pr: add default values for -s and -S separator options

src/uu/pr/src/pr.rs

@@ -276,14 +276,18 @@ pub fn uu_app() -> Command {
                 .short('s')
                 .long(options::COLUMN_CHAR_SEPARATOR)
                 .help(translate!("pr-help-column-char-separator"))
-                .value_name("char"),
+                .value_name("char")
+                .num_args(0..=1)
+                .default_missing_value("\t"),
         )
         .arg(
             Arg::new(options::COLUMN_STRING_SEPARATOR)
                 .short('S')
                 .long(options::COLUMN_STRING_SEPARATOR)
                 .help(translate!("pr-help-column-string-separator"))
-                .value_name("string"),
+                .value_name("string")
+                .num_args(0..=1)
+                .default_missing_value(""),
         )
         .arg(
             Arg::new(options::MERGE)

tests/by-util/test_pr.rs

@@ -627,3 +627,18 @@ fn test_page_header_width() {
     let regex = Regex::new(&pattern).unwrap();
     new_ucmd!().pipe_in("a").succeeds().stdout_matches(&regex);
 }
+
+#[test]
+fn test_separator_options_default_values() {
+    // -s and -S without arguments should use default values (TAB and empty string)
+    new_ucmd!()
+        .args(&["-t", "-2", "-s"])
+        .pipe_in("a\nb\n")
+        .succeeds()
+        .stdout_contains("\t");
+    new_ucmd!()
+        .args(&["-t", "-2", "-S"])
+        .pipe_in("a\nb\n")
+        .succeeds()
+        .stdout_does_not_contain("\t");
+}