To what extent does the variable expansion in a double quote work correctly?

For example, suppose you created a pseudo cp in a shell script in the following format:

#!/bin/sh

content=$(cat"$1")
printf'%s\n'"$content">"$2"

In most cases, the difference between the contents of the file created in the path of the second argument and the original file specified in the path of the first argument disappears, but I wondered if it would work for any file.

Question

• Once a variable receives a (standard) input, expands it in a double quote and writes it somewhere, does the output change from the original input?When will it change?