Nl (Unix)

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

History
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. It first appeared in System V release 2.

The version of  bundled in GNU coreutils was written by Scott Bartram and David MacKenzie.

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.

Syntax
The command has a number of switches:


 * a - number all lines
 * t - number lines with printable text only
 * n - no line numbering
 * string - 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.

Example
The following example numbers only the lines that begin with a capital letter A (matching on the regular expression /^A/). filename is optional. It can be useful as an alternative to grep -n: