Check if file contains string

Grep will give you an array of all found 'monitor's. But you don't want an array, you want a boolean: is there any 'monitor' string in this file? This one reads as little of the file as needed:

if File.open('test.txt').lines.any?{|line| line.include?('monitor')}
  p 'do something'
end

readlines reads the whole file, lines returns an enumerator which does it line by line.

update

#lines are deprecated, Use #each_line instead

  if File.open('test.txt').each_line.any?{|line| line.include?('monitor')}
    p 'do something'
  end

I would use:

if File.readlines("testfile.txt").grep(/monitor/).any?

or

if File.readlines("testfile.txt").any?{ |l| l['monitor'] }

---

Using readlines has scalability issues though as it reads the entire file into an array. Instead, using foreach will accomplish the same thing without the scalability problem:

if File.foreach("testfile.txt").grep(/monitor/).any?

or

if File.foreach("testfile.txt").any?{ |l| l['monitor'] }

See "Why is "slurping" a file not a good practice?" for more information about the scalability issues.

Enumerable#grep does not return a boolean; it returns an array (how would you have access to the matches without passing a block otherwise?).

If no matches are found it returns an empty array, and [] evaluates to true. You'll need to check the size of the array in the if statement, i.e.:

if File.readlines("testfile.txt").grep(/monitor/).size > 0
  # do something
end

The documentation should be your first resource for questions like this.