From: Robert Klemme <shortcutter@...>
Date: 2009-04-30T18:07:35+09:00
Subject: Re: Read and re-write file with one open?

2009/4/30 Gregory Brown <gregory.t.brown@gmail.com>:
> 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.

A variant exploiting Ruby's command line parameters:

11:05:08 Temp$ ruby -e '10.times {|i| puts i}' >| x
11:05:22 Temp$ cat x
0
1
2
3
4
5
6
7
8
9
11:05:23 Temp$ ./x.rb x
11:05:29 Temp$ cat x
<<<0>>>
<<<1>>>
<<<2>>>
<<<3>>>
<<<4>>>
<<<5>>>
<<<6>>>
<<<7>>>
<<<8>>>
<<<9>>>
11:05:34 Temp$ cat x.bak
0
1
2
3
4
5
6
7
8
9
11:05:37 Temp$ cat x.rb
#!/opt/bin/ruby19 -pi.bak

$_.sub! /^/, '<<<'
$_.sub! /$/, '>>>'
11:05:40 Temp$

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/