I have a file like this:
@SRR5874687.1.171.2 length=100 1 /1
CACCGAACTGAAGACATGCGTCATCACCGAAGATTTCAACTAAAGCTGGCATGTGCCATACGTGAATACCACCTGAAGCAACTGGCATAACACCTGGCAT
+
@@@DFFDDHBFHDHGBFG@@C<@F>??CFHIH0??FFIGII<BBC@FCFCHGH.7777=D;AHEFB@?7;;>BEC;@CCCC??ACBCCCCCCC?CC@?CC
@SRR5874687.1.172.2 length=100 2 /1
CTGGACAACGCGCCGCAATATTGCAGCTTATTAGTTTGGTGATGAGAGGACTCGCGCAACCGGCACACGCGGTTTCAGCACGTGCACAACATGTACAGAC
+
?@@FBDDDFHDHHJJJIGHIIJJGGHIGI?FH<DFHJJJCF@GHFHGHIGHHEEEDDDDDDDDDDDDDD@BBBBDDEDDDDDBDDDDDDDDDDDEEEECB
@SRR5874687.1.173.2 length=100 3 /1
GTTAAAGTAGTATTGGTATTAAATTTTGTACGACGTGGGAAAATTAAACCATCTTTTTTAATAAAACGTTTTCTATGGTATTTACGACGTGGTTTAACAC
+
BBCDDDDDHHHHHJJJHIHJHJEJJJJJHEGGEHIHIJJGIHIFHGGEIGIHIIJJJJJIEHGEH@EEBDFFEED@CCCDCEFDDDBDD@DDBDDCCCC>
Within the file, I would like to edit every line starting with "@SRR". In this line after the constant statement "@SRR5874687.1.17X.2", where X is a number that increases by 1 in every next line, I want to add space and X followed by constant /2, so the result would be like this: "@SRR5874687.1.17X.2 X /2 length=100". (for example: "@SRR5874687.1.171.2 1 /2 length=100" and "@SRR5874687.1.172.2 2 /2 length=100")
I am fresh beginner with using awk and regex, but so far I managed to obtain "@SRR5874687.1.17X.2 length=100 X /2" and I only do not know how to put it before "length=100".
I used awk method:
awk '/^@SRR/{$0=$0" "(++i) " /2"}1' test.txt > out.txt