```
fn main() {
    let args: Vec<String> = env::args().collect();

  1 let config = parse_config(&args);

    println!("Searching for {}", 2 config.query);
    println!("In file {}", 3 config.file_path);

    let contents = fs::read_to_string(4 config.file_path)
        .expect("Should have been able to read the file");

    --snip--
}

5 struct Config {
    query: String,
    file_path: String,
}

6 fn parse_config(args: &[String]) -> Config {
  7 let query = args[1].clone();
  8 let file_path = args[2].clone();

    Config { query, file_path }
}
```

Listing 12-6: Refactoring `parse_config` to return an instance of a `Config`
struct

We’ve added a struct named `Config` defined to have fields named `query` and
`file_path` [5]. The signature of `parse_config` now indicates that it returns
a `Config` value [6]. In the body of `parse_config`, where we used to return
string slices that reference `String` values in `args`, we now define `Config`
to contain owned `String` values. The `args` variable in `main` is the owner of
the argument values and is only letting the `parse_config` function borrow
them, which means we’d violate Rust’s borrowing rules if `Config` tried to take
ownership of the values in `args`.

There are a number of ways we could manage the `String` data; the easiest,
though somewhat inefficient, route is to call the `clone` method on the values
[7] [8]. This will make a full copy of the data for the `Config` instance to
own, which takes more time and memory than storing a reference to the string
data. However, cloning the data also makes our code very straightforward
because we don’t have to manage the lifetimes of the references; in this
circumstance, giving up a little performance to gain simplicity is a worthwhile
trade-off.

> ### The Trade-Offs of Using clone
>
> There’s a tendency among many Rustaceans to avoid using `clone` to fix
ownership problems because of its runtime cost. In Chapter 13, you’ll learn how
to use more efficient methods in this type of situation. But for now, it’s okay
to copy a few strings to continue making progress because you’ll make these
copies only once and your file path and query string are very small. It’s
better to have a working program that’s a bit inefficient than to try to
hyperoptimize code on your first pass. As you become more experienced with
Rust, it’ll be easier to start with the most efficient solution, but for now,
it’s perfectly acceptable to call `clone`.

We’ve updated `main` so it places the instance of `Config` returned by
`parse_config` into a variable named `config` [1], and we updated the code that
previously used the separate `query` and `file_path` variables so it now uses
the fields on the `Config` struct instead [2] [3] [4].

Now our code more clearly conveys that `query` and `file_path` are related and
that their purpose is to configure how the program will work. Any code that
uses these values knows to find them in the `config` instance in the fields
named for their purpose.

#### Creating a Constructor for Config

So far, we’ve extracted the logic responsible for parsing the command line
arguments from `main` and placed it in the `parse_config` function. Doing so
helped us see that the `query` and `file_path` values were related, and that
relationship should be conveyed in our code. We then added a `Config` struct to
name the related purpose of `query` and `file_path` and to be able to return
the values’ names as struct field names from the `parse_config` function.

So now that the purpose of the `parse_config` function is to create a `Config`
instance, we can change `parse_config` from a plain function to a function
named `new` that is associated with the `Config` struct. Making this change
will make the code more idiomatic. We can create instances of types in the
standard library, such as `String`, by calling `String::new`. Similarly, by
changing `parse_config` into a `new` function associated with `Config`, we’ll
be able to create instances of `Config` by calling `Config::new`. Listing 12-7
shows the changes we need to make.

Filename: src/main.rs

```
fn main() {
    let args: Vec<String> = env::args().collect();

  1 let config = Config::new(&args);

    --snip--
}

--snip--

2 impl Config {
  3 fn new(args: &[String]) -> Config {
        let query = args[1].clone();
        let file_path = args[2].clone();

        Config { query, file_path }
    }
}
```

Listing 12-7: Changing `parse_config` into `Config::new`

We’ve updated `main` where we were calling `parse_config` to instead call
`Config::new` [1]. We’ve changed the name of `parse_config` to `new` [3] and
moved it within an `impl` block [2], which associates the `new` function with
`Config`. Try compiling this code again to make sure it works.

### Fixing the Error Handling

Now we’ll work on fixing our error handling. Recall that attempting to access
the values in the `args` vector at index 1 or index 2 will cause the program to
panic if the vector contains fewer than three items. Try running the program
without any arguments; it will look like this:

```
$ cargo run
   Compiling minigrep v0.1.0 (file:///projects/minigrep)
    Finished dev [unoptimized + debuginfo] target(s) in 0.0s
     Running `target/debug/minigrep`
thread 'main' panicked at 'index out of bounds: the len is 1 but
the index is 1', src/main.rs:27:21
note: run with `RUST_BACKTRACE=1` environment variable to display
a backtrace
```

The line `index out of bounds: the len is 1 but the index is 1` is an error
message intended for programmers. It won’t help our end users understand what
they should do instead. Let’s fix that now.

#### Improving the Error Message

In Listing 12-8, we add a check in the `new` function that will verify that the
slice is long enough before accessing index 1 and index 2. If the slice isn’t
long enough, the program panics and displays a better error message.

