## 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 instead we’ll use an environment variable. Doing so allows 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 want to add a new `search_case_insensitive` function that we’ll call when the environment variable is on. 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

    # #![allow(unused_variables)]
    #fn main() {
    #[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 than 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 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

    # #![allow(unused_variables)]
    #fn main() {
    fn search_case_insensitive<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
        let query = query.to_lowercase();
        let mut results = Vec::new();

        for line in contents.lines() {
            if line.to_lowercase().contains(&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. Calling `to_lowercase` on the query is necessary so 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.

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 because the signature of `contains` is defined to take a string slice.

Next, we add a call to `to_lowercase` on each `line` before we check whether it contains `query` to lowercase all characters. 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

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 since we aren’t initializing this field anywhere yet:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    pub struct Config {
        pub query: String,
        pub filename: String,
        pub case_sensitive: bool,
    }
    #}

Note that we added the `case_sensitive` field that holds a Boolean. Next, we need the `run` function to check the `case_sensitive` 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. Note this still won’t compile yet:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # use std::error::Error;
    # use std::fs::{self, File};
    # use std::io::prelude::*;
    #
    # fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
    #      vec![]
    # }
    #
    # fn search_case_insensitive<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
    #      vec![]
    # }
    #
    # pub struct Config {
    #     query: String,
    #     filename: String,
    #     case_sensitive: bool,
    # }
    #
    pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
        let contents = fs::read_to_string(config.filename)?;

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

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 want to bring that module into scope with a `use std::env;` line at the top of src/lib.rs. Then we’ll use the `var` method from the `env` module to check for an environment variable named `CASE_INSENSITIVE`, as shown in Listing 12-23:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    use std::env;
    # struct Config {
    #     query: String,
    #     filename: String,
    #     case_sensitive: bool,
    # }

    // --snip--

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

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

            let case_sensitive = env::var("CASE_INSENSITIVE").is_err();

            Ok(Config { query, filename, case_sensitive })
        }
    }
    #}

Listing 12-23: Checking for an environment variable named `CASE_INSENSITIVE`

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

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

We pass the value in the `case_sensitive` variable to the `Config` instance so the `run` function can read that value and decide whether to call `search` or `search_case_insensitive`, 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 0.0 secs
         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 `CASE_INSENSITIVE` set to `1` but with the same query `to`.

If you’re using PowerShell, you will need to set the environment variable and run the program in two commands rather than one:

    $ $env:CASE_INSENSITIVE=1
    $ cargo run to poem.txt

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

    $ CASE_INSENSITIVE=1 cargo run to poem.txt
        Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
         Running `target/debug/minigrep to poem.txt`
    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 insensitivity 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 case insensitive.

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