bash set variable with indirect reference

2 min read 23-10-2024

bash set variable with indirect reference

Mastering Indirect References: Setting Bash Variables with a Twist

In the realm of Bash scripting, the ability to manipulate variables dynamically adds a layer of flexibility and power. This is where the concept of "indirect references" comes into play, allowing you to dynamically access and modify variables based on a string value. Let's delve into this intriguing technique, exploring how it works and providing practical examples.

The Need for Indirect References

Imagine you're building a script that needs to manage numerous variables, each representing a different configuration setting. Instead of cluttering your script with individual variable assignments (like config_1, config_2, etc.), wouldn't it be more elegant to use a single mechanism to access and modify them? This is where indirect references shine.

How Indirect References Work

The core concept is simple: you use a variable (the "reference variable") to store the name of another variable (the "target variable"). You then use this reference variable within a special syntax to access and manipulate the target variable's contents.

Here's the magic syntax:

${!reference_variable}

Let's break it down:

!: This exclamation mark is the key ingredient. It signals to Bash that we are dealing with an indirect reference.
reference_variable: This is the variable holding the name of the target variable you want to access.

Practical Examples

Let's illustrate with a concrete example:

# Setting the reference variable
config_name="config_1"

# Assigning a value to the target variable (config_1) indirectly
config_value="hello world"
${!config_name}="$config_value" 

# Accessing the value stored in the target variable indirectly
echo "${!config_name}"  # Output: hello world

In this code snippet:

config_name holds the name of our target variable (config_1).
The expression ${!config_name} allows us to access config_1 indirectly.
We set the value of config_1 to "hello world" using the indirect reference.
Finally, we retrieve the value of config_1 using another indirect reference.

Beyond Basic Usage: Iterating and Manipulating

Indirect references shine when you need to process multiple variables iteratively. Here's how you could dynamically set the values of multiple configuration variables within a loop:

for i in {1..3}; do
  # Construct the target variable name dynamically
  config_name="config_$i"
  
  # Assign a value to the target variable
  ${!config_name}="Value for config_$i"
done

In this example, we dynamically generate names for the target variables (like config_1, config_2, config_3) within a loop. Then, using indirect references, we assign values to these variables based on their dynamically generated names.

Caveats and Considerations

While powerful, indirect references require careful usage. Here are a few important points:

Unpredictable Errors: Using indirect references with undefined variables can lead to errors. Always ensure the target variable exists before attempting an indirect reference.
Scope and Context: Be mindful of the scope of your variables. Indirect references work within the current context of the script.
Alternatives: If you are managing a large number of variables, consider using arrays or associative arrays as more structured and efficient solutions.

Wrapping Up: Harnessing the Power of Indirect References

Indirect references are a valuable tool in your Bash scripting arsenal. They provide a dynamic way to access and manipulate variables, enhancing script flexibility and streamlining code. By understanding their mechanics and using them cautiously, you can unlock a new level of power and elegance in your scripts.