Substitute environment variables in config file

[LouneCode]

Substitute environment variables in config file

The Substitution of the environment variables in config file enables easy implementation for the dynamic switching from Border Gateway to Relay Gateway ( same matter as in issue/feature request #28). This change will eliminate the need to duplicate configuration file or direct string manipulation/substitution in toml file. The "substitute environment variables" functionality works well in ChirpStack LNS so it might be a good solution also in the ChirpStack-gateway-mesh module?

The proposed implementation can be found, for example in
chirpstack/chirpstack/src/config.rs – line799.

Please see line // substitute environment variables in config file .

pub fn load(config_dir: &Path) -> Result<()> {
    let mut content: String = String::new();

    let paths = fs::read_dir(config_dir)?;
    for path in paths {
        let path = path.unwrap().path();

        if let Some(ext) = path.extension() {
            if ext == "toml" {
                content.push_str(
                    &fs::read_to_string(&path)
                        .context(format!("Read config file: {}", path.display()))?,
                );
            }
        }
    }

    // substitute environment variables in config file
    for (k, v) in env::vars() {
        content = content.replace(&format!("${}", k), &v);
    }

    let conf: Configuration = toml::from_str(&content)?;
    set(conf);

    Ok(())
}