Fix tr argument handling for missing arguments (#1601)

* tests/tr ~ confirm failure for missing argument(s)

* fix/tr ~ mimic GNU error reponse for missing argument(s)

src/uu/tr/src/tr.rs

@@ -218,17 +218,29 @@ pub fn uumain(args: impl uucore::Args) -> i32 {
         return 0;
     }
 
-    if matches.free.is_empty() {
-        usage(&opts);
-        return 1;
-    }
-
     let dflag = matches.opt_present("d");
     let cflag = matches.opts_present(&["c".to_owned(), "C".to_owned()]);
     let sflag = matches.opt_present("s");
     let tflag = matches.opt_present("t");
     let sets = matches.free;
 
+    if sets.is_empty() {
+        show_error!(
+            "missing operand\nTry `{} --help` for more information.",
+            NAME
+        );
+        return 1;
+    }
+
+    if !(dflag || sflag) && sets.len() < 2 {
+        show_error!(
+            "missing operand after ‘{}’\nTry `{} --help` for more information.",
+            sets[0],
+            NAME
+        );
+        return 1;
+    }
+
     if cflag && !dflag && !sflag {
         show_error!("-c is only supported with -d or -s");
         return 1;

tests/by-util/test_tr.rs

@@ -116,3 +116,23 @@ fn test_truncate_with_set1_shorter_than_set2() {
         .run()
         .stdout_is("xycde");
 }
+
+#[test]
+fn missing_args_fails() {
+    let (_, mut ucmd) = at_and_ucmd!();
+    let result = ucmd.run();
+
+    assert!(!result.success);
+    assert!(result.stderr.contains("missing operand"));
+}
+
+#[test]
+fn missing_required_second_arg_fails() {
+    let (_, mut ucmd) = at_and_ucmd!();
+    let result = ucmd
+        .args(&["foo"])
+        .run();
+
+    assert!(!result.success);
+    assert!(result.stderr.contains("missing operand after"));
+}