UNIX for Dummies Questions & Answers

Hello everyone,
I would like to know how to compare two files and modify any differences with some other data using shell script.

I think it would be better understood with an example.

I got two files named 'filex' and filey'. 'filex' is constant file without any changes in data. 'filey' is random file and any new data will be added in first line.

data in filex:
a
b
c
d

data in filey:
b
a

Problem is I want to compare first line in 'filey' with each and every line of 'filex' and if there are any matching lines, the matching lines should be shown and non matching lines should b shown as blanks.

In the above example, first line in 'filey' matches with second line in 'filex' so output should be :

_
b
_
_

and if there is new line which is 'd' added to first line of 'filey' then output should be (by comparing with 'filex')

_
b
_
d

I'm trying to use function , which takes files as arguments.

Thanks in advance.

start with grep, this gives you what appears to be close to what you want

Code:

grep -f filey  filex

this may be closer but it will be slow on big files, it is really inefficient:

Code:

while read recordy
do
      grep -q "$recordy" filex
      if [[ $? -eq 0 ]] ; then
            echo "$recordy"
      else
            echo " "
      fi
done  < filey

Hey Jim thanks for your reply. I could get matching lines between two files.
But I cant replace non-matching lines with "_" (under-score) in 'filex'. any help on this would be appreciated.