feat: Default env variables

README.md

@@ -185,6 +185,27 @@ let command = cmd!(
 assert_eq!(format!("{command:?}"), r#"NEW_VAR="new_var" TEST="test" "echo" "test""#);
 ```
 
+#### Conditional
+You can have a default value set for an environment variable.
+
+```rust
+use comlexr::cmd;
+
+const NEW_VAR: &str = "NEW_VAR";
+std::env::set_var("TEST", "realvalue");
+
+let command = cmd!(
+    env {
+        "TEST":? "test",
+        NEW_VAR: "new_var"
+    };
+    "echo",
+    "test",
+);
+
+assert_eq!(format!("{command:?}"), r#"NEW_VAR="new_var" "echo" "test""#);
+```
+
 #### Current Directory and Env Variable Order Matters
 Environment variable declarations **MUST** come after the current directory declaration.
 

macro/src/cmd.rs

@@ -437,25 +437,51 @@ impl ToTokens for EnvVars {
 struct EnvVar {
     key: Value,
     value: Value,
+    conditional: bool,
 }
 
 impl Parse for EnvVar {
     fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
         let key = input.parse()?;
+
         _ = input.parse::<Token![:]>()?;
+
+        let conditional = if input.lookahead1().peek(Token![?]) {
+            input.parse::<Token![?]>()?;
+            true
+        } else {
+            false
+        };
+
         let value = input.parse()?;
 
-        Ok(Self { key, value })
+        Ok(Self {
+            key,
+            value,
+            conditional,
+        })
     }
 }
 
 impl ToTokens for EnvVar {
     fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
-        let Self { key, value } = self;
+        let Self {
+            key,
+            value,
+            conditional,
+        } = self;
 
+        if *conditional {
+            tokens.extend(quote! {
+                if ::std::env::var(#key).ok().is_none() {
+                    _c.env(#key, #value);
+                }
+            });
+        } else {
             tokens.extend(quote! { _c.env(#key, #value); });
         }
     }
+}
 
 struct CurrentDir(Option<Value>);
 

macro/src/lib.rs

@@ -56,6 +56,26 @@ mod pipe;
 /// assert_eq!(format!("{command:?}"), r#"NEW_VAR="new_var" TEST="test" "echo" "test""#);
 /// ```
 ///
+/// #### Conditional
+/// You can have a default value set for an environment variable.
+///
+/// ```rust
+/// # use comlexr_macro::cmd;
+/// const NEW_VAR: &str = "NEW_VAR";
+/// std::env::set_var("TEST", "realvalue");
+///
+/// let command = cmd!(
+///     env {
+///         "TEST":? "test",
+///         NEW_VAR: "new_var"
+///     };
+///     "echo",
+///     "test",
+/// );
+///
+/// assert_eq!(format!("{command:?}"), r#"NEW_VAR="new_var" "echo" "test""#);
+/// ```
+///
 /// ### Current Directory and Environment Variable Order
 /// ```
 /// # use comlexr_macro::cmd;

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!(