Question

I am trying to do a simple find/replace on all text files in a directory, modifying any instance of [RAVEN_START: by inserting a string (in this case ‘raven was here’) before the line.

Here is the entire ruby program:

#!/usr/bin/env ruby
require 'rubygems'
require 'fileutils' #for FileUtils.mv('your file', 'new location')

class RavenParser

  rawDir = Dir.glob("*.txt")
  count = 0
  rawDir.each do |ravFile|
    #we have selected every text file, so now we have to search through the file
    #and make the needed changes.
    rav = File.open(ravFile, "r+") do |modRav|
      #Now we've opened the file, and we need to do the operations.
        if modRav            
        lines = File.open(modRav).readlines
        lines.each { |line|
        if line.match /\[RAVEN_START:.*\]/ 
          line.gsub!(/\[RAVEN_START:/, 'raven was here '+line)
          count = count + 1
        end
        }
         printf("Total Changed: %d\n",count)
      else
         printf("No txt files found. \n")
      end
    end
    #end of file replacing instructions.
  end

  # S
end

The program runs and compiles fine, but when I open up the text file, there has been no change to any of the text within the file. count increments properly (that is, it is equal to the number of instances of [RAVEN_START: across all the files), but the actual substitution is failing to take place (or at least not saving the changes).

Is my syntax on the gsub! incorrect? Am I doing something else wrong?

Answer 1

You’re reading the data, updating it, and then neglecting to write it back to the file. You need something like:

# And save the modified lines.
File.open(modRav, 'w') { |f| f.puts lines.join("\n") }

immediately before or after this:

printf("Total Changed: %d\n",count)

As DMG notes below, just overwriting the file isn’t properly paranoid as you could be interrupted in the middle of the write and lose data. If you want to be paranoid (which all of us should be because they really are out to get us), then you want to write to a temporary file and then do an atomic rename to replace the original file the new one. A rename generally only works when you stay within a single file system as there is no guarantee that the OS’s temp directory (which Tempfile uses by default) will be on the same file system as modRav so File.rename might not even be an option with a Tempfile unless precautions are taken. But the Tempfile constructor takes a tmpdir parameter so we’re saved:

modRavDir = File.dirname(File.realpath(modRav))
tmp       = Tempfile.new(modRav, modRavDir)
tmp.write(lines.join("\n"))
tmp.close
File.rename(tmp.path, modRav)

You might want to stick that in a separate method (safe_save(modRav, lines) perhaps) to avoid further cluttering your block.