From: timr <timrandg@...>
Date: 2009-05-02T04:10:07+09:00
Subject: Re: Read and re-write file with one open?

On Apr 29, 9:27 pm, Adam Bender <aben...@gmail.com> wrote:
> [Note:  parts of this message were removed to make it a legal post.]
>
> 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?
>
> Thanks,
>
> Adam

I think File objects have a truncate method you can use after reading
(f.truncate(0) #the file is now blank) to delete the contents.

file.truncate(integer) => 0

Truncates file to at most integer bytes. The file must be opened for
writing. Not available on all platforms.

   f = File.new("out", "w")
   f.syswrite("1234567890")   #=> 10
   f.truncate(5)              #=> 0
   f.close()                  #=> nil
   File.size("out")           #=> 5