Add line numbers to a text file in Unix

To add line numbers to a file in Unix, use the nl command. For example, given an input file cherry.txt,

Loveliest of trees, the cherry now
Is hung with bloom along the bough,
And stands about the woodland ride
Wearing white for Eastertide.

the command

nl cherry.txt

gives output

     1	Loveliest of trees, the cherry now
     2	Is hung with bloom along the bough,
     3	And stands about the woodland ride
     4	Wearing white for Eastertide.

The command

nl -n ln cherry.txt

puts the numbers on the left:

1     	Loveliest of trees, the cherry now
2     	Is hung with bloom along the bough,
3     	And stands about the woodland ride
4     	Wearing white for Eastertide.

It is also possible to start from a different number. The command

nl -v 100 cherry.txt

gives the output

   100	Loveliest of trees, the cherry now
   101	Is hung with bloom along the bough,
   102	And stands about the woodland ride
   103	Wearing white for Eastertide.

The separator between the number and the text line can also be altered:

nl -s " -> " cherry.txt

gives the output

     1 -> Loveliest of trees, the cherry now
     2 -> Is hung with bloom along the bough,
     3 -> And stands about the woodland ride
     4 -> Wearing white for Eastertide.

It is possible to set the width of the numbers using -w <number>.

nl -v 999 -w 3

gives the output

999	Loveliest of trees, the cherry now
000	Is hung with bloom along the bough,
001	And stands about the woodland ride
002	Wearing white for Eastertide.

The numbers wrap around so they never exceed the width.