[SOLVED] Replace a full line every Nth line in a text file

Issue

I'm trying to perform something that should be simple but I'm not quite understanding how to do it. I have a file that looks like this:

@A01182:104:HKNG5DSX3:3:1101:3947:1031 1:N:0:CATTGCCT+NATCTCAG
CNTCATAGCTGGTTGCACAGTTAACGTCGTTCAGGCCACGTTCCAGACCGTAGTTTGCCAGCGTCAGATCATAAACGGTGGTCACCAGGGCGGTGCTGCCA
+
F#FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF:FFFFF,FFFFFFFFFFFFFFFFFFFFFFFFFF
@A01182:104:HKNG5DSX3:3:1101:7997:1031 1:N:0:CATTGCCT+NATCTCAG
GNCGATCCCTTCGCTGCTGCTGGCAATTATCGTTGTAGCGTTTGCCGGACCGAGTTTGTCTCACGCCATGTTTGCTGTCTGGCTGGCGCTGCTGCCGCGTA
+
F#FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF:FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF:,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
@A01182:104:HKNG5DSX3:3:1101:5547:1047 1:N:0:CATTGCCT+NATCTCAG
GGTGATGATTGTCTTTGGCGCAACGTTAATGAAAGATGCGCCGAAGCAGGAAGTGAAAACCAGCAATGGTGTGGTGGAGAAGGACTACACCCTGGCAGAGT
+
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF:FFFFFFFF:FFFFFFFFFFFFFFFFFFFFFFFFFF
@A01182:104:HKNG5DSX3:3:1101:20726:1063 1:N:0:CATTGCCT+GATCTCAG
GGGACGCCCATTACGCTGGTGAATCTGGCAACCCATACCAGCGCCCTGCCCCGTGAACAGCCCGGTGGCGCGGCACATCGTCCGGTATTTGTCTGGCCAAC
+
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF:FFFFFFFFFFFFFFF

and goes on for a lot of lines (The actual file is 2.5 Gb). What I want to do is to replace every fourth line (all those that have a lot of F's) for another string, the same for all.

I have tried with sed but I don't seem to be able to get the script right since I produce and output without changes.

Any help would be really appreciated!

Solution

I'd use awk for this

awk '
    NR % 4 == 0 {print "new string"; next} 
    {print}
' file > file.new && mv file.new file

Answered By - glenn jackman

Answer Checked By - David Goodson (WPSolving Volunteer)