feat: implement -b / --number-nonblank for cat -b compatibility

Adds a `-b` / `--number-nonblank` flag that numbers only non-blank
lines.  Blank lines (those containing only the line terminator) are
printed without a number and don't advance the counter, matching
`cat -b` behaviour.

- `-b` implies `--style=numbers` (same as `-n`), but only counts
  non-blank lines
- `-b` takes precedence over `-n`: both `-bn` and `-nb` behave like `-b`
- `-bp` disables numbering (plain mode wins)
- Works with files, stdin, and piped output (like `-n`)

Closes #3856.

CHANGELOG.md

@@ -8,6 +8,7 @@
 
 ## Features
 
+- Add `-b` / `--number-nonblank` flag to only number non-blank lines, for `cat -b` compatibility. Closes #3856, see #3857 (@MeGaurav4)
 - Add a `--sanitize=<auto|always|never>` flag for safe display of untrusted input. It implies `--strip-ansi` at the same value and additionally substitutes terminal-active control bytes (cursor moves, charset switches, beep, etc.) and Unicode bidi / zero-width formatting characters with the Unicode replacement character (U+FFFD). Mitigates Trojan-Source-style spoofing (CVE-2021-42574). See #3729 (@curious-rabbit)
 - Map justfile, Justfile, .justfile, and *.justfile to Makefile syntax highlighting, see #3623 (@zachvalenta)
 - Preserve `--diff` change markers and snip separators when `--plain` is set. Closes #3630, see #3643 (@mvanhorn)

doc/long-help.txt

@@ -83,6 +83,16 @@ Options:
   -n, --number
           Only show line numbers, no other decorations. This is an alias for '--style=numbers'
 
+  -b, --number-nonblank
+          Only show line numbers for non-blank lines, no other decorations. This is an alias for
+          '--style=numbers'. Non-blank lines are lines that contain any character before the line
+          ending, including spaces and tabs. When used together with --number (-n),
+          --number-nonblank (-b) takes precedence.
+          
+          Example:
+            printf 'alpha\n\nbeta\n' | bat -b
+            numbers 'alpha' and 'beta', but skips the empty line.
+
       --color <when>
           Specify when to use colored output. The automatic mode only enables colors if an
           interactive terminal is detected - colors are automatically disabled if the output goes to

doc/short-help.txt

@@ -33,6 +33,8 @@ Options:
           Truncate all lines longer than screen width. Alias for '--wrap=never'.
   -n, --number
           Show line numbers (alias for '--style=numbers').
+  -b, --number-nonblank
+          Show line numbers for non-blank lines only (alias for '--style=numbers').
       --color <when>
           When to use colors (*auto*, never, always).
       --italic-text <when>

src/bin/bat/app.rs

@@ -59,6 +59,10 @@ pub struct App {
     /// (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,
+    /// True if -b / --number-nonblank 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 -b`.
+    number_nonblank_from_cli: bool,
 }
 
 impl App {
@@ -99,6 +103,29 @@ impl App {
             false
         });
 
+        // Check if the -b / --number-nonblank option was passed on the command line
+        // (before merging with config file and environment variables).
+        // This is needed to honor the -b flag when piping output, similar to `cat -b`.
+        // The same combined-flag logic applies as for -n above.
+        let number_nonblank_from_cli = wild::args_os().any(|arg| {
+            let arg_str = arg.to_string_lossy();
+            if arg_str == "-b" || arg_str == "--number-nonblank" {
+                return true;
+            }
+            // Handle combined short flags
+            if arg_str.starts_with('-') && !arg_str.starts_with("--") && arg_str.len() > 2 {
+                let chars: Vec<char> = arg_str.chars().skip(1).collect();
+                let b_pos = chars.iter().position(|&c| c == 'b');
+                let p_pos = chars.iter().position(|&c| c == 'p');
+                if let Some(b) = b_pos {
+                    if p_pos.is_none() || b > p_pos.unwrap() {
+                        return true;
+                    }
+                }
+            }
+            false
+        });
+
         let matches = Self::matches(interactive_output)?;
 
         if matches.get_flag("help") {
@@ -139,6 +166,7 @@ impl App {
             matches,
             interactive_output,
             number_from_cli,
+            number_nonblank_from_cli,
         })
     }
 
