What is Variable Interpolation in the Shell Scripting
Variable interpolation is the process of substituting the value of a variable with its contents. In the bash shell, variables are created using the syntax <variable-name=value>. The value assigned to a variable can be a string, a number, or any other data type.
When referencing a variable in the shell, the syntax <$variable-name> is used. Variable interpolation occurs when this syntax is used in a command or script, and the value of the variable is substituted in its place. For example, if the variable “name” is assigned the value “Mark”, the command “echo $name” will output “mark”.
Variable interpolation can also be combined with other shell commands and operators to manipulate variables. For example, the syntax “${variable-name:-default-value}” can be used to provide a default value if the variable is not set. This is useful when writing scripts that need to handle missing or undefined variables. To future illustrate, I have given two examples that demonstrate the use of variable interpolation:
Example 1: Concatenating Strings
In this example, variable interpolation is used to concatenate two strings. The “first name” and “last name” variables are defined and then concatenated using the “$” syntax.
First_Name="Mark"
Last_Name="Twin"
Full_Name="$First_Name $Last_Name"
echo "Full Name: $Full_Name"
Here is the output of the shell script that concatenates two strings using variable interpolation:
Example 2: Checking for Undefined Variables
In this example, variable interpolation is used to check if a variable is undefined. The “file name” variable is checked to see if it is set. If it is not set, the default value “test_file.sh” is used instead.
if [ -z ${file_name+x} ]; then
file_name="default_file.txt"
fi
echo "File Name: $file_name"
Here is the output of the shell script that declares a variable and adds a value to it if it is not added using string interpolation:
Conclusion
Variable interpolation is a powerful feature in the bash shell that allows users to reference and manipulate values stored in shell variables. It is an essential skill for anyone working with shell scripts or system administration. The examples provided in this article demonstrate how variable interpolation can be used to concatenate strings and check for undefined variables. By mastering variable interpolation, shell scripters, and system administrators can write more efficient and reliable scripts.