Filename: src/main.rs

```
--snip--
fn new(args: &[String]) -> Config {
    if args.len() < 3 {
        panic!("not enough arguments");
    }
    --snip--
```

Listing 12-8: Adding a check for the number of arguments

This code is similar to the `Guess::new` function we wrote in Listing 9-13,
where we called `panic!` when the `value` argument was out of the range of
valid values. Instead of checking for a range of values here, we’re checking
that the length of `args` is at least `3` and the rest of the function can
operate under the assumption that this condition has been met. If `args` has
fewer than three items, this condition will be `true`, and we call the `panic!`
macro to end the program immediately.

With these extra few lines of code in `new`, let’s run the program without any
arguments again to see what the error looks like now:

```
$ cargo run
   Compiling minigrep v0.1.0 (file:///projects/minigrep)
    Finished dev [unoptimized + debuginfo] target(s) in 0.0s
     Running `target/debug/minigrep`
thread 'main' panicked at 'not enough arguments',
src/main.rs:26:13
note: run with `RUST_BACKTRACE=1` environment variable to display
a backtrace
```

This output is better: we now have a reasonable error message. However, we also
have extraneous information we don’t want to give to our users. Perhaps the
technique we used in Listing 9-13 isn’t the best one to use here: a call to
`panic!` is more appropriate for a programming problem than a usage problem, as
discussed in Chapter 9. Instead, we’ll use the other technique you learned
about in Chapter 9—returning a `Result` that indicates either success or an
error.

#### Returning a Result Instead of Calling panic!

We can instead return a `Result` value that will contain a `Config` instance in
the successful case and will describe the problem in the error case. We’re also
going to change the function name from `new` to `build` because many
programmers expect `new` functions to never fail. When `Config::build` is
communicating to `main`, we can use the `Result` type to signal there was a
problem. Then we can change `main` to convert an `Err` variant into a more
practical error for our users without the surrounding text about `thread
'main'` and `RUST_BACKTRACE` that a call to `panic!` causes.

Listing 12-9 shows the changes we need to make to the return value of the
function we’re now calling `Config::build` and the body of the function needed
to return a `Result`. Note that this won’t compile until we update `main` as
well, which we’ll do in the next listing.

Filename: src/main.rs

```
impl Config {
    fn build(args: &[String]) -> Result<Config, &'static str> {
        if args.len() < 3 {
            return Err("not enough arguments");
        }

        let query = args[1].clone();
        let file_path = args[2].clone();

        Ok(Config { query, file_path })
    }
}
```

Listing 12-9: Returning a `Result` from `Config::build`

Our `build` function returns a `Result` with a `Config` instance in the success
case and an `&'static str` in the error case. Our error values will always be
string literals that have the `'static` lifetime.

We’ve made two changes in the body of the function: instead of calling `panic!`
when the user doesn’t pass enough arguments, we now return an `Err` value, and
we’ve wrapped the `Config` return value in an `Ok`. These changes make the
function conform to its new type signature.

Returning an `Err` value from `Config::build` allows the `main` function to
handle the `Result` value returned from the `build` function and exit the
process more cleanly in the error case.

#### Calling Config::build and Handling Errors

To handle the error case and print a user-friendly message, we need to update
`main` to handle the `Result` being returned by `Config::build`, as shown in
Listing 12-10. We’ll also take the responsibility of exiting the command line
tool with a nonzero error code away from `panic!` and instead implement it by
hand. A nonzero exit status is a convention to signal to the process that
called our program that the program exited with an error state.

Filename: src/main.rs

```
1 use std::process;

fn main() {
    let args: Vec<String> = env::args().collect();

  2 let config = Config::build(&args).3 unwrap_or_else(|4 err| {
      5 println!("Problem parsing arguments: {err}");
      6 process::exit(1);
    });

    --snip--
```

Listing 12-10: Exiting with an error code if building a `Config` fails

In this listing, we’ve used a method we haven’t covered in detail yet:
`unwrap_or_else`, which is defined on `Result<T, E>` by the standard library
[2]. Using `unwrap_or_else` allows us to define some custom, non-`panic!` error
handling. If the `Result` is an `Ok` value, this method’s behavior is similar
to `unwrap`: it returns the inner value that `Ok` is wrapping. However, if the
value is an `Err` value, this method calls the code in the *closure*, which is
an anonymous function we define and pass as an argument to `unwrap_or_else`
[3]. We’ll cover closures in more detail in Chapter 13. For now, you just need
to know that `unwrap_or_else` will pass the inner value of the `Err`, which in
this case is the static string `"not enough arguments"` that we added in
Listing 12-9, to our closure in the argument `err` that appears between the
vertical pipes [4]. The code in the closure can then use the `err` value when
it runs.

