From: Gregory Brown <gregory.t.brown@...>
Date: 2009-04-30T14:09:41+09:00
Subject: Re: Read and re-write file with one open?

On Thu, Apr 30, 2009 at 12:27 AM, Adam Bender <abender@gmail.com> wrote:
> I would like to write a Ruby script that opens a text file, performs a gsub
> on each line, and then overwrites the file with the updated contents.  Right
> now I open the file twice: once to read and once to write.  The reason for
> this is that if I try to perform both operations on the same IO object by
> calling io.rewind and writing from the beginning, and the substituted word
> is shorter than what it is replacing, some portion of the end of the
> original file remains.  Is there an idiom for "clearing" the contents of the
> file before writing?

Yes, but typically this is done by creating a new file and then
renaming it to replace the old one.

Here's an example from my upcoming book "Ruby Best Practices"[0].  It
naively strips comments from source files.
You can modify it to fit your needs.

----------

require "tempfile"
require "fileutils"

temp = Tempfile.new("without_comments")
File.foreach(ARGV[0]) do |line|
  temp << line unless line =~ /^\s*#/
end
temp.close

FileUtils.cp(ARGV[0],"#{ARGV[0]}.bak") # comment out if you don't want backups.
FileUtils.mv(temp.path,ARGV[0])

----------

-greg

PS: sorry for the shameless book plug, but it really might be helpful
for questions like these. :)


[0] http://rubybestpractices.com