Hot to trim every nth line by a different value?

Question

I would like to trim the last XY characters of every 4th line. The cut off should be the different between the character count from line 4 and 2, and line 8 and 6.

For example: line 4 (29 characters) - line 2 (20 characters) = 9. So the last 9 characters of line 4 should be removed.

Input:

@V300059044L3C001R0010004402
AAGTAGATATCATGGAGCCG
+
FFFGFGGFGFGFFGFFGFFGGGGGFFFGG
@V300059044L3C001R0010009240
AAAGGGAGGGAGAATAAT
+
GFFGFEGFGFGEFDFGGEFFGGEDEGEGF

Output:

@V300059044L3C001R0010004402
AAGTAGATATCATGGAGCCG
+
FFFGFGGFGFGFFGFFGFFG
@V300059044L3C001R0010009240
AAAGGGAGGGAGAATAAT
+
GFFGFEGFGFGEFDFGGE

score 1 · Accepted Answer · answered Dec 17 '20 at 15:02

1

Running

awk 'NR%4==0 {$0=substr($0,1,a)} NR%2==0 {a=length($0)}  {print $0}' input.txt

on input.txt yields

@V300059044L3C001R0010004402
AAGTAGATATCATGGAGCCG
+
FFFGFGGFGFGFFGFFGFFG
@V300059044L3C001R0010009240
AAAGGGAGGGAGAATAAT
+
GFFGFEGFGFGEFDFGGE

answered Dec 17 '20 at 15:02

F. Knorr

3,045
15
22

Thanks a lot!!! This command remove by default the characters at the end of the line? – gnikixam Dec 17 '20 at 15:12
characters are only removed at every 4th line – F. Knorr Dec 17 '20 at 15:13
Yeah right, but why does this command remove them starting at the end of the 4th line. Couldn't it also remove the characters starting at the beginning of the line? – gnikixam Dec 17 '20 at 15:19
no, the `substr`-method takes the entire line `$0` and preserves `a` characters starting from the `1`st – F. Knorr Dec 17 '20 at 18:14

Hot to trim every nth line by a different value?

1 Answers1