Number Lines

nl writes each FILE to standard output, with line numbers added.
With no FILE, or when FILE is -, it read standard input.

[Example]
~ nl population.txt
    1  China
    2  India
    3  United States 

~ nl -s : population.txt
    1:China
    2:India
    3:United States
~ nl -n rz --number-width=4 population.txt
    0001  China
    0002  India
    0003  United States 


[Options]

-s : seprator
-n : number format
ln - left justified, no leading zeros
rn - right justified, no leading zeros
rz - right justified, leading zeros