feat: Default env variables

tests/cmd.rs

@@ -1,6 +1,8 @@
 extern crate comlexr;
 extern crate rstest;
 
+use std::env;
+
 use comlexr::cmd;
 use rstest::rstest;
 
@@ -35,6 +37,37 @@ fn env_vars() {
     assert_eq!(format!("{command:?}"), r#"TEST="test" "echo" "test""#);
 }
 
+#[test]
+fn conditional_env_vars() {
+    env::set_var("TEST", "realvalue");
+    env::set_var("TEST2", "won't see");
+
+    let command = cmd!(
+        env {
+            "TEST":? "test",
+            "TEST2": "test2"
+        };
+        "echo",
+        "test",
+    );
+
+    assert_eq!(format!("{command:?}"), r#"TEST2="test2" "echo" "test""#);
+
+    let command = cmd!(
+        env {
+            "TEST": "test",
+            "TEST2": "test2"
+        };
+        "echo",
+        "test",
+    );
+
+    assert_eq!(
+        format!("{command:?}"),
+        r#"TEST="test" TEST2="test2" "echo" "test""#
+    );
+}
+
 #[test]
 fn cd_env_vars() {
     let command = cmd!(