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.
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.
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 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 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/lib.rs
use std::error::Error;
use std::fs;
pub struct Config {
pub query: String,
pub file_path: String,
pub ignore_case: bool,
}
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();
Ok(Config { query, file_path })
}
}
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(())
}
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)
);
}
}
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.
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.
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` 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.