# recursive-env

Look up env variables that are built from other env vars

## Why

Shell env vars should evaluate recursively

```bash
$ TEST1="test1"
$ TEST2="${TEST1} test2"
$ echo $test
test1 test2
```

But sometimes tooling sets env vars without evaluating them first.
Take this simple program:

```rust
fn main() {
    env::set_var("TEST1", "test1");
    env::set_var("TEST2", "${TEST1} test2");
    for (key, value) in env::vars() {
        println!("{key}: {value}");
    }
}
// TODO: rewrite examples here with asserts. Run theses as part of tests
```

This will set 'TEST2' to the literal string "${TEST1} test2" instead of evaluating to "test1 test2".

```bash
$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
    Running `target/debug/test`
TEST1: test1
TEST2: ${TEST1} test2
```

The solution is to get env vars through `recursive_env::var()` instead of `std::env::var()`

```rust
use recursive_env::lookup;
fn main() {
    env::set_var("TEST1", "test1");
    env::set_var("TEST2", "${TEST1} test 2");
    for (key, _) in env::vars() {
        let fixed = lookup(key)
        println!("{key}: {fixed}");
    }
}
```

```bash
$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
    Running `target/debug/test`
TEST1: test1
TEST2: test1 test2
```

## Caveats

- Sub expressions are not evaluated

```rust
use recursive_env::lookup;
fn main() {
    let key = "KEY";
    let value = String::from("$(subexpression)");
    env::set_var(&key, &value);
    assert_eq!(lookup::var(key).unwrap(), value);
}
```