Shell Programming and Scripting

nawk '
    # While processing records from file a (9000 lines)
    FILENAME=="file_a.txt" {
        # Record key value that should be excluded from file b
        Keys[$1]++
    }
    
    # While processing records from file b (50000)
    FILENAME=="file_b.txt" {
        # Look up key value in keys collected from file a
        if (Keys[$1] == 0) {
            # If the key is not found in the key array, save in the delta file
            #print > "deltas.txt"
            print $0
        }
    }
' file_a.txt file_b.txt

FILENAME=="file_b.txt" && ! Keys[$1] { print > "delta.txt" }