Mastering Environment Variable Substitution in Bash Scripts

Описание: Discover how to effectively use `environment variables` in your bash scripts for dynamic configuration and automation.
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This video is based on the question https://stackoverflow.com/q/71958106/ asked by the user 'shashantrika' ( https://stackoverflow.com/u/1958669/ ) and on the answer https://stackoverflow.com/a/71958890/ provided by the user 'glenn jackman' ( https://stackoverflow.com/u/7552/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering Environment Variable Substitution in Bash Scripts

When working with bash scripts, you may often find yourself needing to access and use environment variables. While it might sound straightforward, there can be tricky scenarios where variable substitution doesn't work as intended. In this post, we'll tackle one such common problem and provide a practical solution.

The Problem: Environment Variable Substitution in a Bash Script

Imagine you are writing a simple bash script to parse a property file (dev.properties) to compute a string based on environment variable values. Here's an example of the property file:

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You have already set your environment variable as follows:

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However, when trying to print the value of local_jdbcUrl within your bash script using printenv local_jdbcUrl, you're not getting the expected output. Instead, you're seeing something like:

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This indicates that the substitution is not working inside your script. So how can you resolve this issue?

The Solution: Using envsubst

The tool you want to leverage to handle environment variable substitution is called envsubst. This utility substitutes the value of environment variables into your text, allowing you to dynamically compose strings and commands within your scripts.

How to Use envsubst

Here’s a basic usage example to demonstrate envsubst in action. Suppose you have a file containing a placeholder for an environment variable:

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Using envsubst, you can substitute the ${HOME} variable like this:

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Output:

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Applying envsubst to Your Bash Script

To implement this solution in your bash script, follow these steps:

Modify your script to use envsubst before printing the value of your property:

Replace the line:

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with:

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Run your script again. You should now see the actual value of the environment variable instead of the placeholder.

Example of the Updated Bash Script

Here's how the relevant part of your script will look after the changes:

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Conclusion

Using envsubst gives you a robust and flexible way to handle environment variable substitution in your bash scripts. As you continue to build more complex scripts for automation, being comfortable with tools like envsubst will make your life much easier.

By employing this method, you can ensure that your scripts dynamically use the most up-to-date values from your environment, enhancing the overall functionality of your bash scripts. Happy scripting!