We’ve added a new `use` line to bring `process` from the standard library into
scope [1]. The code in the closure that will be run in the error case is only
two lines: we print the `err` value [5] and then call `process::exit` [6]. The
`process::exit` function will stop the program immediately and return the
number that was passed as the exit status code. This is similar to the
`panic!`-based handling we used in Listing 12-8, but we no longer get all the
extra output. Let’s try it:

```
$ cargo run
   Compiling minigrep v0.1.0 (file:///projects/minigrep)
    Finished dev [unoptimized + debuginfo] target(s) in 0.48s
     Running `target/debug/minigrep`
Problem parsing arguments: not enough arguments
```

Great! This output is much friendlier for our users.

### Extracting Logic from main

Now that we’ve finished refactoring the configuration parsing, let’s turn to
the program’s logic. As we stated in “Separation of Concerns for Binary
Projects” on page XX, we’ll extract a function named `run` that will hold all
the logic currently in the `main` function that isn’t involved with setting up
configuration or handling errors. When we’re done, `main` will be concise and
easy to verify by inspection, and we’ll be able to write tests for all the
other logic.

Listing 12-11 shows the extracted `run` function. For now, we’re just making
the small, incremental improvement of extracting the function. We’re still
defining the function in *src/main.rs*.

Filename: src/main.rs

```
fn main() {
    --snip--

    println!("Searching for {}", config.query);
    println!("In file {}", config.file_path);

    run(config);
}

fn run(config: Config) {
    let contents = fs::read_to_string(config.file_path)
        .expect("Should have been able to read the file");

    println!("With text:\n{contents}");
}

--snip--
```

Listing 12-11: Extracting a `run` function containing the rest of the program
logic

The `run` function now contains all the remaining logic from `main`, starting
from reading the file. The `run` function takes the `Config` instance as an
argument.

#### Returning Errors from the run Function

With the remaining program logic separated into the `run` function, we can
improve the error handling, as we did with `Config::build` in Listing 12-9.
Instead of allowing the program to panic by calling `expect`, the `run`
function will return a `Result<T, E>` when something goes wrong. This will let
us further consolidate the logic around handling errors into `main` in a
user-friendly way. Listing 12-12 shows the changes we need to make to the
signature and body of `run`.

Filename: src/main.rs

```
1 use std::error::Error;

--snip--

2 fn run(config: Config) -> Result<(), Box<dyn Error>> {
    let contents = fs::read_to_string(config.file_path)3 ?;

    println!("With text:\n{contents}");

  4 Ok(())
}
```

Listing 12-12: Changing the `run` function to return `Result`

We’ve made three significant changes here. First, we changed the return type of
the `run` function to `Result<(), Box<dyn Error>>` [2]. This function
previously returned the unit type, `()`, and we keep that as the value returned
in the `Ok` case.

For the error type, we used the *trait object* `Box<dyn Error>` (and we’ve
brought `std::error::Error` into scope with a `use` statement at the top [1]).
We’ll cover trait objects in Chapter 17. For now, just know that `Box<dyn
Error>` means the function will return a type that implements the `Error`
trait, but we don’t have to specify what particular type the return value will
be. This gives us flexibility to return error values that may be of different
types in different error cases. The `dyn` keyword is short for *dynamic*.

Second, we’ve removed the call to `expect` in favor of the `?` operator [3], as
we talked about in Chapter 9. Rather than `panic!` on an error, `?` will return
the error value from the current function for the caller to handle.

Third, the `run` function now returns an `Ok` value in the success case [4].
We’ve declared the `run` function’s success type as `()` in the signature,
which means we need to wrap the unit type value in the `Ok` value. This
`Ok(())` syntax might look a bit strange at first, but using `()` like this is
the idiomatic way to indicate that we’re calling `run` for its side effects
only; it doesn’t return a value we need.

When you run this code, it will compile but will display a warning:

```
warning: unused `Result` that must be used
  --> src/main.rs:19:5
   |
19 |     run(config);
   |     ^^^^^^^^^^^^
   |
   = note: `#[warn(unused_must_use)]` on by default
   = note: this `Result` may be an `Err` variant, which should be
handled
```

Rust tells us that our code ignored the `Result` value and the `Result` value
might indicate that an error occurred. But we’re not checking to see whether or
not there was an error, and the compiler reminds us that we probably meant to
have some error-handling code here! Let’s rectify that problem now.

#### Handling Errors Returned from run in main

We’ll check for errors and handle them using a technique similar to one we used
with `Config::build` in Listing 12-10, but with a slight difference:

Filename: src/main.rs

```
fn main() {
    --snip--

    println!("Searching for {}", config.query);
    println!("In file {}", config.file_path);

    if let Err(e) = run(config) {
        println!("Application error: {e}");
        process::exit(1);
    }
}
```

We use `if let` rather than `unwrap_or_else` to check whether `run` returns an
`Err` value and to call `process::exit(1)` if it does. The `run` function
doesn’t return a value that we want to `unwrap` in the same way that
`Config::build` returns the `Config` instance. Because `run` returns `()` in
the success case, we only care about detecting an error, so we don’t need
`unwrap_or_else` to return the unwrapped value, which would only be `()`.

