shuf: Do not read input when -n0 is given

This is explicitly tested by some suites, including the GNU test suite.

src/uu/shuf/src/shuf.rs

@@ -92,6 +92,16 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         },
     };
 
+    if options.head_count == 0 {
+        // Do not attempt to read the random source or the input file.
+        // However, we must touch the output file, if given:
+        if let Some(s) = options.output {
+            File::create(&s[..])
+                .map_err_context(|| format!("failed to open {} for writing", s.quote()))?;
+        }
+        return Ok(());
+    }
+
     match mode {
         Mode::Echo(args) => {
             let mut evec = args.iter().map(String::as_bytes).collect::<Vec<_>>();

tests/by-util/test_shuf.rs

@@ -147,6 +147,81 @@ fn test_head_count() {
     );
 }
 
+#[test]
+fn test_zero_head_count_pipe() {
+    let result = new_ucmd!().arg("-n0").pipe_in(vec![]).succeeds();
+    // Output must be completely empty, not even a newline!
+    result.no_output();
+}
+
+#[test]
+fn test_zero_head_count_pipe_explicit() {
+    let result = new_ucmd!().arg("-n0").arg("-").pipe_in(vec![]).succeeds();
+    result.no_stderr();
+
+    let result_seq: &str = result.stdout_str();
+    assert!(
+        result_seq.is_empty(),
+        "Output must be completely empty, not even newline"
+    );
+}
+
+#[test]
+fn test_zero_head_count_file_unreadable() {
+    let result = new_ucmd!()
+        .arg("-n0")
+        .arg("/invalid/unreadable")
+        .pipe_in(vec![])
+        .succeeds();
+    result.no_stderr();
+
+    let result_seq: &str = result.stdout_str();
+    assert!(
+        result_seq.is_empty(),
+        "Output must be completely empty, not even newline"
+    );
+}
+
+#[test]
+fn test_zero_head_count_file_touch_output() {
+    new_ucmd!()
+        .arg("-n0")
+        .arg("-o")
+        .arg("/invalid/unwritable")
+        .pipe_in(vec![])
+        .fails()
+        .stderr_contains("failed to open '/invalid/unwritable' for writing:");
+}
+
+#[test]
+fn test_zero_head_count_echo() {
+    let result = new_ucmd!()
+        .arg("-n0")
+        .arg("-e")
+        .arg("hello")
+        .pipe_in(vec![])
+        .succeeds();
+    result.no_stderr();
+
+    let result_seq: &str = result.stdout_str();
+    assert!(
+        result_seq.is_empty(),
+        "Output must be completely empty, not even newline"
+    );
+}
+
+#[test]
+fn test_zero_head_count_range() {
+    let result = new_ucmd!().arg("-n0").arg("-i4-8").succeeds();
+    result.no_stderr();
+
+    let result_seq: &str = result.stdout_str();
+    assert!(
+        result_seq.is_empty(),
+        "Output must be completely empty, not even newline"
+    );
+}
+
 #[test]
 fn test_head_count_multi_big_then_small() {
     let repeat_limit = 5;