Command Line – mdfind -onlyin Wildcard Without Pipe

You need to escape your quotes like so:

mdfind \"I love Apple\" -onlyin ~/Documents

This results in just the one document being found:

~/Documents/test1.txt

Without escaping them, I don't think the quotes actually get passed to the mdfind command, they're just interpreted by your shell to say that I love Apple is a single argument. With the backslash-escaping, the argument then includes the quote characters.

You could also just use grep:

grep -qFi nintendo file.txt && echo found

-q = quiet, -F = fixed strings (disable regex), -i = case-insensitive.

This would search recursively (-r) and list the names of matching files (-l):

grep -rlFi nintendo ~/Notes

You can also restrict mdfind to specific metadata attributes, but it doesn't really help in this case:

mdfind 'kMDItemTextContent="*nintendo*"i' -onlyin ~/Notes/