how to merge similar lines in linux

Question

I have a file test.txt on my linux system which has data in following format :

first second third fourth 10  
first second third fourth 20  
fifth sixth sevent         


        

Answer 1

Reuse of my solution (J4F)

cat file.txt | sort | while read L;
do
  y=`echo $L | rev | cut -f2- -d' ' | rev`;
  {
    test "$x" = "$y" && echo -n " `echo $L | awk '{print $NF}'`";
  } || 
  {
    x="$y";echo -en "\n$L"; 
  };
done