nl

Additional Notes

nl numbers the lines of a file. It is the line-numbering counterpart of cat -n, but offers much more control over the numbering format, including the ability to number only non-empty lines, use different numbering styles, custom separators, and page-wise numbering (headers, footers, and body).

It is useful for printing source code with line numbers, creating numbered reference lists, or formatting documents where line numbers are needed. The number format, width, and separator are all configurable.

Syntax

nl [options] [file...]

Parameters

• file: One or more files to number. Reads from stdin if no file is given.

Common Options

• -b style, --body-numbering=style: Line numbering style for the body.
• a: Number all lines.
• t: Number only non-empty lines (default).
• n: No line numbering.
• pREGEX: Number only lines matching a regular expression.
• -n format, --number-format=format: Number format.
• ln: Left-justified, no leading zeros.
• rn: Right-justified, no leading zeros (default).
• rz: Right-justified, with leading zeros.
• -w width, --number-width=width: Set the number column width (default 6).
• -s string, --number-separator=string: String to separate the number from the text (default is tab).
• -i number, --line-increment=number: Increment line numbers by number (default 1).
• -v number, --starting-line-number=number: Starting line number (default 1).
• -p, --page-increment: Do not reset line numbers at page boundaries.
• -l n, --join-blank-lines=n: Group n blank lines into one.
• -d xy, --section-delimiter=xy: Set the delimiter characters for page sections (default :\:).

Page Sections

nl treats input as composed of three sections: header, body, and footer. Each section starts with a delimiter line:

• \:\:\:: Start of header.
• \:\:: Start of body (also resets line numbers).
• \:: Start of footer.

Examples

nl file.txt

Number lines of file.txt, skipping blank lines (default).

nl -b a file.txt

Number all lines, including blank ones (like cat -n).

nl -b a -w 3 -s ": " file.txt

Number all lines; use a 3-character wide number with : as separator.

nl -b p^# config.conf

Number only lines that start with # (comments).

nl -n rz -w 4 file.txt

Number with zero-padded, right-justified format (width 4).

nl -v 10 -i 5 file.txt

Start numbering at 10, increment by 5.

nl -b t -s ". " notes.txt

Number non-empty lines with a period and space as separator.

Practical Notes

• cat -n is simpler but less flexible; use nl when you need control over formatting.
• The default style (-b t) skips empty lines, which is useful for source code.
• For quick line numbering of files that also show non-printing characters, use cat -An.
• The official nl behavior is defined by POSIX, so it should work identically across different Unix systems.