How to cut every nth line of file A while read value from file B? - while-loop

Question

I would like to cut every nth line in file A by the value of file B, line by line.
File A looks like:
#V300059044L3C001R0010004402
AAGTAGATATCATGGAGCCGTTG
+
FGFFGFFGFFGGGGGFFFGGFGF
#V300059044L3C001R0010009240
AAAGGGAGGGAGAATAATGG
+
FGEFDFGGEFFGGEDEGEGF
...
File B looks like:
12
5
4
19
...
I want to cut the fourth line from file A with the first value of file b, the 8th line with the 2nd value of file b and so on.
I am using the command below which cuts line 1, 2, 3, ... instead of 4, 8, 12.
for i in *fileA; do fileB=${i/A/B}; while IFS= read -r a && IFS= read -ru3 b; do printf %s\\n "${b:a}" & [ $( jobs | wc -l ) -ge $( sysctl -n hw.ncpu ) ] && wait; done <$i 3< $fileB > output; done
I used the last part to hurry up the command & [ $( jobs | wc -l ) -ge $( sysctl -n hw.ncpu ) ] && wait (it takes a few hours ...)

Answer 1

If your Linux distribution does not install Ed by default, then you will want to do that for this script. It's a very small program, so there's no reason not to have it.
#!/bin/sh -x
cp file-a.txt stack
next () {
[ -s stack ] && main
end
}
main() {
ed -s stack < file-a.ed
ed -s 4lines.txt < 4lines.ed
next
}
end() {
paste new-a.txt file-b.txt > merged-file.txt
rm -v ./*.ed
rm -v ./4lines.txt
rm -v ./stack
rm -v ./new-a.txt
exit 0
}
cat >> file-a.ed << EOF
1,4W 4lines.txt
1,4d
wq
EOF
cat >> 4lines.ed << EOF
1,3d
1W new-a.txt
1d
wq
EOF
next