head: fix -c0 and -n0 on directories to match GNU behavior

When zero bytes or zero lines are requested, there is nothing to read,
so we should not check whether the path is a directory. GNU head
succeeds in this case because it never attempts to open/read the file.

Previously, uutils head would stat the file and reject directories
even when no reading was needed, causing a spurious 'Is a directory'
error.

Fixes #12215

Author: sylvestre

src/uu/head/src/head.rs

@@ -447,15 +447,12 @@ fn uu_head(options: &HeadOptions) -> UResult<()> {
 
             Ok(())
         } else {
-            if Path::new(file).is_dir() {
-                show!(USimpleError::new(
-                    1,
-                    translate!("head-error-reading-file", "name" => file.quote(), "err" => "Is a directory")
-                ));
-                continue;
-            }
-            let mut file_handle = match File::open(file) {
-                Ok(f) => f,
+            // Stat the path first so we know whether to print the header.
+            // GNU head prints "==> name <==" for existing files and
+            // directories, but NOT for nonexistent ones — those produce
+            // only an error message.
+            let metadata = match Path::new(file).metadata() {
+                Ok(m) => m,
                 Err(err) => {
                     show!(err.map_err_context(
                         || translate!("head-error-cannot-open", "name" => file.quote())
@@ -470,7 +467,32 @@ fn uu_head(options: &HeadOptions) -> UResult<()> {
                 write!(stdout, "==> ")?;
                 print_verbatim(file).unwrap();
                 writeln!(stdout, " <==")?;
+                first = false;
             }
+            // When 0 bytes or 0 lines are requested, there is nothing to
+            // read, so we should succeed on directories just like GNU head
+            // does. Skip opening the file entirely in that case (also
+            // avoids platform differences: on Windows, `File::open` on a
+            // directory fails with "Permission denied").
+            let zero_output = matches!(options.mode, Mode::FirstBytes(0) | Mode::FirstLines(0));
+            if metadata.is_dir() {
+                if !zero_output {
+                    show!(USimpleError::new(
+                        1,
+                        translate!("head-error-reading-file", "name" => file.quote(), "err" => "Is a directory")
+                    ));
+                }
+                continue;
+            }
+            let mut file_handle = match File::open(file) {
+                Ok(f) => f,
+                Err(err) => {
+                    show!(err.map_err_context(
+                        || translate!("head-error-cannot-open", "name" => file.quote())
+                    ));
+                    continue;
+                }
+            };
             head_file(&mut file_handle, options)?;
             Ok(())
         };

tests/by-util/test_head.rs

@@ -933,3 +933,50 @@ fn test_do_not_attempt_to_read_a_directory() {
         .fails_with_code(1)
         .stderr_contains("error reading '.'");
 }
+
+/// Regression test for https://github.com/uutils/coreutils/issues/12215
+/// `head -c0 <directory>` should succeed (nothing to read), matching GNU.
+#[test]
+fn test_zero_bytes_on_directory_succeeds() {
+    new_ucmd!().args(&["-c", "0", "."]).succeeds().no_output();
+}
+
+/// `head -n0 <directory>` should also succeed.
+#[test]
+fn test_zero_lines_on_directory_succeeds() {
+    new_ucmd!().args(&["-n", "0", "."]).succeeds().no_output();
+}
+
+/// GNU `head` prints the `==> name <==` header for every file argument
+/// (including directories) when invoked with multiple files. With non-zero
+/// output, directories also produce an "Is a directory" error after the
+/// header.
+#[cfg(not(windows))]
+#[test]
+fn test_directory_header_with_multiple_files() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+    at.mkdir("d");
+    at.write("f", "hello\n");
+    ts.ucmd()
+        .args(&["-c", "5", "d", "f"])
+        .fails_with_code(1)
+        .stdout_is("==> d <==\n\n==> f <==\nhello")
+        .stderr_contains("Is a directory");
+}
+
+/// With `-c 0` (zero output), the directory header is still printed but no
+/// error is emitted.
+#[cfg(not(windows))]
+#[test]
+fn test_directory_header_with_multiple_files_zero_output() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+    at.mkdir("d");
+    at.write("f", "hello\n");
+    ts.ucmd()
+        .args(&["-c", "0", "d", "f"])
+        .succeeds()
+        .stdout_is("==> d <==\n\n==> f <==\n")
+        .no_stderr();
+}