How to uppercase first letter of the first word in each sentence using shell script?

How to uppercase first letter of the first word in each sentence using
shell script?

I am trying to capitalize each first letter of the first word in each
sentence from a txt file called input.txt and I want this input file to be
a argument of the shell script
./script.sh input.txt
sample input file:
i am Andrew. you are Jhon. here we are, forever.
result file:
I am Andrew. You are Jhon. Here we are, forever.