handle an existing shebang in uv init --script

crates/uv-scripts/Cargo.toml

@@ -16,11 +16,13 @@ uv-pep508 = { workspace = true }
 uv-pypi-types = { workspace = true }
 uv-redacted = { workspace = true }
 uv-settings = { workspace = true }
+uv-warnings = { workspace = true }
 uv-workspace = { workspace = true }
 
 fs-err = { workspace = true, features = ["tokio"] }
 indoc = { workspace = true }
 memchr = { workspace = true }
+regex = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 thiserror = { workspace = true }
 toml = { workspace = true }

crates/uv-scripts/src/lib.rs

@@ -14,6 +14,7 @@ use uv_pep508::PackageName;
 use uv_pypi_types::VerbatimParsedUrl;
 use uv_redacted::DisplaySafeUrl;
 use uv_settings::{GlobalOptions, ResolverInstallerOptions};
+use uv_warnings::warn_user;
 use uv_workspace::pyproject::Sources;
 
 static FINDER: LazyLock<Finder> = LazyLock::new(|| Finder::new(b"# /// script"));
@@ -238,11 +239,25 @@ impl Pep723Script {
         let metadata = serialize_metadata(&default_metadata);
 
         let script = if let Some(existing_contents) = existing_contents {
+            let (mut shebang, contents) = extract_shebang(&existing_contents)?;
+            if !shebang.is_empty() {
+                shebang.push_str("\n#\n");
+                // If the shebang doesn't contain `uv`, it's probably something like
+                // `#! /usr/bin/env python`, which isn't going to respect the inline metadata.
+                // Issue a warning for users who might not know that.
+                // TODO: There are a lot of mistakes we could consider detecting here, like
+                // `uv run` without `--script` when the file doesn't end in `.py`.
+                if !regex::Regex::new(r"\buv\b").unwrap().is_match(&shebang) {
+                    warn_user!(
+                        "If you execute {} directly, it might ignore its inline metadata.\nConsider replacing its shebang with: {}",
+                        file.to_string_lossy().cyan(),
+                        "#!/usr/bin/env -S uv run --script".cyan(),
+                    );
+                }
+            }
             indoc::formatdoc! {r"
-            {metadata}
-            {content}
-            ",
-            content = String::from_utf8(existing_contents).map_err(|err| Pep723Error::Utf8(err.utf8_error()))?}
+            {shebang}{metadata}
+            {contents}" }
         } else {
             indoc::formatdoc! {r#"
             {metadata}

crates/uv-warnings/src/lib.rs

@@ -24,7 +24,7 @@ pub fn disable() {
 /// Warn a user, if warnings are enabled.
 #[macro_export]
 macro_rules! warn_user {
-    ($($arg:tt)*) => {
+    ($($arg:tt)*) => {{
         use $crate::anstream::eprintln;
         use $crate::owo_colors::OwoColorize;
 
@@ -33,7 +33,7 @@ macro_rules! warn_user {
             let formatted = message.bold();
             eprintln!("{}{} {formatted}", "warning".yellow().bold(), ":".bold());
         }
-    };
+    }};
 }
 
 pub static WARNINGS: LazyLock<Mutex<FxHashSet<String>>> = LazyLock::new(Mutex::default);
@@ -42,7 +42,7 @@ pub static WARNINGS: LazyLock<Mutex<FxHashSet<String>>> = LazyLock::new(Mutex::d
 /// message.
 #[macro_export]
 macro_rules! warn_user_once {
-    ($($arg:tt)*) => {
+    ($($arg:tt)*) => {{
         use $crate::anstream::eprintln;
         use $crate::owo_colors::OwoColorize;
 
@@ -54,5 +54,5 @@ macro_rules! warn_user_once {
                 }
             }
         }
-    };
+    }};
 }

crates/uv/tests/it/init.rs

@@ -929,6 +929,65 @@ fn init_script_file_conflicts() -> Result<()> {
     Ok(())
 }
 
+// Init script should not trash an existing shebang.
+#[test]
+fn init_script_shebang() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let script_path = context.temp_dir.child("script.py");
+
+    let contents = "#! /usr/bin/env python3\nprint(\"Hello, world!\")";
+    fs_err::write(&script_path, contents)?;
+    uv_snapshot!(context.filters(), context.init().arg("--script").arg("script.py"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: If you execute script.py directly, it might ignore its inline metadata.
+    Consider replacing its shebang with: #!/usr/bin/env -S uv run --script
+    Initialized script at `script.py`
+    ");
+    let resulting_script = fs_err::read_to_string(&script_path)?;
+    assert_snapshot!(resulting_script, @r#"
+    #! /usr/bin/env python3
+    #
+    # /// script
+    # requires-python = ">=3.12"
+    # dependencies = []
+    # ///
+
+    print("Hello, world!")
+    "#
+    );
+
+    // If the shebang already contains `uv`, the result is the same, but we suppress the warning.
+    let contents = "#!/usr/bin/env -S uv run --script\nprint(\"Hello, world!\")";
+    fs_err::write(&script_path, contents)?;
+    uv_snapshot!(context.filters(), context.init().arg("--script").arg("script.py"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Initialized script at `script.py`
+    ");
+    let resulting_script = fs_err::read_to_string(&script_path)?;
+    assert_snapshot!(resulting_script, @r#"
+    #!/usr/bin/env -S uv run --script
+    #
+    # /// script
+    # requires-python = ">=3.12"
+    # dependencies = []
+    # ///
+
+    print("Hello, world!")
+    "#
+    );
+
+    Ok(())
+}
+
 /// Run `uv init --lib` with an existing py.typed file
 #[test]
 fn init_py_typed_exists() -> Result<()> {

docs/guides/scripts.md

@@ -241,10 +241,12 @@ Declaration of dependencies is also supported in this context, for example:
 
 ```python title="example"
 #!/usr/bin/env -S uv run --script
+#
 # /// script
 # requires-python = ">=3.12"
 # dependencies = ["httpx"]
 # ///
+
 import httpx
 
 print(httpx.get("https://example.com"))