ln: refuse to replace a directory with --backup, -f or -i

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

@@ -30,6 +30,7 @@ ln-error-extra-operand = extra operand {$operand}
   Try '{$program} --help' for more information.
 ln-error-could-not-update = Could not update {$target}: {$error}
 ln-error-will-not-overwrite = will not overwrite just-created {$target} with {$source}
+ln-error-cannot-overwrite-directory = {$dest}: cannot overwrite directory
 ln-prompt-replace = replace {$file}?
 ln-cannot-backup = cannot backup {$file}
 ln-failed-to-access = failed to access {$file}

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

@@ -31,6 +31,7 @@ ln-error-extra-operand = opérande supplémentaire {$operand}
   Essayez « {$program} --help » pour plus d'informations.
 ln-error-could-not-update = Impossible de mettre à jour {$target} : {$error}
 ln-error-will-not-overwrite = ne remplacera pas le fichier {$target} qui vient d'être créé par {$source}
+ln-error-cannot-overwrite-directory = {$dest} : impossible d'écraser le répertoire
 ln-prompt-replace = remplacer {$file} ?
 ln-cannot-backup = impossible de sauvegarder {$file}
 ln-failed-to-access = échec d'accès à {$file}

src/uu/ln/src/ln.rs

@@ -71,6 +71,9 @@ pub enum LnError {
 
     #[error("{}", translate!("ln-failed-to-create-hard-link-dir", "source" => _0.to_string_lossy()))]
     FailedToCreateHardLinkDir(PathBuf),
+
+    #[error("{}", translate!("ln-error-cannot-overwrite-directory", "dest" => _0.maybe_quote()))]
+    CannotOverwriteDirectory(PathBuf),
 }
 
 impl UError for LnError {
@@ -405,6 +408,13 @@ pub fn link(src: &Path, dst: &Path, settings: &Settings) -> LnResult<()> {
     };
 
     if dst.is_symlink() || dst.exists() {
+        // Never move a directory aside or replace it to make room for a link,
+        // matching GNU. A symlink to a directory is a link and is replaced.
+        if (settings.backup != BackupMode::None || settings.overwrite != OverwriteMode::NoClobber)
+            && dst.symlink_metadata().is_ok_and(|m| m.is_dir())
+        {
+            return Err(LnError::CannotOverwriteDirectory(dst.to_owned()));
+        }
         backup_path = backup_control::get_backup_path(settings.backup, dst, &settings.suffix);
         if settings.backup != BackupMode::None && !settings.symbolic {
             // `ln --backup f f`: the backup would move `f` away before the

tests/by-util/test_ln.rs

@@ -1091,6 +1091,63 @@ fn test_backup_simple_numbered_same_file() {
     }
 }
 
+#[test]
+fn test_backup_force_interactive_refuse_directory() {
+    for args in [
+        vec!["-s", "-b", "-t", "site", "src/static"],
+        vec!["-s", "--backup=numbered", "-t", "site", "src/static"],
+        vec!["-s", "-S", ".bak", "-t", "site", "src/static"],
+        vec!["-s", "-f", "-b", "-t", "site", "src/static"],
+        vec!["-s", "-r", "-b", "-t", "site", "src/static"],
+        vec!["-s", "-f", "-t", "site", "src/static"],
+        vec!["-s", "-i", "-t", "site", "src/static"],
+        vec!["-s", "-b", "-T", "src/static", "site/static"],
+        vec!["-b", "-t", "site", "static"],
+    ] {
+        let (at, mut ucmd) = at_and_ucmd!();
+        at.mkdir_all("site/static");
+        at.write("site/static/app.js", "original");
+        at.mkdir_all("src/static");
+        at.touch("static");
+
+        // ln refuses before reading the answer, so it may exit before the
+        // input is written.
+        ucmd.args(&args)
+            .pipe_in("y\n")
+            .ignore_stdin_write_error()
+            .fails_with_code(1)
+            .stderr_contains("static: cannot overwrite directory");
+
+        assert!(at.dir_exists("site/static"));
+        assert_eq!(at.read("site/static/app.js"), "original");
+        assert!(!at.dir_exists("site/static~"));
+        assert!(!at.dir_exists("site/static.~1~"));
+        assert!(!at.dir_exists("site/static.bak"));
+    }
+}
+
+#[test]
+fn test_backup_symlink_to_directory() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.mkdir_all("site");
+    at.mkdir_all("src/static");
+    at.mkdir("other");
+    at.relative_symlink_dir("../other", "site/static");
+
+    ucmd.args(&["-s", "-b", "-t", "site", "src/static"])
+        .succeeds()
+        .no_stderr();
+
+    assert_eq!(
+        at.read_symlink("site/static"),
+        "src/static".replace('/', MAIN_SEPARATOR_STR)
+    );
+    assert_eq!(
+        at.read_symlink("site/static~"),
+        "../other".replace('/', MAIN_SEPARATOR_STR)
+    );
+}
+
 #[test]
 fn test_backup_simple_same_file_into_directory() {
     let (at, mut ucmd) = at_and_ucmd!();