The bodies of the `if let` and the `unwrap_or_else` functions are the same in
both cases: we print the error and exit.

### Splitting Code into a Library Crate

Our `minigrep` project is looking good so far! Now we’ll split the
*src/main.rs* file and put some code into the *src/lib.rs* file. That way, we
can test the code and have a *src/main.rs* file with fewer responsibilities.

Let’s move all the code that isn’t in the `main` function from *src/main.rs* to
*src/lib.rs*:

* The `run` function definition
* The relevant `use` statements
* The definition of `Config`
* The `Config::build` function definition

The contents of *src/lib.rs* should have the signatures shown in Listing 12-13
(we’ve omitted the bodies of the functions for brevity). Note that this won’t
compile until we modify *src/main.rs* in Listing 12-14.

Filename: src/lib.rs

```
use std::error::Error;
use std::fs;

pub struct Config {
    pub query: String,
    pub file_path: String,
}

impl Config {
    pub fn build(
        args: &[String],
    ) -> Result<Config, &'static str> {
        --snip--
    }
}

pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
    --snip--
}
```

Listing 12-13: Moving `Config` and `run` into *src/lib.rs*

We’ve made liberal use of the `pub` keyword: on `Config`, on its fields and its
`build` method, and on the `run` function. We now have a library crate that has
a public API we can test!

Now we need to bring the code we moved to *src/lib.rs* into the scope of the
binary crate in *src/main.rs*, as shown in Listing 12-14.

Filename: src/main.rs

```
use std::env;
use std::process;

use minigrep::Config;

fn main() {
    --snip--
    if let Err(e) = minigrep::run(config) {
        --snip--
    }
}
```

Listing 12-14: Using the `minigrep` library crate in *src/main.rs*

We add a `use minigrep::Config` line to bring the `Config` type from the
library crate into the binary crate’s scope, and we prefix the `run` function
with our crate name. Now all the functionality should be connected and should
work. Run the program with `cargo run` and make sure everything works correctly.

Whew! That was a lot of work, but we’ve set ourselves up for success in the
future. Now it’s much easier to handle errors, and we’ve made the code more
modular. Almost all of our work will be done in *src/lib.rs* from here on out.

Let’s take advantage of this newfound modularity by doing something that would
have been difficult with the old code but is easy with the new code: we’ll
write some tests!

## Developing the Library’s Functionality with Test-Driven Development

Now that we’ve extracted the logic into *src/lib.rs* and left the argument
collecting and error handling in *src/main.rs*, it’s much easier to write tests
for the core functionality of our code. We can call functions directly with
various arguments and check return values without having to call our binary
from the command line.

In this section, we’ll add the searching logic to the `minigrep` program using
the test-driven development (TDD) process with the following steps:

1. Write a test that fails and run it to make sure it fails for the reason you
expect.
1. Write or modify just enough code to make the new test pass.
1. Refactor the code you just added or changed and make sure the tests continue
to pass.
1. Repeat from step 1!

Though it’s just one of many ways to write software, TDD can help drive code
design. Writing the test before you write the code that makes the test pass
helps to maintain high test coverage throughout the process.

We’ll test-drive the implementation of the functionality that will actually do
the searching for the query string in the file contents and produce a list of
lines that match the query. We’ll add this functionality in a function called
`search`.

### Writing a Failing Test

Because we don’t need them anymore, let’s remove the `println!` statements from
*src/lib.rs* and *src/main.rs* that we used to check the program’s behavior.
Then, in *src/lib.rs*, we’ll add a `tests` module with a test function, as we
did in Chapter 11. The test function specifies the behavior we want the
`search` function to have: it will take a query and the text to search, and it
will return only the lines from the text that contain the query. Listing 12-15
shows this test, which won’t compile yet.

Filename: src/lib.rs

```
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn one_result() {
        let query = "duct";
        let contents = "\
Rust:
safe, fast, productive.
Pick three.";

        assert_eq!(
            vec!["safe, fast, productive."],
            search(query, contents)
        );
    }
}
```

Listing 12-15: Creating a failing test for the `search` function we wish we had

This test searches for the string `"duct"`. The text we’re searching is three
lines, only one of which contains `"duct"` (note that the backslash after the
opening double quote tells Rust not to put a newline character at the beginning
of the contents of this string literal). We assert that the value returned from
the `search` function contains only the line we expect.

We aren’t yet able to run this test and watch it fail because the test doesn’t
even compile: the `search` function doesn’t exist yet! In accordance with TDD
principles, we’ll add just enough code to get the test to compile and run by
adding a definition of the `search` function that always returns an empty
vector, as shown in Listing 12-16. Then the test should compile and fail
because an empty vector doesn’t match a vector containing the line `"safe,
fast, productive."`.

