Can You Indirectly Assign a Variable in Bash?
In Bash scripting, assigning values to variables is a fundamental operation. But what if you need to assign a value to a variable whose name is itself stored in another variable? This is where indirect variable assignment comes in handy. It allows you to manipulate variables dynamically, adding a layer of flexibility to your scripts.
The Challenge of Indirect Assignment
Imagine you have a variable named var_name holding the name of another variable, say my_value. How do you assign a value to my_value using the information stored in var_name?
Directly assigning a value to my_value using var_name won't work:
var_name="my_value"
var_name="Hello" # This assigns "Hello" to var_name, not my_value
Here, var_name itself gets the value "Hello", not my_value.
The Solution: Indirect Variable Assignment
Bash offers a clever solution using indirect variable expansion:
var_name="my_value"
${!var_name}="Hello" # Indirectly assigns "Hello" to my_value
Let's break down this line:
${!var_name}: This is the key. The exclamation mark (!) tells Bash to treatvar_namenot as a literal variable name, but as the name of the variable we want to access. So, Bash effectively replacesvar_namewith its value, which is "my_value", resulting in${!my_value}.="Hello": This assigns the value "Hello" to the variable identified by${!my_value}, which ismy_value.
Now, my_value holds the value "Hello" as intended.
Example: Managing Configuration
Imagine you have a configuration file with multiple settings, each with its own variable name. You can use indirect assignment to access these settings based on their keys stored in another variable:
# Configuration file (config.txt):
username=user1
password=secret1
# Script:
key="username"
config_value=$(grep "$key=" config.txt | cut -d= -f2)
echo "The username is: ${!key}"
key="password"
config_value=$(grep "$key=" config.txt | cut -d= -f2)
echo "The password is: ${!key}"
This script reads the configuration file, extracts the values based on the keys stored in key, and uses indirect assignment to display the corresponding settings.
When to Use Indirect Assignment
Indirect variable assignment is a powerful technique for:
- Dynamic variable manipulation: Accessing and manipulating variables whose names are determined at runtime.
- Configuration management: Reading and working with configuration files where keys and values are associated.
- Creating flexible scripts: Adapting to different environments or input parameters.
Key Points to Remember
- The
!symbol is crucial: It signifies indirect variable expansion. - Double quotes matter: If
var_namecontains spaces or special characters, enclose it in double quotes within the!expression. - Use with caution: Overuse of indirect assignment can make your scripts less readable and maintainable.
Conclusion
Indirect variable assignment in Bash offers a powerful way to dynamically manage variables, adding a layer of flexibility and dynamic behavior to your scripts. By understanding its mechanics and usage, you can unlock new possibilities for writing more advanced and adaptable Bash scripts.