kill -1 should trigger an error

https://github.com/uutils/coreutils/issues/9699

src/uu/kill/src/kill.rs

@@ -137,8 +137,8 @@ pub fn uu_app() -> Command {
 }
 
 fn handle_obsolete(args: &mut Vec<String>) -> Option<usize> {
-    // Sanity check
-    if args.len() > 2 {
+    // Sanity check - need at least the program name and one argument
+    if args.len() >= 2 {
         // Old signal can only be in the first argument position
         let slice = args[1].as_str();
         if let Some(signal) = slice.strip_prefix('-') {

tests/by-util/test_kill.rs

@@ -395,3 +395,27 @@ fn test_kill_with_signal_and_table() {
         .arg("-t")
         .fails();
 }
+
+/// Test that `kill -1` (signal without PID) reports "no process ID" error
+/// instead of being misinterpreted as pid=-1 which would kill all processes.
+/// This matches GNU kill behavior.
+#[test]
+fn test_kill_signal_only_no_pid() {
+    // Test with -1 (SIGHUP)
+    new_ucmd!()
+        .arg("-1")
+        .fails()
+        .stderr_contains("no process ID specified");
+
+    // Test with -9 (SIGKILL)
+    new_ucmd!()
+        .arg("-9")
+        .fails()
+        .stderr_contains("no process ID specified");
+
+    // Test with -TERM
+    new_ucmd!()
+        .arg("-TERM")
+        .fails()
+        .stderr_contains("no process ID specified");
+}