You are basically trying to use the second file as an index. You could filter the file first, to keep only the wanted lines, and then process the result line-by-line.

It's probably easier to build an awk array of line numbers to process.

BEGIN { array_len=0 } { array[array_len++] = $1 } END { for(i=0;i<array_len;i++) { do stuff with array[i]th line of file1 } } 

You are basically trying to use the second file as an index. You could filter the file first, to keep only the wanted lines, and then process the result line-by-line.

It's probably easier to build an awk array of line numbers to process.

awk 'NR==FNR { array[FNR] = $1 } NR!=FNR { file1_lines[FNR] = $0 } END { for(linenum in array) { print file1_lines[array[linenum]] } }' file2 file1 

You can redirect this to another file and then process the whole of it without filtering.