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 true even if the word monitor does not exist in the file
if File.readlines("testfile.txt").grep(/monitor/)
do something
end
Should it be something like == "nil"? I'm quite new to ruby and not sure of what the outputs would be.
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.
any?, since this will end as soon as it finds the first match instead of processing every line just to see if any matched.
true on the first run and false on the second. You need to rewind the file before running it again.
rewind. readlines will open and close the file automatically so it always starts at the beginning and reads to the end.
any?, though to quickly clarify, the approach to return as soon as you have a match is any? { ... } rather than grep(...).any?. Learned this the hard way, should pay closer attention :)
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.
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
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
I generally skip ruby for the command-line utilities as they tend to be faster.
`grep "monitor" "testfile.txt" > /dev/null`
$?.success #=> true if zero exit status, false otherwise.
readlinescan really cause problems if the file is larger than the available memory. See "Why is slurping a file bad?" for more information.