using if and a boolean function in bash script: if condition evaluates to false when function returns true

Question

is_dir_empty(){

    for file in "$1"
    do
        if [ "$file" != "$1" ]; then
            return 0
        fi
    done
    echo "return 1"
    return 1
}

file="/home/tmp/*.sh"

if is_dir_empty "$file"; then
    echo "empty"
else echo "not empty"
fi

it outputs

return 1
not empty

so is_dir_empty returned 1 but if condition evaluated to false somehow.... why?

Answer 1

Because shell scripts follow the Unix convention of expecting utilities to return zero for success and non-zero for failure, so boolean conditions are inverted.