nl is a command that allows you to number the lines. Through some flags you can decide how to filter this command.
We have a file named example.txt that contains 5 elements, to test this command: 
Car
Computer
Robot
Smartphone
Videogame
The syntax is: 
 nl [flags][file] 
nl example.txt
    1  Car
    2  Computer
    3  Robot
    4  Smartphone
    5  Videogame
As you can see, the command has numbered the lines.

Flags

  • -b (regex): Specify the lines to be numbered 
    nl -b p^[cv] example.txt
    1  Car
    2  Computer
    3  Videogame
       Robot
       Smartphone
    The part after the flag is used to number only the lines that start with c and v.
  • -bn: The flag n goes with b, doesn't number any lines 
    nl -bn example.txt
    Car
    Computer
    Videogame
    Robot
    Smartphone
  • -s: Usually nl separes the number of the line from the text with a tab, with -s flag you can choose another separator, in this example we will use "=". 
    nl -s= example.txt
    1=Car
    2=Computer
    3=Videogame
    4=Robot
    5=Smartphone