nl

Number non-blank lines in a file

$ nl [path/to/file]

$ [command] | nl

$ nl -b a [path/to/file]

$ nl -b p'[pattern]' [path/to/file]

$ nl -i [increment] [path/to/file]

$ nl -n [rz|ln|rn]

$ nl -w [col_width] [path/to/file]

$ nl -s "[separator]" [path/to/file]

nl [options] [file]

nl numbers lines from files or standard input. By default, it numbers only non-blank lines, preserving blank lines in the output without numbers.
The command recognizes logical page sections (header, body, footer) delimited by special character sequences, allowing different numbering styles for each section. Most commonly, it's used simply to add line numbers to file contents.
Output format is customizable: numbers can be left or right justified, padded with spaces or zeros, and separated from text by any string.

-b, --body-numbering _style_

Numbering style: a (all), t (non-blank, default), n (none), pregex (pattern)

Increment for line numbers (default: 1)

Format: ln (left, no zeros), rn (right, no zeros), rz (right, with zeros)

Width of line number column (default: 6)

String between number and line (default: TAB)

First line number (default: 1)

Characters for logical page delimiters

Different from cat -n, which numbers all lines including blank ones. The logical page feature (sections) is rarely used but can cause unexpected behavior if input contains the delimiter characters (\\: by default).

nl originated in System V Unix and is specified by POSIX. It has been part of the GNU coreutils since the project's inception.

cat(1), pr(1), head(1), tail(1)