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Old 05-12-2005, 11:32 AM   #1
umk
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Distribution: debian (woody)
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Problem using sed to replace string in file


Hi everybody,

I'm trying to write a script which among others has to replace string A in file F with string B. Despite reading the sed man page and some googled stuff, I haven't managed to do this. First, I tried the following command:

sed -e 's/A/B/w F' F

but this simply erases everything from F.

Question 1: why is this?

Then I did the following:

sed -e 's/A/B/g' F >> F.new | rm -f F | mv F.new F

This does what I want (replace A with B in F) but by brute force.

Question 2: Surely there must be a "right" way of doing this. What is it?

Thanks, umk
 
Old 05-12-2005, 11:43 AM   #2
Technoslave
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You could try something like

sed -i -e 's/dont want/i want' filename

This will replace the string listed above in the file and leave the file as is.
 
Old 05-12-2005, 12:00 PM   #3
umk
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I've tried it out, but it doesn't do what you say it does. In fact it doesn't do anything but give an error message. Thanks anyway, umk
 
Old 05-12-2005, 01:39 PM   #4
Technoslave
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Well, give an actual example of what it is you're trying to do. It might be something in the string you're trying to replace and just not escaping out of properly.
 
Old 05-12-2005, 05:37 PM   #5
tibob
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Use perl

Hi,

the problem is, you can't use sed to replace in a file, you have to use a temporary file for this.

I use perl for such replaces:

Code:
perl -pi'*.bak' -e 's/stringA/stringB/g' file.txt
Note that it creates a .bak backup file.

For a whole path,
Code:
perl -pi'*.bak' -e 's/stringA/stringB/g' `find path -name '*.txt'`
Cheers,

tibob
 
Old 05-13-2005, 04:31 AM   #6
umk
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Techonslave,

I've realised that I was using an old version of sed which didn't allow the -i option. I've now got a new version and the command
sed -i 's/old/new/g' filename
does exactly what I want, namely replace old with new in file "filename".

Tibob,

Thanks for the perl tip. When I saw the command line I remembered that I actually used it in one of my first scritps, but then forgot about it. Thanks, umk
 
Old 02-09-2006, 08:52 AM   #7
Marcel Fritzenwallne
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Registered: Feb 2006
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$String replacement doesn't work

Hi,

I want to replace a string in a file, the problem is that the replacement is a variable, and this doesn't work. I could not get it working with sed nor with perl.

Can anyone help me?
----------------------------------------------------------
perl -pi'.bak' -e 's/ABC/$VAR/g' file.txt
----------------------------------------------------------
sed 's/ABC/$VAR/' file.txt
----------------------------------------------------------
--> it doesnt work with $VAR, it doesn't take the variable inside, it replaces "ABC" with the string "$VAR", and this is not what I want.

Thanks a lot!
Marcel


Quote:
Originally Posted by tibob
Hi,

the problem is, you can't use sed to replace in a file, you have to use a temporary file for this.

I use perl for such replaces:

Code:
perl -pi'*.bak' -e 's/stringA/stringB/g' file.txt
Note that it creates a .bak backup file.

For a whole path,
Code:
perl -pi'*.bak' -e 's/stringA/stringB/g' `find path -name '*.txt'`
Cheers,

tibob
 
Old 02-09-2006, 09:47 AM   #8
Marcel Fritzenwallne
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Registered: Feb 2006
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Hi,

Somenone could help me finding the solution, thanks RedHat Mailinglist!
----------------------------------------------------------
sed -i "s/ABC/$VAR/" file.txt
sed -i 's/ABC/'$VAR'/' file.txt
perl -pi'.bak' -e "s/ABC/$VAR/g" file.txt
----------------------------------------------------------
Marcel

Quote:
Originally Posted by Marcel Fritzenwallne
Hi,

I want to replace a string in a file, the problem is that the replacement is a variable, and this doesn't work. I could not get it working with sed nor with perl.

Can anyone help me?
----------------------------------------------------------
perl -pi'.bak' -e 's/ABC/$VAR/g' file.txt
----------------------------------------------------------
sed 's/ABC/$VAR/' file.txt
----------------------------------------------------------
--> it doesnt work with $VAR, it doesn't take the variable inside, it replaces "ABC" with the string "$VAR", and this is not what I want.

Thanks a lot!
Marcel
 
Old 02-13-2006, 08:22 PM   #9
kushalkoolwal
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Me too! Even I am trying the same but with no luck. This is what I am trying with
Code:
perl -i -wpe "s/hdc/$IDEDEVICE/g" /home/test.txt
No luck
 
Old 10-28-2009, 05:40 AM   #10
adjs1157
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perl/sed replace "in place" with variables

Code:
find . -name '*.txt' -print -exec sed -i s/"${OLD}"/"${NEW}"/g {} \;
or
Code:
find . -name '*.txt' -print -exec perl -pi -e s/"${OLD}"/"${NEW}"/g {} \;
 
Old 02-16-2011, 03:18 AM   #11
shwetha sagar
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Registered: Feb 2011
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sed -i 's/ABC/'$VAR'/g' file.txt

will work..

Last edited by shwetha sagar; 02-16-2011 at 03:19 AM. Reason: not double quotes..
 
1 members found this post helpful.
Old 01-31-2012, 04:39 PM   #12
mohit1611
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Registered: Jan 2012
Posts: 1

Rep: Reputation: Disabled
Smile

Quote:
Originally Posted by shwetha sagar View Post
sed -i 's/ABC/'$VAR'/g' file.txt

will work..
Thanks Swetha. It works
 
Old 02-01-2012, 09:39 AM   #13
David the H.
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It's not generally recommended to resurrect old threads, unless you have something important to add to the discussion. Although I suppose you could argue that this qualifies, just barely.

The big issue about using variables in sed is really the same as in many other situations. You have to protect parts of the line that contain spaces or other reserved characters from the shell, while still allowing the variable to expand. In this case, we just need to ensure that the sed expression is passed to the command as a single argument when it is run, with the variable substituted.

The simplest way is to just double-quote the whole thing. Double-quotes escape everything except for "$,`,\" (and "!" history substitution in interactive shells). This allows variable and command substitutions to expand and certain backslash escapes to work, while protecting everything else. So as long as your expression doesn't contain any of the above characters except that of the variable, you can use this:

Code:
sed -i "s/ABC/$VAR/g" file.txt
If there are other reserved characters that need protecting, then you have to hard-quote (or otherwise escape) the parts that need protecting, and un-quote the variable, such as shwetha sagar just posted. But there's a drawback to this. If the string in the variable happens to contain whitespace, then the resulting expression would be word-split into multiple arguments after the substitution was made, and the resulting command would be broken. To guard against that, you should also double-quote the variable to protect the contents.

Let's imagine that there's a "$" in the middle of the matching expression, and a space inside the variable, for example:

Code:
sed -i 's/AB$CD/'"$VAR"'/g' file.txt
Notice how this is getting kind of hard to read. Double-quoting is generally better. Remember you can still use backslash escapes!

Code:
sed -i "s/AB\$CD/$VAR/g" file.txt
For more on handling shell arguments, variables, and whitespace, see here:
http://mywiki.wooledge.org/Arguments
http://mywiki.wooledge.org/WordSplitting
http://mywiki.wooledge.org/Quotes

Last edited by David the H.; 02-01-2012 at 09:42 AM.
 
  


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