Add just_executable() function

README.adoc

@@ -585,6 +585,20 @@ $ just system-info
 This is an x86_64 machine
 ```
 
+- `just_excutable()` - Path to the running just executable.
+
+For example:
+
+```make
+itself:
+    @echo The executable is at: {{just_excutable()}}
+```
+
+```
+$ just itself
+The executable is at: /bin/just
+```
+
 ==== Environment Variables
 
 - `env_var(key)` – Retrieves the environment variable with name `key`, aborting if it is not present.

src/function.rs

@@ -18,6 +18,7 @@ lazy_static! {
     ("invocation_directory", Nullary(invocation_directory)),
     ("env_var", Unary(env_var)),
     ("env_var_or_default", Binary(env_var_or_default)),
+    ("just_executable", Nullary(just_executable)),
   ]
   .into_iter()
   .collect();
@@ -123,3 +124,18 @@ fn env_var_or_default(
     Ok(value) => Ok(value),
   }
 }
+
+fn just_executable(_context: &FunctionContext) -> Result<String, String> {
+  let exe_path = std::env::current_exe()
+    .map_err(|e| format!("Error getting just executable: {}", e))?;
+
+  exe_path
+    .to_str()
+    .map(str::to_owned)
+    .ok_or_else(|| {
+      format!(
+        "Just executable path is not valid unicode: {}",
+        exe_path.to_string_lossy()
+      )
+    })
+}