# Problem with gsub! in file

**URL:** <https://www.ruby-forum.com/t/problem-with-gsub-in-file/163735>\
**Category:** Ruby\
**Created:** [March 24, 2009, 2:35pm UTC](https://www.ruby-forum.com/t/problem-with-gsub-in-file/163735 "2009-03-24T14:35:31Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![JenC](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/71c47a/32.png) [@JenC](https://www.ruby-forum.com/u/JenC)\
**Post date:** [March 24, 2009, 2:35pm UTC](https://www.ruby-forum.com/t/problem-with-gsub-in-file/163735/1 "2009-03-24T14:35:31Z")

</div>

I am new to Ruby and trying to do a find and replace on a text file  
based on a regular expression. When I step through the code line by  
line, it appears that the appropriate line in the file is changed, but  
when I exit the function, the file hasn’t been modified. Are in-place  
edits of files allowed? can you see what it wrong here?

Here is the body of the function:

def ReplaceGuid(guid)  
r = nil  
sconfProj = File.open(“myfile.vcproj”, “r+”).each do |line|  
m = @guidExp.match(line)  
if !m.nil?  
str = m[1]  
r = %r{#{str}}  
line.gsub!(r, guid)  
break  
end  
end  
end

Many thanks,  
Jen

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<div class="post-metadata">

**Author:** ![JenC](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/71c47a/32.png) [@JenC](https://www.ruby-forum.com/u/JenC)\
**Post date:** [March 24, 2009, 4:00pm UTC](https://www.ruby-forum.com/t/problem-with-gsub-in-file/163735/2 "2009-03-24T16:00:53Z")

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OK, I see now it’s not trivial to read and write from a file at the  
same time, and instead I should probably just drop to the command line  
and call  
`ruby -pe 'gsub(/foo/, "bar")' < myfile.vcproj`

When i do this, the text of the file spits out to stdio, and it  
appears the substitution has been made, but when I open the file in a  
text editor, it hasn’t been changed.  
Am I missing something?  
I already checked file permissions and everything looks fine

Thanks,  
Jen

`

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<div class="post-metadata">

**Author:** ![JenC](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/71c47a/32.png) [@JenC](https://www.ruby-forum.com/u/JenC)\
**Post date:** [March 24, 2009, 4:17pm UTC](https://www.ruby-forum.com/t/problem-with-gsub-in-file/163735/3 "2009-03-24T16:17:27Z")

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JenC wrote:

> OK, I see now it’s not trivial to read and write from a file at the  
> same time, and instead I should probably just drop to the command line  
> and call  
> `ruby -pe 'gsub(/foo/, "bar")' < myfile.vcproj`
> 
> When i do this, the text of the file spits out to stdio, and it  
> appears the substitution has been made, but when I open the file in a  
> text editor, it hasn’t been changed.  
> Am I missing something?  
> I already checked file permissions and everything looks fine
> 
> Thanks,  
> Jen
> 
> `

You’ll want ruby -pie ‘gsub(/foo/,“bar”)’ myfile.vcproj

Or, if you want to keep a backup of the original (and you probably do,  
until you know this works the way you expect), you can add an extension  
of your choice to the -i flag:

ruby -p -i.bak e ‘gsub(/foo/,“bar”)’ myfile.vcproj

I’ve actually never used this on Windows, you’ll have to try it and see.

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<div class="post-metadata">

**Author:** ![JenC](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/71c47a/32.png) [@JenC](https://www.ruby-forum.com/u/JenC)\
**Post date:** [March 24, 2009, 4:19pm UTC](https://www.ruby-forum.com/t/problem-with-gsub-in-file/163735/4 "2009-03-24T16:19:16Z")

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Adam G. wrote:

> ruby -p -i.bak e ‘gsub(/foo/,“bar”)’ myfile.vcproj

Er, sorry, make that ruby -p -i.bak -e ‘gsub(/foo/,“bar”)’  
myfile.vcproj

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<div class="post-metadata">

**Author:** ![JenC](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/71c47a/32.png) [@JenC](https://www.ruby-forum.com/u/JenC)\
**Post date:** [March 24, 2009, 5:31pm UTC](https://www.ruby-forum.com/t/problem-with-gsub-in-file/163735/5 "2009-03-24T17:31:57Z")

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Thanks Adam. I tried this:  
ruby -p -i.bak -e ‘gsub(/foo/,“bar”)’ myfile.vcproj

but them I get a whole load of warnings:  
-e:1: warning: Can’t do inplace edit for stdio

and the file still doesn’t change… any ideas?

best,  
Jen

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<div class="post-metadata">

**Author:** ![JenC](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/71c47a/32.png) [@JenC](https://www.ruby-forum.com/u/JenC)\
**Post date:** [March 24, 2009, 5:49pm UTC](https://www.ruby-forum.com/t/problem-with-gsub-in-file/163735/6 "2009-03-24T17:49:28Z")

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JenC wrote:

> Thanks Adam. I tried this:  
> ruby -p -i.bak -e ‘gsub(/foo/,“bar”)’ myfile.vcproj
> 
> but them I get a whole load of warnings:  
> -e:1: warning: Can’t do inplace edit for stdio
> 
> and the file still doesn’t change… any ideas?
> 
> best,  
> Jen

Are you sure you did just ‘myfile.vcproj’ and not ‘\< myfile.vcproj’?

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<div class="post-metadata">

**Author:** ![JenC](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/71c47a/32.png) [@JenC](https://www.ruby-forum.com/u/JenC)\
**Post date:** [March 25, 2009, 2:28pm UTC](https://www.ruby-forum.com/t/problem-with-gsub-in-file/163735/7 "2009-03-25T14:28:12Z")

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> > ```
> > sconfProj = File.open("myfile.vcproj", "r+").each do |line|
> > 
> > ```
> > 
> > Many thanks,  
> > Jen

The problem is this: you’re reading the contents of your file into  
memory, changing the copy in memory and expecting it to propagate to the  
physical file. Try this: (though I haven’t tested or run it, so there  
might be some errors)

def ReplaceGuid(guid)  
outfile = File.new(“Some\_filename”, ‘w’)  
found\_match = false  
IO.read(“myfile.vcproj”).each\_line do |line|  
if (@guidExp =~ (line)) && !found\_match  
line.gsub!($1, guid)  
found\_match = true  
end  
outfile.puts line  
end  
outfile.close  
end

That reads in the original and writes the output to a new file. I’m  
assuming you wanted to stop at the first match, which is what would  
happen in your original code.

HTH,  
Michael

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<div class="post-metadata">

**Author:** ![JenC](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/71c47a/32.png) [@JenC](https://www.ruby-forum.com/u/JenC)\
**Post date:** [March 25, 2009, 9:56am UTC](https://www.ruby-forum.com/t/problem-with-gsub-in-file/163735/8 "2009-03-25T09:56:03Z")

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> Are you sure you did just ‘myfile.vcproj’ and not ‘\< myfile.vcproj’?  
> aha! that was it!

Thanks so much!

On Mar 24, 5:46 pm, Adam G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > Jen