stat: warn once when QUOTING_STYLE env has an unknown value

src/uu/stat/locales/en-US.ftl

@@ -56,6 +56,7 @@ stat-after-help = Valid format sequences for files (without `--file-system`):
 ## Error messages
 
 stat-error-invalid-quoting-style = Invalid quoting style: {$style}
+stat-warning-invalid-env-quoting-style = ignoring invalid value of environment variable QUOTING_STYLE: '{$style}'
 stat-error-missing-operand = missing operand
   Try 'stat --help' for more information.
 stat-error-invalid-directive = {$directive}: invalid directive

src/uu/stat/locales/fr-FR.ftl

@@ -92,6 +92,7 @@ stat-word-birth = Créé
 ## Messages d'erreur
 
 stat-error-invalid-quoting-style = Style de guillemets invalide : {$style}
+stat-warning-invalid-env-quoting-style = valeur invalide de la variable d'environnement QUOTING_STYLE ignorée : '{$style}'
 stat-error-missing-operand = opérande manquant
   Essayez 'stat --help' pour plus d'informations.
 stat-error-invalid-directive = {$directive} : directive invalide

src/uu/stat/src/stat.rs

@@ -451,16 +451,32 @@ fn quote_file_name(file_name: &str, quoting_style: &QuotingStyle) -> String {
     }
 }
 
+fn warn_invalid_quoting_style(style: &str) {
+    use std::sync::atomic::{AtomicBool, Ordering};
+    static WARNED: AtomicBool = AtomicBool::new(false);
+    if !WARNED.swap(true, Ordering::Relaxed) {
+        show_error!(
+            "{}",
+            translate!("stat-warning-invalid-env-quoting-style", "style" => style.to_string())
+        );
+    }
+}
+
 fn get_quoted_file_name(
     display_name: &str,
     file: &OsString,
     file_type: FileType,
     from_user: bool,
 ) -> Result<String, i32> {
-    let quoting_style = env::var("QUOTING_STYLE")
-        .ok()
-        .and_then(|style| style.parse().ok())
-        .unwrap_or_default();
+    let quoting_style = match env::var("QUOTING_STYLE") {
+        Ok(style) => style.parse().unwrap_or_else(|_| {
+            // Match GNU coreutils 9.11: warn (once) when QUOTING_STYLE is set
+            // to a value we don't understand, then fall back to the default.
+            warn_invalid_quoting_style(&style);
+            QuotingStyle::default()
+        }),
+        Err(_) => QuotingStyle::default(),
+    };
 
     if file_type.is_symlink() {
         let quoted_display_name = quote_file_name(display_name, &quoting_style);

tests/by-util/test_stat.rs

@@ -3,6 +3,8 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
+// spell-checker:ignore crème brûlée
+
 use uutests::at_and_ucmd;
 use uutests::new_ucmd;
 use uutests::unwrap_or_return;
@@ -447,6 +449,52 @@ fn test_quoting_style_locale() {
         .stdout_only("\'\"\'\n");
 }
 
+#[test]
+fn test_quoting_style_invalid_env() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+    at.touch("baguette");
+    at.touch("Croissant");
+    at.touch("Escargot");
+
+    let needle = "ignoring invalid value of environment variable QUOTING_STYLE";
+
+    // A bogus value triggers exactly one warning across multiple files and the
+    // output falls back to the default (shell-escape) style.
+    let res = ts
+        .ucmd()
+        .env("QUOTING_STYLE", "fromage")
+        .args(&["-c", "nom=[%N]", "baguette", "Croissant", "Escargot"])
+        .succeeds();
+    res.stdout_is("nom=['baguette']\nnom=['Croissant']\nnom=['Escargot']\n");
+    assert_eq!(res.stderr_str().matches(needle).count(), 1);
+
+    // An empty value is also invalid and must be reported with empty quotes.
+    ts.ucmd()
+        .env("QUOTING_STYLE", "")
+        .args(&["-c", "%N", "baguette"])
+        .succeeds()
+        .stdout_is("'baguette'\n")
+        .stderr_is("stat: ignoring invalid value of environment variable QUOTING_STYLE: ''\n");
+
+    // %%%N: a literal '%' followed by the quoted name, fallback style applies.
+    ts.ucmd()
+        .env("QUOTING_STYLE", "soufflé")
+        .args(&["-c", "%%%N", "baguette"])
+        .succeeds()
+        .stdout_is("%'baguette'\n")
+        .stderr_is(
+            "stat: ignoring invalid value of environment variable QUOTING_STYLE: 'soufflé'\n",
+        );
+
+    // When the format never consults %N, QUOTING_STYLE must not be parsed at all.
+    ts.ucmd()
+        .env("QUOTING_STYLE", "crème-brûlée")
+        .args(&["-c", "taille=%s genre:%F brut=%n", "baguette"])
+        .succeeds()
+        .no_stderr();
+}
+
 #[test]
 fn test_printf_octal_1() {
     let ts = TestScenario::new(util_name!());