@@ -454,7 +482,8 @@ impl App {
                     .map(|s| s.as_str())
                     == Some("always")
                 || self.matches.get_flag("force-colorization")
-                || self.number_from_cli),
+                || self.number_from_cli
+                || self.number_nonblank_from_cli),
             tab_width: self
                 .matches
                 .get_one::<String>("tabs")
@@ -495,6 +524,8 @@ impl App {
             ),
             quiet_empty: self.matches.get_flag("quiet-empty"),
             unbuffered: self.matches.get_flag("unbuffered"),
+            number_nonblank: self.matches.get_flag("number-nonblank")
+                || self.number_nonblank_from_cli,
             theme: theme(self.theme_options()).to_string(),
             visible_lines: match self.matches.try_contains_id("diff").unwrap_or_default()
                 && self.matches.get_flag("diff")
@@ -613,6 +644,13 @@ impl App {
             ])));
         }
 
+        // Only line numbers for non-blank lines if `--number-nonblank`.
+        if self.matches.get_flag("number-nonblank") || self.number_nonblank_from_cli {
+            return Some(StyleComponents(HashSet::from([
+                StyleComponent::LineNumbers,
+            ])));
+        }
+
         // Plain if `--plain` is specified at least once.
         if self.matches.get_count("plain") > 0 {
             let mut components = HashSet::from([StyleComponent::Plain]);

src/bin/bat/clap_app.rs

@@ -97,6 +97,7 @@ pub fn build_app(interactive_output: bool) -> Command {
             Arg::new("plain")
                 .overrides_with("plain")
                 .overrides_with("number")
+                .overrides_with("number-nonblank")
                 .short('p')
                 .long("plain")
                 .action(ArgAction::Count)
@@ -277,6 +278,23 @@ pub fn build_app(interactive_output: bool) -> Command {
                      '--style=numbers'",
                 ),
         )
+        .arg(
+            Arg::new("number-nonblank")
+                .long("number-nonblank")
+                .overrides_with("number-nonblank")
+                .short('b')
+                .action(ArgAction::SetTrue)
+                .help("Show line numbers for non-blank lines only (alias for '--style=numbers').")
+                .long_help(
+                    "Only show line numbers for non-blank lines, no other decorations. This is an \
+                     alias for '--style=numbers'. Non-blank lines are lines that contain any \
+                     character before the line ending, including spaces and tabs. When used \
+                     together with --number (-n), --number-nonblank (-b) takes precedence.\n\n\
+                     Example:\n  \
+                     printf 'alpha\\n\\nbeta\\n' | bat -b\n  \
+                     numbers 'alpha' and 'beta', but skips the empty line.",
+                ),
+        )
         .arg(
             Arg::new("color")
                 .long("color")

src/config.rs

@@ -119,6 +119,10 @@ pub struct Config<'a> {
 
     /// Whether or not to use unbuffered input reading for streaming use cases
     pub unbuffered: bool,
+
+    /// Only number non-blank lines (like `cat -b`). Has no effect if `style_components` doesn't
+    /// include `LineNumbers`.
+    pub number_nonblank: bool,
 }
 
 #[cfg(all(feature = "minimal-application", feature = "paging"))]

src/controller.rs

@@ -276,10 +276,21 @@ impl Controller<'_> {
             }
             if !reached_eof {
                 if reader.read_line(&mut current_line_buffer)? {
-                    // Fill the buffer
-                    buffered_lines
-                        .push_back((mem::take(&mut current_line_buffer), current_line_number));
-                    current_line_number += 1;
+                    // Fill the buffer. In number-nonblank mode, don't advance the
+                    // line counter for empty lines (content-free lines that contain
+                    // only the line terminator).
+                    if self.config.number_nonblank
+                        && current_line_buffer
+                            .iter()
+                            .all(|&b| b == b'\r' || b == b'\n')
+                    {
+                        buffered_lines
+                            .push_back((mem::take(&mut current_line_buffer), current_line_number));
+                    } else {
+                        buffered_lines
+                            .push_back((mem::take(&mut current_line_buffer), current_line_number));
+                        current_line_number += 1;
+                    }
                 } else {
                     // No more data to read
                     reached_eof = true;