Filename: src/lib.rs

```
pub fn search<'a>(
    query: &str,
    contents: &'a str,
) -> Vec<&'a str> {
    vec![]
}
```

Listing 12-16: Defining just enough of the `search` function so our test will
compile

Notice that we need to define an explicit lifetime `'a` in the signature of
`search` and use that lifetime with the `contents` argument and the return
value. Recall in Chapter 10 that the lifetime parameters specify which argument
lifetime is connected to the lifetime of the return value. In this case, we
indicate that the returned vector should contain string slices that reference
slices of the argument `contents` (rather than the argument `query`).

In other words, we tell Rust that the data returned by the `search` function
will live as long as the data passed into the `search` function in the
`contents` argument. This is important! The data referenced *by* a slice needs
to be valid for the reference to be valid; if the compiler assumes we’re making
string slices of `query` rather than `contents`, it will do its safety checking
incorrectly.

If we forget the lifetime annotations and try to compile this function, we’ll
get this error:

```
error[E0106]: missing lifetime specifier
  --> src/lib.rs:31:10
   |
29 |     query: &str,
   |            ----
30 |     contents: &str,
   |               ----
31 | ) -> Vec<&str> {
   |          ^ expected named lifetime parameter
   |
   = help: this function's return type contains a borrowed value, but the
signature does not say whether it is borrowed from `query` or `contents`
help: consider introducing a named lifetime parameter
   |
28 ~ pub fn search<'a>(
29 ~     query: &'a str,
30 ~     contents: &'a str,
31 ~ ) -> Vec<&'a str> {
   |
```

Rust can’t possibly know which of the two arguments we need, so we need to tell
it explicitly. Because `contents` is the argument that contains all of our text
and we want to return the parts of that text that match, we know `contents` is
the argument that should be connected to the return value using the lifetime
syntax.

Other programming languages don’t require you to connect arguments to return
values in the signature, but this practice will get easier over time. You might
want to compare this example with the examples in “Validating References with
Lifetimes” on page XX.

Now let’s run the test:

```
$ cargo test
   Compiling minigrep v0.1.0 (file:///projects/minigrep)
    Finished test [unoptimized + debuginfo] target(s) in 0.97s
     Running unittests src/lib.rs (target/debug/deps/minigrep-9cd200e5fac0fc94)

running 1 test
test tests::one_result ... FAILED

failures:

---- tests::one_result stdout ----
thread 'tests::one_result' panicked at 'assertion failed: `(left == right)`
  left: `["safe, fast, productive."]`,
 right: `[]`', src/lib.rs:47:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    tests::one_result

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out;
finished in 0.00s

error: test failed, to rerun pass '--lib'
```

Great, the test fails, exactly as we expected. Let’s get the test to pass!

### Writing Code to Pass the Test

Currently, our test is failing because we always return an empty vector. To fix
that and implement `search`, our program needs to follow these steps:

1. Iterate through each line of the contents.
1. Check whether the line contains our query string.
1. If it does, add it to the list of values we’re returning.
1. If it doesn’t, do nothing.
1. Return the list of results that match.

Let’s work through each step, starting with iterating through lines.

#### Iterating Through Lines with the lines Method

Rust has a helpful method to handle line-by-line iteration of strings,
conveniently named `lines`, that works as shown in Listing 12-17. Note that
this won’t compile yet.

Filename: src/lib.rs

```
pub fn search<'a>(
    query: &str,
    contents: &'a str,
) -> Vec<&'a str> {
    for line in contents.lines() {
        // do something with line
    }
}
```

Listing 12-17: Iterating through each line in `contents`

The `lines` method returns an iterator. We’ll talk about iterators in depth in
Chapter 13, but recall that you saw this way of using an iterator in Listing
3-5, where we used a `for` loop with an iterator to run some code on each item
in a collection.

#### Searching Each Line for the Query

Next, we’ll check whether the current line contains our query string.
Fortunately, strings have a helpful method named `contains` that does this for
us! Add a call to the `contains` method in the `search` function, as shown in
Listing 12-18. Note that this still won’t compile yet.

Filename: src/lib.rs

```
pub fn search<'a>(
    query: &str,
    contents: &'a str,
) -> Vec<&'a str> {
    for line in contents.lines() {
        if line.contains(query) {
            // do something with line
        }
    }
}
```

Listing 12-18: Adding functionality to see whether the line contains the string
in `query`

At the moment, we’re building up functionality. To get the code to compile, we
need to return a value from the body as we indicated we would in the function
signature.

#### Storing Matching Lines

To finish this function, we need a way to store the matching lines that we want
to return. For that, we can make a mutable vector before the `for` loop and
call the `push` method to store a `line` in the vector. After the `for` loop,
we return the vector, as shown in Listing 12-19.

Filename: src/lib.rs

```
pub fn search<'a>(
    query: &str,
    contents: &'a str,
) -> Vec<&'a str> {
    let mut results = Vec::new();

    for line in contents.lines() {
        if line.contains(query) {
            results.push(line);
        }
    }

    results
}
```

Listing 12-19: Storing the lines that match so we can return them

Now the `search` function should return only the lines that contain `query`,
and our test should pass. Let’s run the test:

```
$ cargo test
--snip--
running 1 test
test tests::one_result ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0
filtered out; finished in 0.00s
```

Our test passed, so we know it works!

