Check if file contains string

Question

So I found this question on here, but I\'m having an issue with the output and how to handle it with an if statement. This is what I have, but it\'s always saying that it\'s tru

Answer 1

if anyone is looking for a solution to display last line of a file where that string occurs just do

File.readlines('dir/testfile.txt').select{|l| l.match /monitor/}.last 

example

file:

monitor 1
monitor 2
something else

you'll get

monitor 2

Answer 2

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.

Answer 3

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.

Answer 4

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.