[signa11]

> ... because it only numbers non-empty lines...

oh, that's just a man page lookup away. use 'nl -ba' which numbers all the lines. default style is '-bt' which numbers only non-empty ones, as you have experienced...

[jwilk]

Even with -ba (which I could never remember), nl does too much magic:

  $ printf 'foo\n\:\nbar\n' | cat -n
       1	foo
       2	\:
       3	bar
  
  $ printf 'foo\n\:\nbar\n' | nl -ba
       1	foo
  
         bar