How does shell determine if a parameter is empty?

In a shell script, you can use the $# variable to get the number of parameters, and use $1, $2, $3, etc. to get specific parameter values. To check if a parameter is empty, you can use conditional statements.

Here is a sample code for checking if the first parameter is empty:

if [ -z "$1" ]; then
    echo "参数为空"
else
    echo "参数不为空"
fi

In the above code, the -z option is used to determine if a string is empty. If $1 is empty, it will output “parameter is empty”, otherwise it will output “parameter is not empty”.

To determine if multiple parameters are empty, you can check each parameter one by one, such as $1, $2, $3, and so on.

if [ -z "$1" ] && [ -z "$2" ]; then
    echo "参数1和参数2均为空"
elif [ -z "$1" ]; then
    echo "参数1为空，参数2不为空"
elif [ -z "$2" ]; then
    echo "参数1不为空，参数2为空"
else
    echo "参数1和参数2均不为空"
fi

In the above code, the -z option is used to determine if the string is empty. It uses multiple if statements to check different scenarios and output corresponding information.