Say I have a file:Line1Line2Line3Line4Line5 Line6Line7 Line8 Line9Line10Line11Line12I want to get:Line1Line2Line3Line7 Line8 Line9Line13Line14Line15It means that I want the odd number of three lines (including).I have some ideas like for (i=1, i<=FR, i+5), {sed -n,'i, i+1p' } < inputCould you tell me the structure of for loop in sed. And how to correct this? 
How to extract 1st, 3rd, 5th of group of three lines by sed or awk?
text processing;sed;awk