nl

From Initq

nl is a line numbering utility. nl alone will number all lines except blank.

[alibaba@ohnonono junk]$ nl shakes.txt
     1  to


     2  be
     3  or
     4  not
     5  to
     6  be

[alibaba@ohnonono junk]$ nl -b a shakes.txt
     1  to
     2
     3  be
     4  or
     5  not
     6  to
     7  be