How to ensure only one file is returned by checking for newline

You can use pattern matching:

[[ $FILE == *$'\n'* ]] && echo More than one line

If $str contains new line you can check it by,

if [ $(echo "$str" | wc -l) -gt 1 ];
then
     // perform operation if it has new lines
else
     // no new lines.
fi

To refer to your example: Note that filenames could contain newlines, too.

A safe way to count files would be

find -name "pattern_*.sh" -printf '\n' | wc -c

This avoids printing the filename and prints only a newline instead.

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