The nl command reads lines from the named file , or the standard input if no file is named, and reproduces the lines on the standard output. Lines are numbered on the left in accordance with the command options in effect.
Line numbering is reset at the start of each logical page.
Empty sections are valid. Different line numbering options are independently available for header, body, and footer. Unless optioned otherwise, nl assumes the text being read is in a single logical page body. Only one file may be named. The specified default is used when the option is not entered on the command line.