src/decorations.rs

@@ -45,6 +45,14 @@ impl Decoration for LineNumberDecoration {
         continuation: bool,
         _printer: &InteractivePrinter,
     ) -> DecorationText {
+        if line_number == 0 {
+            // Blank line in number-nonblank mode: show empty space instead of a number.
+            return DecorationText {
+                text: self.color.paint(" ".repeat(self.width())).to_string(),
+                width: self.width(),
+            };
+        }
+
         if continuation {
             if line_number >= self.cached_wrap_invalid_at {
                 let new_width = self.cached_wrap.width + 1;

src/printer.rs

@@ -738,10 +738,17 @@ impl Printer for InteractivePrinter<'_> {
 
         // Line decorations.
         if self.panel_width > 0 {
+            let display_line_number =
+                if self.config.number_nonblank && line.trim_end_matches(['\r', '\n']).is_empty() {
+                    0
+                } else {
+                    line_number
+                };
+
             let decorations = self
                 .decorations
                 .iter()
-                .map(|d| d.generate(line_number, false, self));
+                .map(|d| d.generate(display_line_number, false, self));
 
             for deco in decorations {
                 write!(handle, "{} ", deco.text)?;

tests/integration_tests.rs

@@ -213,6 +213,69 @@ fn numbers_honored_from_cli_when_preceeded_by_plain_in_loop_through_mode() {
         .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 number_nonblank_style() {
+    bat()
+        .arg("empty_lines.txt")
+        .arg("-b")
+        .arg("--decorations=always")
+        .assert()
+        .success()
+        .stdout("   1 line 1\n     \n     \n     \n   2 line 5\n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n   3 line 20\n   4 line 21\n     \n     \n   5 line 24\n     \n   6 line 26\n     \n     \n     \n   7 line 30\n");
+}
+
+#[test]
+fn number_nonblank_from_cli_in_loop_through_mode() {
+    bat()
+        .arg("empty_lines.txt")
+        .arg("-b")
+        .assert()
+        .success()
+        .stdout("   1 line 1\n     \n     \n     \n   2 line 5\n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n   3 line 20\n   4 line 21\n     \n     \n   5 line 24\n     \n   6 line 26\n     \n     \n     \n   7 line 30\n");
+}
+
+#[test]
+fn number_nonblank_takes_precedence_over_number() {
+    // -bn should behave like -b
+    bat()
+        .arg("empty_lines.txt")
+        .arg("-bn")
+        .arg("--decorations=always")
+        .assert()
+        .success()
+        .stdout("   1 line 1\n     \n     \n     \n   2 line 5\n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n   3 line 20\n   4 line 21\n     \n     \n   5 line 24\n     \n   6 line 26\n     \n     \n     \n   7 line 30\n");
+
+    // -nb should also behave like -b
+    bat()
+        .arg("empty_lines.txt")
+        .arg("-nb")
+        .arg("--decorations=always")
+        .assert()
+        .success()
+        .stdout("   1 line 1\n     \n     \n     \n   2 line 5\n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n     \n   3 line 20\n   4 line 21\n     \n     \n   5 line 24\n     \n   6 line 26\n     \n     \n     \n   7 line 30\n");
+}
+
+#[test]
+fn number_nonblank_ignored_when_followed_by_plain() {
+    bat()
+        .arg("empty_lines.txt")
+        .arg("-bp")
+        .arg("--decorations=auto")
+        .assert()
+        .success()
+        .stdout("line 1\n\n\n\nline 5\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nline 20\nline 21\n\n\nline 24\n\nline 26\n\n\n\nline 30\n");
+}
+
+#[test]
+fn piped_output_with_number_nonblank_flag() {
+    bat()
+        .arg("-b")
+        .write_stdin("hello\n\nworld\n")
+        .assert()
+        .success()
+        .stdout("   1 hello\n     \n   2 world\n");
+}
+
 #[test]
 fn line_range_2_3() {
     bat()