feat: add env variable resolving to preprocessing

src/preprocess.rs

@@ -4,10 +4,11 @@ use regex::Regex;
 use std::env;
 
 const ENV_SET: Lazy<Regex> = Lazy::new(|| Regex::new(r#"(?P<key>\w+)=(?P<value>\w*)"#).unwrap());
+const ENV_VARIABLE: Lazy<Regex> = Lazy::new(|| Regex::new(r"(\\\$|\$(?P<env>\w+))").unwrap());
 
 /// Returns `Ok(Some(...))` if a command should be triggered after calling this functions. If it
 /// returns `Ok(None)`, do not execute a command.
-pub fn preprocess(line: String) -> Result<Option<String>, ShellError> {
+pub fn preprocess(mut line: String) -> Result<Option<String>, ShellError> {
     if let Some(capture) = ENV_SET.captures(&line) {
         let key = capture.name("key").unwrap().as_str();
         let value = capture
@@ -18,6 +19,16 @@ pub fn preprocess(line: String) -> Result<Option<String>, ShellError> {
         return Ok(None);
     }
 
+    for capture in ENV_VARIABLE.captures_iter(&line.clone()) {
+        let variable_name = capture.name("env").unwrap().as_str();
+        if variable_name.is_empty() {
+            continue;
+        }
+        let value = env::var(variable_name).unwrap_or_default();
+
+        line = ENV_VARIABLE.replacen(&line, 1, &value).to_string();
+    }
+
     Ok(Some(line))
 }