SED - Stream editor - Changing one letter --> 's/i/I/'

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SED - Stream editor

https://www.linuxfordevices.com/tutorials/linux/sed-command-in-linux#:~:text=see%20next%20example).-,Ignore%20case%20while%20replacing,be%20used%20with%20sed%20command.
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:'
#The sed command stands for stream editor, performs editing operations on text coming from standard input or a file
#‘cat filegrep.txt | sed ‘s/i/I/’’, here cat command is used to get the content of the file and after the pipe ‘|’ sign, 
with the ‘sed’ keyword we specify the operation which is substitution this case.
But this editing is not permanent. 
It remains only in display, but in actual, file content remains the same.
SED options------>
-n: ignore duplicates
-e: search for multiple expressions and print
p -print
s- substitute
d- delete
-i - case sensitive
'
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SED - Changing one letter  --> 's/i/I/'
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[adimulamvenkat19851609@cxln4 ~]$ cat filegrep.txt
This is Linux 2000
This is Windows 3000
This is MAC 4000
This is Linux 2000
This is Windows 3000
This is MAC 4000
This is Linux 2000
This is Windows 3000
This is MAC 4000
This is Linux 2000
This is Windows 3000
This is MAC 4000

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#! /bin/bash
echo "enter filename to substitute using sed"
read fileName

if [[ -f $fileName ]]
then
    cat filegrep.txt | sed 's/i/I/' | head -3 
   
else
    echo "$fileName doesn't exist"
fi

[prabhucloudxlab@cxln4 ~]$ sh hello.sh
enter filename to substitute using sed
filegrep.txt

ThIs is Linux 2000
ThIs is Windows 3000
ThIs is MAC 4000
ThIs is Linux 2000
ThIs is Windows 3000
ThIs is MAC 4000
ThIs is Linux 2000
ThIs is Windows 3000
ThIs is MAC 4000
ThIs is Linux 2000
ThIs is Windows 3000
ThIs is MAC 4000



: '

Note:
cat filegrep.txt | sed 's/i/I/' | head -3  
[prabhucloudxlab@cxln4 ~]$ sh hello.sh
enter filename to substitute using sed
filegrep.txt
ThIs is Linux 2000
ThIs is Windows 3000
ThIs is MAC 4000
'