How to Set an Environment Variable in Bash That Executes a Command Only Once

Learn how to mimic the behavior of a single-execution environment variable in Bash with this simple guide, using shell functions and caching techniques.
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This video is based on the question https://stackoverflow.com/q/69139341/ asked by the user 'Ivan' ( https://stackoverflow.com/u/4913489/ ) and on the answer https://stackoverflow.com/a/69139542/ provided by the user 'John Bollinger' ( https://stackoverflow.com/u/2402272/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: One-Time Execution in Bash

When working in Bash, you might find yourself needing to execute a command and use its output as an environment variable. However, the catch is that you want this command to run only once—the first time you access the variable—and then reuse the result for all subsequent accesses. This scenario raises a common question: Can I set an environment variable that executes a command only the first time it's accessed?

Unfortunately, the answer is no. In Bash, a variable’s value is assigned when it is established, not when it is read. However, you can achieve similar functionality using a shell function. In this guide, we will delve into how to create a function that executes a command once, stores the result, and reuses it in the future.

The Solution: Using a Shell Function

To accomplish the task of creating a variable that outputs the result of a command only on its first access, we can create a shell function. Let’s break this down into manageable steps.

Step 1: Create the Function

You can define a function my_pwd that performs the following:

Check if a variable (_saved_pwd) is empty.

If it is, run the pwd command to get the current working directory and store the result in _saved_pwd.

If it is not empty, simply output the cached value.

Here’s how the function looks:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Export the Function

To make the my_pwd function available in your Bash session, you need to export it:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Using the Function

Once you've defined and exported the my_pwd function, you can use it just like any other command. Running my_pwd the first time will display the current working directory, and any subsequent calls will return the cached value.

Here is how you would use it in your terminal:

[[See Video to Reveal this Text or Code Snippet]]

Summary of Steps

Define a shell function that executes a command and caches its result.

Export the function to make it accessible in your session.

Access the function to get the cached output on subsequent calls.

Conclusion

Although you cannot directly create a variable in Bash that executes a command only when accessed, using a shell function allows you to mimic that behavior effectively. This strategy is particularly useful for scenarios where command execution might be resource-intensive, and caching the result can significantly improve performance.

With the provided function, you can store the value of commands that you need to access efficiently multiple times in your Bash scripts.

By following the steps above, you can easily set up a mechanism in Bash to cache results of commands for efficient reuse!

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