如何正确比较Bash中的变量和字符串？

str1="Hello"
str2="World"
if [ "$str1" == "$str2" ]; then
    echo "Strings are equal"
else
    echo "Strings are not equal"
fi

str1="Hello"
str2="World"
if [ "$str1" != "$str2" ]; then
    echo "Strings are not equal"
else
    echo "Strings are equal"
fi

str=""
if [ "$str" == "" ]; then
    echo "String is empty"
else
    echo "String is not empty"
fi

str1="Hello"
str2="World"
if [ ${#str1} -gt ${#str2} ]; then
    echo "str1 is longer than str2"
elif [ ${#str1} -lt ${#str2} ]; then
    echo "str1 is shorter than str2"
else
    echo "str1 and str2 have the same length"
fi

str="Hello World"
if [[ "$str" == *"Hello"* ]]; then
    echo "String contains 'Hello'"
else
    echo "String does not contain 'Hello'"
fi