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Learn how to effectively set environment variables using shell scripts and ensure they persist in your current terminal session.
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This video is based on the question https://stackoverflow.com/q/63016867/ asked by the user 'Alexandr Zhukov' ( https://stackoverflow.com/u/13818254/ ) and on the answer https://stackoverflow.com/a/63018029/ provided by the user 'Paul Hodges' ( https://stackoverflow.com/u/8656552/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Set Environment Variables from a Shell Script

Setting environment variables in a Unix-like operating system can sometimes be tricky, especially when using shell scripts. You might encounter issues where the environment variables you set within a script don’t seem to apply in your current terminal session. In this post, we’ll explore this problem and provide a step-by-step solution to ensure your environment variables are correctly set.

The Problem

Imagine you have a shell script named set_env.sh, which is intended to export an environment variable. Its contents might look something like this:

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When you run this script from the terminal using:

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you find that the expected environment variable LD_LIBRARY_PATH is not set. In fact, running the following command:

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returns nothing. This can be perplexing, but understanding how shell processes work will clarify why this happens.

Understanding Shell Processes

When you execute a script (like set_env.sh) with ./set_env.sh, you are running it in a child process. This child process executes the commands in the script, but when it finishes, it discards all changes made to the environment variables. Consequently, the parent process (your current terminal session) remains unaffected.

Key Points:

A child process cannot modify the environment of its parent process.

Once the child process terminates, any changes it made to environment variables are lost.

The Solution

To effectively set environment variables using your shell script so they persist in the current terminal session, you need to use a method that allows the script to directly modify the current shell environment.

Use the source Command

Instead of executing the script in a new shell instance, you should source it. This reads and executes the content of the script in the context of the current shell session.

You can do this in two ways:

Using the source command:

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Using a dot (.) which is a shorthand for source:

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What Happens When You Source a Script

When you source a script:

The commands in the script are executed in the current shell.

Any environment variables exported will remain in the current shell session.

Conclusion

Setting environment variables from within shell scripts requires an understanding of how processes work in the shell. By recognizing that a child process cannot alter the parent's environment, you can effectively use the source command to ensure that your variables persist in your terminal.

Now, the next time you need to set environment variables from a script, remember to source it rather than executing it directly. Happy scripting!