Shell Programming and Scripting

if(!(NR%2))      -->  it will go to even NR

else{s=$1;gsub(s,"",$0);a[s]=$0}   -->  When NR is odd - get $1 to "s" and store it in array with index as $1. And remove $1 from $0

p=$1;gsub(p,"",$0);  --> when NR is even - Here same as previous line, get $1 to "p" and remove $1 from $0

for(x in a)  --> Get the index present it array a

{print s,p,$0,a[x];delete a[x]}   -->   print the output as you want(which i assumed what it is) and delete array element.