Nesting Awk and variable comparison [duplicate]

Question

I found myself having to learn a bit of awk, and it's really cool, but I have a small problem I am facing.

I have a script that has an output that is designed to be piped. Let's call it matrix.

I also have a local text file blacklist.txt over which I have complete control and can format any way I want. It only contains 1 column (or one set of values)

I want to run the script, pipe it, compare one of the resulting column values with each line (or comma separated) value in the text file, and if it's a match executing something else.

I want to run the following (with some pseudo code):

matrix username |    // this returns an output and is piped
// I then want to awk in a blacklist of usernames from my file,
//not sure if I should use getline
awk '{print $1}' blacklist.txt
// And now, I would like to compare the two, and execute another command
if($4 from matrix == $1 from blacklist.txt) {
  matrix doSomething
}
else {
  print "Not valid"
}

Any pointers would be greatly appreciated. Thank you.

Answer 1

This can be acomplished using two features of awk:

1. arrays are indexed by strings
2. the built-in variable FILENAME

This script assumes default field separators, and that blacklist.txt contains one field per line. Adjust according to your needs.

#!/bin/sh

awk '
    FILENAME == "blacklist.txt" {
        blacklist[$0] = 1
        next
    }
    blacklist[$4] {
        print $4, $0
    }
' blacklist.txt -