How to run grep inside awk?

Question

Suppose I have a file input.txt with few columns and few rows, the first column is the key, and a directory dir with files which contain some of these

Answer 1

Use process substitution to create a keyword "file" that you can pass to grep via the -f option:

grep -f <(awk '{print $1}' input.txt) dir/*

This will search each file in dir for lines containing keywords printed by the awk command. It's equivalent to

awk '{print $1}' input.txt > tmp.txt
grep -f tmp.txt dir/*