SED - Changing all letters --> 's/i/I/g' --> Global Replacement

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SED - Changing all letters --> 's/i/I/g' --> Global Replacement
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[adimulamvenkat19851609@cxln4 ~]$ cat>hello.sh

#! /bin/bash
#Changing all letters --> 's/i/I/g'

echo "enter filename to substitute using sed"
read fileName

if [[ -f $fileName ]]
then
    cat filegrep.txt | sed 's/i/I/g' | head -5 > newfile.txt
else
    echo "$fileName doesn't exist"
fi



[adimulamvenkat19851609@cxln4 ~]$ sh hello.sh
enter filename to substitute using sed
filegrep.txt



[adimulamvenkat19851609@cxln4 ~]$ cat newfile.txt
ThIs Is LInux 2000
ThIs Is WIndows 3000
ThIs Is MAC 4000
ThIs Is LInux 2000
ThIs Is WIndows 3000