At this point, we could consider opportunities for refactoring the
implementation of the search function while keeping the tests passing to
maintain the same functionality. The code in the search function isn’t too bad,
but it doesn’t take advantage of some useful features of iterators. We’ll
return to this example in Chapter 13, where we’ll explore iterators in detail,
and look at how to improve it.

#### Using the search Function in the run Function

Now that the `search` function is working and tested, we need to call `search`
from our `run` function. We need to pass the `config.query` value and the
`contents` that `run` reads from the file to the `search` function. Then `run`
will print each line returned from `search`:

Filename: src/lib.rs

```
pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
    let contents = fs::read_to_string(config.file_path)?;

    for line in search(&config.query, &contents) {
        println!("{line}");
    }

    Ok(())
}
```

We’re still using a `for` loop to return each line from `search` and print it.

Now the entire program should work! Let’s try it out, first with a word that
should return exactly one line from the Emily Dickinson poem: *frog*.

```
$ cargo run -- frog poem.txt
   Compiling minigrep v0.1.0 (file:///projects/minigrep)
    Finished dev [unoptimized + debuginfo] target(s) in 0.38s
     Running `target/debug/minigrep frog poem.txt`
How public, like a frog
```

Cool! Now let’s try a word that will match multiple lines, like *body*:

```
$ cargo run -- body poem.txt
    Finished dev [unoptimized + debuginfo] target(s) in 0.0s
     Running `target/debug/minigrep body poem.txt`
I'm nobody! Who are you?
Are you nobody, too?
How dreary to be somebody!
```

And finally, let’s make sure that we don’t get any lines when we search for a
word that isn’t anywhere in the poem, such as *monomorphization*:

```
$ cargo run -- monomorphization poem.txt
    Finished dev [unoptimized + debuginfo] target(s) in 0.0s
     Running `target/debug/minigrep monomorphization poem.txt`
```

Excellent! We’ve built our own mini version of a classic tool and learned a lot
about how to structure applications. We’ve also learned a bit about file input
and output, lifetimes, testing, and command line parsing.

To round out this project, we’ll briefly demonstrate how to work with
environment variables and how to print to standard error, both of which are
useful when you’re writing command line programs.

## Working with Environment Variables

We’ll improve `minigrep` by adding an extra feature: an option for
case-insensitive searching that the user can turn on via an environment
variable. We could make this feature a command line option and require that
users enter it each time they want it to apply, but by instead making it an
environment variable, we allow our users to set the environment variable once
and have all their searches be case insensitive in that terminal session.

### Writing a Failing Test for the Case-Insensitive search Function

We first add a new `search_case_insensitive` function that will be called when
the environment variable has a value. We’ll continue to follow the TDD process,
so the first step is again to write a failing test. We’ll add a new test for
the new `search_case_insensitive` function and rename our old test from
`one_result` to `case_sensitive` to clarify the differences between the two
tests, as shown in Listing 12-20.

Filename: src/lib.rs

```
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn case_sensitive() {
        let query = "duct";
        let contents = "\
Rust:
safe, fast, productive.
Pick three.
Duct tape.";

        assert_eq!(
            vec!["safe, fast, productive."],
            search(query, contents)
        );
    }

    #[test]
    fn case_insensitive() {
        let query = "rUsT";
        let contents = "\
Rust:
safe, fast, productive.
Pick three.
Trust me.";

        assert_eq!(
            vec!["Rust:", "Trust me."],
            search_case_insensitive(query, contents)
        );
    }
}
```

Listing 12-20: Adding a new failing test for the case-insensitive function
we’re about to add

Note that we’ve edited the old test’s `contents` too. We’ve added a new line
with the text `"Duct tape."` using a capital *D* that shouldn’t match the query
`"duct"` when we’re searching in a case-sensitive manner. Changing the old test
in this way helps ensure that we don’t accidentally break the case-sensitive
search functionality that we’ve already implemented. This test should pass now
and should continue to pass as we work on the case-insensitive search.

The new test for the case-*insensitive* search uses `"rUsT"` as its query. In
the `search_case_insensitive` function we’re about to add, the query `"rUsT"`
should match the line containing `"Rust:"` with a capital *R* and match the
line `"Trust me."` even though both have different casing from the query. This
is our failing test, and it will fail to compile because we haven’t yet defined
the `search_case_insensitive` function. Feel free to add a skeleton
implementation that always returns an empty vector, similar to the way we did
for the `search` function in Listing 12-16 to see the test compile and fail.

### Implementing the search_case_insensitive Function

The `search_case_insensitive` function, shown in Listing 12-21, will be almost
the same as the `search` function. The only difference is that we’ll lowercase
the `query` and each `line` so that whatever the case of the input arguments,
they’ll be the same case when we check whether the line contains the query.

