edit: this is failing if a line is repeated more than once, see @showdead's excellent explanation the shortest I've got so far is perl -lnE'say if!++$#$_'

edit: this is failing if a line is repeated more than once, see @showdead's excellent explanation the shortest I've got so far is perl -lnE'say if!++$#$_' --- I don't understand what's happening with $#$_ but seems like something I should look into, thanks :) you could remove n as p is used and would be same no. of characters as perl -ne 'print if $#$_++' you could save one more by removing space between e switch and single quote