cut: fix -s flag ignored when delimiter is newline

When using newline as the delimiter (-d $'\n') with -s (only-delimited),
cut should suppress lines that do not contain the delimiter. However,
cut_fields_newline_char_delim() was not checking the only_delimited flag,
causing it to always output even when -s was specified.

The fix uses read_until() instead of split() to read segments. Unlike
split(), read_until() includes the delimiter in the buffer when found,
allowing us to detect whether a delimiter was actually present or if
we just hit EOF.

This commit:
- Adds only_delimited parameter to cut_fields_newline_char_delim()
- Uses read_until() to detect delimiter presence while reading
- If no delimiter found and only_delimited is true, returns early
- Adds test case for newline delimiter with -s flag

Fixes #10012

Author: rynewang

src/uu/cut/src/cut.rs

@@ -265,12 +265,34 @@ fn cut_fields_newline_char_delim<R: Read, W: Write>(
     reader: R,
     out: &mut W,
     ranges: &[Range],
+    only_delimited: bool,
     newline_char: u8,
     out_delim: &[u8],
 ) -> UResult<()> {
-    let buf_in = BufReader::new(reader);
+    let mut buf_in = BufReader::new(reader);
+    let mut segments: Vec<Vec<u8>> = Vec::new();
+    let mut found_delimiter = false;
+
+    // Read segments using read_until, which includes the delimiter in the buffer
+    // This lets us detect whether a delimiter was actually found
+    loop {
+        let mut segment = Vec::new();
+        if buf_in.read_until(newline_char, &mut segment)? == 0 {
+            break;
+        }
+        // If segment ends with delimiter, we found one - remove it from segment
+        if segment.last() == Some(&newline_char) {
+            found_delimiter = true;
+            segment.pop();
+        }
+        segments.push(segment);
+    }
+
+    // With -s (only_delimited), suppress output if no delimiter found
+    if only_delimited && !found_delimiter {
+        return Ok(());
+    }
 
-    let segments: Vec<_> = buf_in.split(newline_char).filter_map(|x| x.ok()).collect();
     let mut print_delim = false;
 
     for &Range { low, high } in ranges {
@@ -303,7 +325,14 @@ fn cut_fields<R: Read, W: Write>(
     match field_opts.delimiter {
         Delimiter::Slice(delim) if delim == [newline_char] => {
             let out_delim = opts.out_delimiter.unwrap_or(delim);
-            cut_fields_newline_char_delim(reader, out, ranges, newline_char, out_delim)
+            cut_fields_newline_char_delim(
+                reader,
+                out,
+                ranges,
+                field_opts.only_delimited,
+                newline_char,
+                out_delim,
+            )
         }
         Delimiter::Slice(delim) => {
             let matcher = ExactMatcher::new(delim);

tests/by-util/test_cut.rs

@@ -301,6 +301,25 @@ fn test_newline_as_delimiter_with_output_delimiter() {
         .stdout_only_bytes("a:b\n");
 }
 
+#[test]
+fn test_newline_as_delimiter_with_only_delimited() {
+    // When input has no newline delimiter and -s is specified,
+    // the line should be suppressed (no output)
+    new_ucmd!()
+        .args(&["-f1", "-d", "\n", "-s"])
+        .pipe_in("abc")
+        .succeeds()
+        .stdout_only_bytes("");
+
+    // When input has newline delimiter and -s is specified,
+    // it should output the selected field
+    new_ucmd!()
+        .args(&["-f1", "-d", "\n", "-s"])
+        .pipe_in("line1\nline2")
+        .succeeds()
+        .stdout_only_bytes("line1\n");
+}
+
 #[test]
 fn test_multiple_delimiters() {
     new_ucmd!()