UNIX for Dummies Questions & Answers

Hi
I have a large txt file on my AIX server and I need to replace some text using two other files. So filename1 has about 500 lines similar to:

Code:

txtcode SYStem100

I have the string I want to change in string2 and the new stringname in string3. Does anyone know a way of doing this? I have awk,sed,perl,vi etc.
So it will be

Code:

edit filename1 change string2 to string3

Any help much appreciated as usual!

G

---------- Post updated at 11:48 AM ---------- Previous update was at 10:21 AM ----------

Here's a section of the file and also the two string files:

Code:

MEMNAME="PREBK001"

Plus lots of other coding...

file string1 contains all of the existing MEMNAME

Code:

MEMNAME="PREBK001"

file string2 contains what I want the new ones to be (Basically all lower case)

Code:

MEMNAME="prebk001"

I can't do a global change because the MEMNAME exists in other parts of the file in Upper Case which is valid.
Cheers

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---------- Post updated at 15:30 ---------- Previous update was at 15:20 ----------

Not sure how that is to be interpreted: Will MEMNAME be the key for the replacement, or will it be MEMNAME="PREBK001"? If MEMNAME with upper case is valid in other spots, how is the one to be replaced being identified?