Filename: src/lib.rs

```
pub fn search_case_insensitive<'a>(
    query: &str,
    contents: &'a str,
) -> Vec<&'a str> {
  1 let query = query.to_lowercase();
    let mut results = Vec::new();

    for line in contents.lines() {
        if 2 line.to_lowercase().contains(3 &query) {
            results.push(line);
        }
    }

    results
}
```

Listing 12-21: Defining the `search_case_insensitive` function to lowercase the
query and the line before comparing them

First we lowercase the `query` string and store it in a shadowed variable with
the same name [1]. Calling `to_lowercase` on the query is necessary so that no
matter whether the user’s query is `"rust"`, `"RUST"`, `"Rust"`, or `"rUsT"`,
we’ll treat the query as if it were `"rust"` and be insensitive to the case.
While `to_lowercase` will handle basic Unicode, it won’t be 100% accurate. If
we were writing a real application, we’d want to do a bit more work here, but
this section is about environment variables, not Unicode, so we’ll leave it at
that here.

Note that `query` is now a `String` rather than a string slice because calling
`to_lowercase` creates new data rather than referencing existing data. Say the
query is `"rUsT"`, as an example: that string slice doesn’t contain a lowercase
`u` or `t` for us to use, so we have to allocate a new `String` containing
`"rust"`. When we pass `query` as an argument to the `contains` method now, we
need to add an ampersand [3] because the signature of `contains` is defined to
take a string slice.

Next, we add a call to `to_lowercase` on each `line` to lowercase all
characters [2]. Now that we’ve converted `line` and `query` to lowercase, we’ll
find matches no matter what the case of the query is.

Let’s see if this implementation passes the tests:

```
running 2 tests
test tests::case_insensitive ... ok
test tests::case_sensitive ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0
filtered out; finished in 0.00s
```

Great! They passed. Now, let’s call the new `search_case_insensitive` function
from the `run` function. First we’ll add a configuration option to the `Config`
struct to switch between case-sensitive and case-insensitive search. Adding
this field will cause compiler errors because we aren’t initializing this field
anywhere yet:

Filename: src/lib.rs

```
pub struct Config {
    pub query: String,
    pub file_path: String,
    pub ignore_case: bool,
}
```

We added the `ignore_case` field that holds a Boolean. Next, we need the `run`
function to check the `ignore_case` field’s value and use that to decide
whether to call the `search` function or the `search_case_insensitive`
function, as shown in Listing 12-22. This still won’t compile yet.

Filename: src/lib.rs

```
pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
    let contents = fs::read_to_string(config.file_path)?;

    let results = if config.ignore_case {
        search_case_insensitive(&config.query, &contents)
    } else {
        search(&config.query, &contents)
    };

    for line in results {
        println!("{line}");
    }

    Ok(())
}
```

Listing 12-22: Calling either `search` or `search_case_insensitive` based on
the value in `config.ignore_case`

Finally, we need to check for the environment variable. The functions for
working with environment variables are in the `env` module in the standard
library, so we bring that module into scope at the top of *src/lib.rs*. Then
we’ll use the `var` function from the `env` module to check to see if any value
has been set for an environment variable named `IGNORE_CASE`, as shown in
Listing 12-23.

Filename: src/lib.rs

```
use std::env;
--snip--

impl Config {
    pub fn build(
        args: &[String]
    ) -> Result<Config, &'static str> {
        if args.len() < 3 {
            return Err("not enough arguments");
        }

        let query = args[1].clone();
        let file_path = args[2].clone();

        let ignore_case = env::var("IGNORE_CASE").is_ok();

        Ok(Config {
            query,
            file_path,
            ignore_case,
        })
    }
}
```

Listing 12-23: Checking for any value in an environment variable named
`IGNORE_CASE`

Here, we create a new variable, `ignore_case`. To set its value, we call the
`env::var` function and pass it the name of the `IGNORE_CASE` environment
variable. The `env::var` function returns a `Result` that will be the
successful `Ok` variant that contains the value of the environment variable if
the environment variable is set to any value. It will return the `Err` variant
if the environment variable is not set.

We’re using the `is_ok` method on the `Result` to check whether the environment
variable is set, which means the program should do a case-insensitive search.
If the `IGNORE_CASE` environment variable isn’t set to anything, `is_ok` will
return `false` and the program will perform a case-sensitive search. We don’t
care about the *value* of the environment variable, just whether it’s set or
unset, so we’re checking `is_ok` rather than using `unwrap`, `expect`, or any
of the other methods we’ve seen on `Result`.

We pass the value in the `ignore_case` variable to the `Config` instance so the
`run` function can read that value and decide whether to call
`search_case_insensitive` or `search`, as we implemented in Listing 12-22.

