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

Here is one way to do it:

awk ' {
  last=$NF; $NF=""
  if($0==previous) {
    tail=tail " " last
  }
  else {
    if(previous!="") {
      if(split(tail,foo)==1) tail=tail " 0"
      print previous tail
    }
    previous=$0
    tail=last
  }
}
END {
    if(previous!="") print previous tail
}
'