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PROBLEM: You wish to number a file but skipping headers etc.  (Why
number lines? Useful when you wish to do a complicated merge+sort).
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$ cat Rabbits
%Rabbit_Name Rabbit_Index
Parvi 10
Laxmi 5
Tubby 8
Padma 6
Rusty 6
cntrl D

$ nl -b p"^[^%]" Rabbits             
      	%Rabbit_Name Rabbit_Index
     1	Parvi 10
     2	Laxmi 5
     3	Tubby 8
     4	Padma 6
     5	Rusty 6

#Note that what follows after "p" is a regular expression (whence it
#is quoted). Note the regular expression. (While at it, the
#expression "[^]" is not only not what you think(!) but also makes
#no sense. [Worth pondering].