## Working with Environment Variables

We’ll improve the `minigrep` binary 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 Case-Insensitive Search

We first add a new `search_case_insensitive` function to the `minigrep` library 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

    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
    }

    #[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<'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
    }

    pub 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
    }

    #[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 12-21: 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 new variable with the same name, shadowing the original `query`. 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 percent 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 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. 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:

    $ cargo test
       Compiling minigrep v0.1.0 (file:///projects/minigrep)
        Finished `test` profile [unoptimized + debuginfo] target(s) in 1.33s
         Running unittests src/lib.rs (target/debug/deps/minigrep-9cd200e5fac0fc94)

    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

         Running unittests src/main.rs (target/debug/deps/minigrep-9cd200e5fac0fc94)

    running 0 tests

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

       Doc-tests minigrep

    running 0 tests

    test result: ok. 0 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/main.rs

    use std::env;
    use std::error::Error;
    use std::fs;
    use std::process;

    use minigrep::{search, search_case_insensitive};

    // --snip--

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

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

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

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

    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 })
        }
    }

    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(())
    }

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/main.rs

    use std::env;
    use std::error::Error;
    use std::fs;
    use std::process;

    use minigrep::{search, search_case_insensitive};

    // --snip--

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

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

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

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

    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 })
        }
    }

    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, which is already in scope at the top of src/main.rs. 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/main.rs

    use std::env;
    use std::error::Error;
    use std::fs;
    use std::process;

    use minigrep::{search, search_case_insensitive};

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

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

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

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

    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();

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

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

    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 12-23: 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 that 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` profile [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.