Input
aaaa bbbb cccc Output
aaaa < _jupijuice_ > bbbb < _jupijuice_ > cccc < _jupijuice_ > In simple words, replace \n with \n< jupijuice >\n
In regular expressions you can use $ to match end of line:
sed 's/$/\n< _jupijuice_ >/' input.txt \n in other sed than GNU will failed if in replacment pattern, you need a real new line in this case.sed 'a\ < _jupijuice_ >' YourFile append a line <...> after each line