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:

1. Defining a Variable: You first define a variable and assign it a value. For example:

   MY_VAR="Hello, World!"
   

2. 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!
   

3. 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!
   

4. 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
   

5. 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.