fix(numfmt): format output on error messages

src/uu/numfmt/src/format.rs

@@ -452,6 +452,26 @@ fn format_string(
     ))
 }
 
+/// Encodes a byte slice as a string, representing non-UTF-8 bytes and non-printable ASCII
+/// bytes as octal escapes. Valid UTF-8 multi-byte characters pass through unchanged.
+/// Used to safely format invalid input in error messages.
+pub(crate) fn escape_line(line: &[u8]) -> String {
+    let mut result = String::new();
+    for chunk in line.utf8_chunks() {
+        for c in chunk.valid().chars() {
+            if c.is_ascii() && !c.is_ascii_graphic() && !c.is_ascii_whitespace() {
+                result.push_str(&format!("\\{:03o}", c as u8));
+            } else {
+                result.push(c);
+            }
+        }
+        for &b in chunk.invalid() {
+            result.push_str(&format!("\\{b:03o}"));
+        }
+    }
+    result
+}
+
 fn split_bytes<'a>(input: &'a [u8], delim: &'a [u8]) -> impl Iterator<Item = &'a [u8]> {
     let mut remainder = Some(input);
     std::iter::from_fn(move || {
@@ -488,7 +508,7 @@ pub fn write_formatted_with_delimiter<W: std::io::Write>(
         if field_selected {
             // Field must be valid UTF-8 for numeric conversion
             let field_str = std::str::from_utf8(field)
-                .map_err(|_| translate!("numfmt-error-invalid-number", "input" => String::from_utf8_lossy(field).into_owned().quote()))?
+                .map_err(|_| translate!("numfmt-error-invalid-number", "input" => escape_line(field).quote()))?
                 .trim_start();
             let formatted = format_string(field_str, options, None)?;
             writer.write_all(formatted.as_bytes()).unwrap();

src/uu/numfmt/src/numfmt.rs

@@ -4,7 +4,7 @@
 // file that was distributed with this source code.
 
 use crate::errors::NumfmtError;
-use crate::format::{write_formatted_with_delimiter, write_formatted_with_whitespace};
+use crate::format::{escape_line, write_formatted_with_delimiter, write_formatted_with_whitespace};
 use crate::options::{
     DEBUG, DELIMITER, FIELD, FIELD_DEFAULT, FORMAT, FROM, FROM_DEFAULT, FROM_UNIT,
     FROM_UNIT_DEFAULT, FormatOptions, HEADER, HEADER_DEFAULT, INVALID, InvalidModes, NUMBER,
@@ -99,7 +99,9 @@ fn write_line<W: std::io::Write>(
         // Whitespace mode requires valid UTF-8
         match std::str::from_utf8(&line) {
             Ok(s) => write_formatted_with_whitespace(writer, s, options, eol),
-            Err(_) => Err(translate!("numfmt-error-invalid-input")),
+            Err(_) => Err(
+                translate!("numfmt-error-invalid-number", "input" => escape_line(&line).quote()),
+            ),
         }
     };
 

tests/by-util/test_numfmt.rs

@@ -1272,3 +1272,13 @@ fn test_null_byte_input_multiline() {
         .succeeds()
         .stdout_is("1000\n3000");
 }
+
+#[test]
+fn test_invalid_utf8_input() {
+    // 0xFF is invalid UTF-8
+    new_ucmd!()
+        .pipe_in([b'1', b'0', b'\n', b'\xFF'])
+        .fails_with_code(2)
+        .stdout_is("10\n")
+        .stderr_is("numfmt: invalid number: '\\377'\n");
+}