From: Robert Klemme <shortcutter@...>
Date: 2007-06-14T02:20:06+09:00
Subject: Re: why writing the file is partial

On 13.06.2007 18:58, Li Chen wrote:
> Hi all,
> 
> Thank you for your valuable inputs for my previous post.
> 
> Right now I encounter another odd situation. I want to search some
> strings and replace them all in place. ButI can partially update/replace
> part of them. I wonder if any experts there can explain it.
> 
> Li
> 
> And here are my codes:
> 
> my_direrctory="C:\\flow\\test"
> 
> pat=/2\.000000\\\$P1G\\2\.000000/
> sub='1.000000\$P1G\\\1.000000'
> 
> Dir.chdir(my_direrctory)
> Dir.foreach(my_direrctory) do |filename|       #return a file name
>       next if filename=~/^\./                  # skip . and ..
>       my_file_path=my_direrctory+"\\"+filename #get absolute file name
>       File.open( my_file_path,"r+") do |file|  #pass a file to the block
>          file.each_line do |line|
>                 if pat.match(line)
>                       line.gsub!(pat,sub)  #replace the line in place
>                       file.print line    #write/update the file
>                 end
>          end
>       end
> end
> 
> #####input file format###
> test.001
> xxx\2.000000\$P1G\2.000000\xxx
> xxx\2.000000\$P1G\2.000000\xxx
> xxx\2.000000\$P1G\2.000000\xxx
> xxx\2.000000\$P1G\2.000000\xxx
> 
> 
> ### the updated file###
> test.001
> xxx\2.000000\$P1G\2.000000\xxx
> xxx\1.000000\$P1G\1.000000\xxx
> xxx\2.000000\$P1G\2.000000\xxx
> xxx\1.000000\$P1G\1.000000\xxx
> 

Reading and writing one file at the time is a dangerous thing to do - 
especially if pattern and replacement can have different lengths.  If 
you want to do it properly, then you need to #seek before you switch 
from reading to writing and back AFAIK.  But I'd use a temporary file, 
this is much safer.  Then you can even do it in a one liner:

ruby -i.bak -pe 'gsub /pattern/, "replacement"' your_file1 your_file2

Kind regards

	robert