awk '{if (substr($0, 1, 3) == "HDB" && substr($0, 72, 1) == "1") $0 = substr($0, 1, 71) "2" substr($0, 73); print}' input.txt > output.txt sed -E '/^HDB/{s/^(.{71})1/\12/}' input.txt > output.txt
