feat: return option inside the preprocess result

src/execute.rs

@@ -13,7 +13,11 @@ pub fn interpret(
     config: &mut BuiltinConfig,
     ctrlc_recv: Receiver<()>,
 ) -> Result<(), ShellError> {
-    line = preprocess(line)?;
+    if let Some(l) = preprocess(line)? {
+        line = l
+    } else {
+        return Ok(());
+    }
 
     if line.is_empty() {
         return Err(ShellError::EmptyLine);

src/preprocess.rs

@@ -5,7 +5,9 @@ use std::env;
 
 const ENV_SET: Lazy<Regex> = Lazy::new(|| Regex::new(r#"(?P<key>\w+)=(?P<value>\w*)"#).unwrap());
 
-pub fn preprocess(line: String) -> Result<String, ShellError> {
+/// 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> {
     if let Some(capture) = ENV_SET.captures(&line) {
         let Some(key) = capture.name("key") else {
             return Err(ShellError::MalformedArgs("cannot find key to set env variable".to_string()))
@@ -14,8 +16,9 @@ pub fn preprocess(line: String) -> Result<String, ShellError> {
             .name("value")
             .map_or("".to_string(), |v| v.as_str().to_string());
 
-        env::set_var(key.as_str(), value)
+        env::set_var(key.as_str(), value);
+        return Ok(None);
     }
 
-    Ok(line)
+    Ok(Some(line))
 }