Fix -n flag to show line numbers in loop-through mode

When the -n/--number flag is passed on the command line, bat now shows
line numbers even when piping output to another process (loop-through
mode), similar to how `cat -n` behaves.

This change detects if -n or --number was passed on the CLI (before
merging with config file and environment variables) and disables
loop-through mode in that case, allowing the InteractivePrinter to
add line numbers.

The existing behavior is preserved:
- Styles from config/env are still ignored when piping (unless --decorations=always is set)
- Only the -n flag from CLI enables line numbers in piped mode
- -p and --style options from CLI do not disable loop-through mode

CHANGELOG.md

@@ -9,7 +9,7 @@
 - Fix hang when using `--list-themes` with an explicit pager, see #3457 (@abhinavcool42)
 - Fix negative values of N not being parsed in <N:M> line ranges without `=` flag value separator, see #3442 (@lmmx)
 - Fix broken Docker syntax preventing use of custom assets, see #3476 (@keith-hall)
-- Fix decorations being applied unexpectedly when piping, breaking cat compatibility. See #3496 (@keith-hall)
+- Fix decorations being applied unexpectedly when piping. Now only line numbers explicitly required on the command line should be applied in auto decorations mode for `cat` compatibility. See #3496 (@keith-hall)
 
 ## Other
 - Improve README documentation on pager options passed to less, see #3443 (@injust)

src/bin/bat/app.rs

@@ -46,6 +46,10 @@ enum HelpType {
 pub struct App {
     pub matches: ArgMatches,
     interactive_output: bool,
+    /// True if -n / --number was passed on the command line
+    /// (not from config file or environment variables).
+    /// This is used to honor the flag when piping output, similar to `cat -n`.
+    number_from_cli: bool,
 }
 
 impl App {
@@ -54,6 +58,15 @@ impl App {
         let _ = nu_ansi_term::enable_ansi_support();
 
         let interactive_output = std::io::stdout().is_terminal();
+        
+        // Check if the -n / --number option was passed on the command line
+        // (before merging with config file and environment variables).
+        // This is needed to honor the -n flag when piping output, similar to `cat -n`.
+        let number_from_cli = wild::args_os().any(|arg| {
+            let arg_str = arg.to_string_lossy();
+            arg_str == "-n" || arg_str == "--number"
+        });
+        
         let matches = Self::matches(interactive_output)?;
 
         if matches.get_flag("help") {
@@ -91,6 +104,7 @@ impl App {
         Ok(App {
             matches,
             interactive_output,
+            number_from_cli,
         })
     }
 
@@ -384,7 +398,7 @@ impl App {
                     .map(|s| s.as_str())
                     == Some("always")
                 || self.matches.get_flag("force-colorization")
-                || self.matches.get_flag("number")),
+                || self.number_from_cli),
             tab_width: self
                 .matches
                 .get_one::<String>("tabs")

tests/integration_tests.rs

@@ -166,6 +166,17 @@ fn line_numbers() {
         .stdout("   1 line 1\n   2 line 2\n   3 line 3\n   4 line 4\n   5 line 5\n   6 line 6\n   7 line 7\n   8 line 8\n   9 line 9\n  10 line 10\n");
 }
 
+// Test that -n on command line shows line numbers even when piping (similar to `cat -n`)
+#[test]
+fn line_numbers_from_cli_in_loop_through_mode() {
+    bat()
+        .arg("multiline.txt")
+        .arg("-n")
+        .assert()
+        .success()
+        .stdout("   1 line 1\n   2 line 2\n   3 line 3\n   4 line 4\n   5 line 5\n   6 line 6\n   7 line 7\n   8 line 8\n   9 line 9\n  10 line 10\n");
+}
+
 #[test]
 fn line_range_2_3() {
     bat()
@@ -419,8 +430,8 @@ fn piped_output_with_line_numbers_style_flag() {
 }
 
 #[test]
-#[cfg(not(target_os = "windows"))]
 fn piped_output_with_line_numbers_with_header_grid_style_flag() {
+    // style ignored because non-interactive
     bat()
         .arg("--style=header,grid,numbers")
         .write_stdin("hello\nworld\n")