Shell Programming and Scripting

Suppose there are two files:

Code:

A, format is like:
line1   12
line2   33
line3   6
...

B, format is like:

Code:

>header
taaccctaaccctaaccctaacccaaccccaccccaaccccaaccccaac
ccaaccctaaccctaaccctaacccaaccctaaccctaaccctaacccaa
ccctcaccctcaccctcaccctcaccctcaccctcaccctcaccctaacc
ctaccctaacccctaacccctaacccctaacccctaacccttaaccctaa
ccctaaccctaccctaaccctaaccctaacccctaacccctaaccc

There are only two lines in B files, one starting with ">" and the rest string is one line and they may be very long.

The problem is how to extract the exact character in B's string according to the 2nd columns' information in A, and output the characters in A's order?
I notice the awk substr function might be of help, but I didn't know how to handle with two files.

Thanks!

Code:

awk 'FNR==NR{a[FNR]=$2;next} FNR>1{for(i=1;a[i];i++) print substr($0,a[i],1)}' fileA fileB