How can we use shell to determine if a variable exists?

You can use an if statement with conditional judgement to check if a variable exists. Here are the specific steps:

1. in the event that
2. “Can you please repeat that?”
3. I don’t understand.
4. Enclose the variable in double quotation marks to avoid errors when the variable does not exist.

Here is an example code using if statement to check if a variable exists:

if [ -z "$variable" ]; then
    echo "变量不存在"
else
    echo "变量存在"
fi

In the code above, $variable is the variable name to be checked. If the variable exists and is not empty, output “Variable exists”; if the variable does not exist or is empty, output “Variable does not exist”.