How to number each line in a text file on Linux

Some Linux commands support options that will number the input lines as a side effect, e.g., grep, and cat. The nl command is on the other hand dedicated to the task of numbering lines in a text file. If you want maximum flexibility, sed or perl is your best bet.

Suppose you want to number the lines in the input.txt file which contains:

123

456

789


abc

def

ghi

Below are some ways to number input.txt:

• cat -n
  

  $ cat -n input.txt
       1  123
       2
       3  456
       4
       5  789
       6
       7
       8  abc
       9
      10  def
      11
      12  ghi

  
  
  
• grep -n
  

  $ grep -n '^' input.txt
  1:123
  2:
  3:456
  4:
  5:789
  6:
  7:
  8:abc
  9:
  10:def
  11:
  12:ghi

  
  
  
  
• nl
  

nl inserts the line number at the beginning of each non-empty line. By default, the line number is 6 characters wide, right justified with leading spaces. A tab is inserted by default after the line number.

$ nl input.txt
     1  123

     2  456

     3  789


     4  abc

     5  def

     6  ghi

If your file is small, 6 is perhaps too wide for the line number field. To adjust the width of the line number, use the -w option. To make nl number all lines including blank ones, add -ba option.

$ nl -ba -w 3 input.txt
  1     123
  2
  3     456
  4
  5     789
  6
  7
  8     abc
  9
 10     def
 11
 12     ghi

If you don't want a tab after the line number, you can replace the tab with null (no separator between line number and rest of line), or any string you want. Use -s '' for no separator or -s ' ' for a space.

$ nl -ba -w 3  -s ' ' input.txt
  1 123
  2
  3 456
  4
  5 789
  6
  7
  8 abc
  9
 10 def
 11
 12 ghi

If you prefer left justifying the line numbers, set the field width to 1 (or use -n ln option).

$ nl -ba -w 1  input.txt
1       123
2
3       456
4
5       789
6
7
8       abc
9
10      def
11
12      ghi

nl is flexible enough to only number lines that match a regular expression. This is done by specifying the option -b p followed by a regular expression on the command line.

Number only those lines that start with either the character 1 or 4.

$ nl -b 'p^[14]' -w 3 -s ' ' input.txt
  1 123

  2 456

    789


    abc

    def

    ghi

Note that we specify -s ' ' to use a single space as the separator between line number and body text. This is used to preserve text alignment (the default tab will cause output to look messy).

Number only those lines that contain the "words" 12 or ef.

$ nl -b 'p12\|ef' -w 3 -s ' ' input.txt
  1 123

    456

    789


    abc

  2 def

    ghi