How does variable expansion work?

Variable expansion in programming and scripting languages, such as Bash, refers to the process of replacing a variable with its value when the variable is referenced. Here’s how it typically works:

Defining a Variable: You first define a variable and assign it a value. For example:
```
MY_VAR="Hello, World!"
```
Referencing a Variable: When you want to use the value of the variable, you reference it by prefixing it with a dollar sign ($):
```
echo $MY_VAR
```
This command will output:
```
Hello, World!
```
Expansion in Strings: Variable expansion can also occur within strings. For example:
```
echo "The value is: $MY_VAR"
```
This will output:
```
The value is: Hello, World!
```
Curly Braces: To avoid ambiguity, especially when concatenating variables or when a variable name is adjacent to other characters, you can use curly braces:
```
echo "Value: ${MY_VAR}123"
```
This will output:
```
Value: Hello, World!123
```
Command Substitution: You can also use variable expansion with command substitution to assign the output of a command to a variable:
```
CURRENT_DIR=$(pwd)
echo "Current directory: $CURRENT_DIR"
```

Variable expansion is a fundamental concept in scripting that allows for dynamic and flexible code.