nl is a Unix

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

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

It 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 a number of command line options.

Example

$ 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

Regular expression

In computing, a regular expression provides a concise and flexible means for "matching" strings of text, such as particular characters, words, or patterns of characters. Abbreviations for "regular expression" include "regex" and "regexp"...

 /^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

Grep

grep is a command-line text-search utility originally written for Unix. The name comes from the ed command g/re/p...

:
$ cat somefile
aaa
bbb
ccc
ddd
$ nl somefile | grep ccc
3 ccc

See also

• wc
  Wc (Unix)
  wc is a command in Unix-like operating systems.The program reads either standard input or a list of files and generates one or more of the following statistics: number of bytes, number of words, and number of lines...
  
   - the word count command
• cat
  Cat (Unix)
  The cat command is a standard Unix program used to concatenate and display files. The name is from catenate, a synonym of concatenate.- Specification :...
  
   - concatenate command (-n flag is equivalent to nl -a)
• List of Unix programs