How to change the case of letter of the content of a file in bash shell

Suppose there is a file named file1.txt which contains
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Hello World
This is a simple command
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Now the command will be like this
$cat file1.txt | while read line; do; echo $line | tr "a-z" "A-Z"; done

Now what happens here
first cat prints the content of the file which instead of going to stdout goes as input to the next
command through the PIPE ( | ). Here we runs a loop which reads one line at a time it gets as
input from file1.txt, and again PIPED to another command tr which is a translation command
converting each character from small case ("a-z") to upper ("A-Z"). while loop runs until last line
of the file is reached.

Output is

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hELLO wORLD
tHIS IS A SIMPLE COMMAND
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