nl is a Unix utility for numbering lines, either from a file or from standard input, reproducing output on standard output.

nl is part of the X/Open Portability Guide since issue 2 of 1987. It was inherited into the first version of POSIX.1 and the Single Unix Specification.^[1] It first appeared in System V release 2.^[2]

The version of nl bundled in GNU coreutils was written by Scott Bartram and David MacKenzie.^[3]

The command is available as a separate package for Microsoft Windows as part of the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities.^[4]

The command has a number of switches:

• a - number all lines
• t - number lines with printable text only
• n - no line numbering
• pstring - number only those lines containing the regular expression defined in the string supplied.

The default applied switch is t.

nl also supports some command line options.

 $ nl tf
     1  echo press cr
     2  read cr
     3  done

The following example numbers only the lines that begin with a capital letter A (matching on the regular expression /^A/). filename is optional.

$ nl -b p^A filename
       apple
    1  Apple
       BANANA
    2  Allspice
       strawberry

It can be useful as an alternative to grep -n:

$ cat somefile
aaaa
bbbb
cccc
dddc
$ nl -ba somefile | grep cccc
    3 cccc

• wc (Unix) – the word count command
• cat (Unix) – concatenate command (-n flag is equivalent to nl -a)
• List of Unix commands