Let’s give it a try! First we’ll run our program without the environment
variable set and with the query `to`, which should match any line that contains
the word *to* in all lowercase:

```
$ cargo run -- to poem.txt
   Compiling minigrep v0.1.0 (file:///projects/minigrep)
    Finished dev [unoptimized + debuginfo] target(s) in 0.0s
     Running `target/debug/minigrep to poem.txt`
Are you nobody, too?
How dreary to be somebody!
```

Looks like that still works! Now let’s run the program with `IGNORE_CASE` set
to `1` but with the same query `to`:

```
$ IGNORE_CASE=1 cargo run -- to poem.txt
```

If you’re using PowerShell, you will need to set the environment variable and
run the program as separate commands:

```
PS> $Env:IGNORE_CASE=1; cargo run -- to poem.txt
```

This will make `IGNORE_CASE` persist for the remainder of your shell session.
It can be unset with the `Remove-Item` cmdlet:

```
PS> Remove-Item Env:IGNORE_CASE
```

We should get lines that contain *to* that might have uppercase letters:

```
Are you nobody, too?
How dreary to be somebody!
To tell your name the livelong day
To an admiring bog!
```

Excellent, we also got lines containing *To*! Our `minigrep` program can now do
case-insensitive searching controlled by an environment variable. Now you know
how to manage options set using either command line arguments or environment
variables.

Some programs allow arguments *and* environment variables for the same
configuration. In those cases, the programs decide that one or the other takes
precedence. For another exercise on your own, try controlling case sensitivity
through either a command line argument or an environment variable. Decide
whether the command line argument or the environment variable should take
precedence if the program is run with one set to case sensitive and one set to
ignore case.

The `std::env` module contains many more useful features for dealing with
environment variables: check out its documentation to see what is available.

## Writing Error Messages to Standard Error Instead of Standard Output

At the moment, we’re writing all of our output to the terminal using the
`println!` macro. In most terminals, there are two kinds of output: *standard
output* (`stdout`) for general information and *standard error* (`stderr`) for
error messages. This distinction enables users to choose to direct the
successful output of a program to a file but still print error messages to the
screen.

The `println!` macro is only capable of printing to standard output, so we have
to use something else to print to standard error.

### Checking Where Errors Are Written

First let’s observe how the content printed by `minigrep` is currently being
written to standard output, including any error messages we want to write to
standard error instead. We’ll do that by redirecting the standard output stream
to a file while intentionally causing an error. We won’t redirect the standard
error stream, so any content sent to standard error will continue to display on
the screen.

Command line programs are expected to send error messages to the standard error
stream so we can still see error messages on the screen even if we redirect the
standard output stream to a file. Our program is not currently well behaved:
we’re about to see that it saves the error message output to a file instead!

To demonstrate this behavior, we’ll run the program with `>` and the file path,
*output.txt*, that we want to redirect the standard output stream to. We won’t
pass any arguments, which should cause an error:

```
$ cargo run > output.txt
```

The `>` syntax tells the shell to write the contents of standard output to
*output.txt* instead of the screen. We didn’t see the error message we were
expecting printed to the screen, so that means it must have ended up in the
file. This is what *output.txt* contains:

```
Problem parsing arguments: not enough arguments
```

Yup, our error message is being printed to standard output. It’s much more
useful for error messages like this to be printed to standard error so only
data from a successful run ends up in the file. We’ll change that.

### Printing Errors to Standard Error

We’ll use the code in Listing 12-24 to change how error messages are printed.
Because of the refactoring we did earlier in this chapter, all the code that
prints error messages is in one function, `main`. The standard library provides
the `eprintln!` macro that prints to the standard error stream, so let’s change
the two places we were calling `println!` to print errors to use `eprintln!`
instead.

Filename: src/main.rs

```
fn main() {
    let args: Vec<String> = env::args().collect();

    let config = Config::build(&args).unwrap_or_else(|err| {
        eprintln!("Problem parsing arguments: {err}");
        process::exit(1);
    });

    if let Err(e) = minigrep::run(config) {
        eprintln!("Application error: {e}");
        process::exit(1);
    }
}
```

Listing 12-24: Writing error messages to standard error instead of standard
output using `eprintln!`

Let’s now run the program again in the same way, without any arguments and
redirecting standard output with `>`:

```
$ cargo run > output.txt
Problem parsing arguments: not enough arguments
```

Now we see the error onscreen and *output.txt* contains nothing, which is the
behavior we expect of command line programs.

Let’s run the program again with arguments that don’t cause an error but still
redirect standard output to a file, like so:

```
$ cargo run -- to poem.txt > output.txt
```

We won’t see any output to the terminal, and *output.txt* will contain our
results:

Filename: output.txt

```
Are you nobody, too?
How dreary to be somebody!
```

This demonstrates that we’re now using standard output for successful output
and standard error for error output as appropriate.

## Summary

This chapter recapped some of the major concepts you’ve learned so far and
covered how to perform common I/O operations in Rust. By using command line
arguments, files, environment variables, and the `eprintln!` macro for printing
errors, you’re now prepared to write command line applications. Combined with
the concepts in previous chapters, your code will be well organized, store data
effectively in the appropriate data structures, handle errors nicely, and be
well tested.

Next, we’ll explore some Rust features that were influenced by functional
languages: closures and iterators.