Question

I have a configuration file to which I want to add a string, that looks e.g. like that:

line1
line2
line3
line4

The new string should not be appended but written somewhere into the middle of the file. Therefore I am looking for a specific position (or string) in the file and when it has been found, I insert my new string:

file = File.open(path,"r+")
while (!file.eof?)
  line = file.readline
  if (line.downcase.starts_with?("line1"))
    file.write("Some nice little sentence")
  end
end

The problem is that Ruby overwrites the line in that position with the new text, so the result is the following:

line1
Some nice little sentence
line3
line4

What I want is a “real” insertion:

line1
Some nice little sentence
line2
line3
line4

How can this be achieved?

Answer 1

If it’s a small file I think the easiest way would be to:

• Read the complete file.
• Insert the line.
• Write the new file.

That is the way Rails does it behind the scenes.

Or you can copy it line by line and then overwrite the original with mv like this:

require 'fileutils'

tempfile=File.open("file.tmp", 'w')
f=File.new("file.txt")
f.each do |line|
  tempfile<<line
  if line.downcase=~/^line2/
    tempfile << "Some nice little sentence\n"
  end
end
f.close
tempfile.close

FileUtils.mv("file.tmp", "file.txt")