From: Andrew Johnson <ajohnson@...>
Date: 2006-03-27T07:08:56+09:00
Subject: Re: Ruby-esque approaches to adding a line at the beginning of a file.

On Mon, 27 Mar 2006 05:55:30 +0900, Randy Kramer <rhkramer@gmail.com> wrote:
> On our local lug mail list a discussion came up about using Ruby (as opposed 
> to Perl) for an application that involved inserting a new line at the front 
> of an existing file.

Well, assuming reading the entire file into memory is acceptable,
we can use a fairly standard approach without a temporary file:

  #!/usr/bin/ruby -w

  file  = 'somefile'
  first = "new start\n"

  File.open(file,'r+') do |f|
    contents = f.read
    f.seek 0, IO::SEEK_SET  
    f.truncate 0
    f.puts first, contents
  end

And, if you like, you can wrap the manipulations with #flock

  File.open(file,'r+') do |f|
    f.flock File::LOCK_EX
    contents = f.read
    f.seek 0, IO::SEEK_SET  
    f.truncate 0
    f.puts first, contents
    f.flock File::LOCK_UN
  end

Of course, this is a generalized version for any content manipulation you
might want to do on the contents, not just adding a new first line.

Otherwise, loop over lines, making changes and writing to a temp file, and
afterwards, #rename the temp file back to the original.

Or, use the command line -i switch:

  $ cat somefile 
  one
  two
  three

  $ ruby -pi -e 'puts "zero\n" if $. == 1' somefile 

  $ cat somefile
  zero
  one
  two
  three

regards,
andrew

-- 
Andrew L. Johnson   http://www.siaris.net/
      What have you done to the cat? It looks half-dead.
          -- Schroedinger's wife