Mastering Shell: How to Assign Multiple Variables in Parallel

Описание: Discover how to efficiently assign multiple variables in parallel in shell scripting. Learn the step-by-step solution for capturing software versions without losing any data.
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This video is based on the question https://stackoverflow.com/q/72819606/ asked by the user 'Martin' ( https://stackoverflow.com/u/5037224/ ) and on the answer https://stackoverflow.com/a/72819736/ provided by the user 'Barmar' ( https://stackoverflow.com/u/1491895/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering Shell: How to Assign Multiple Variables in Parallel

When working with shell scripts, you might find yourself needing to retrieve and assign multiple values simultaneously. For instance, you may want to capture the versions of various software packages installed on your system. However, if not done correctly, you can end up with empty variables and lots of confusion. In this post, we will tackle how to assign multiple variables in parallel effectively and ensure that your script runs smoothly.

The Problem: Empty Variable Assignments

Imagine you are trying to run the following script to get the versions of Node.js, npm, Yarn, and curl:

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At first glance, it seems efficient because it runs in parallel. However, you'll likely encounter an issue where all the variable assignments are empty when printed. The reason? Background commands in shell scripts execute in subshells, meaning those variables do not get exported to the parent shell where you're trying to access them.

The Solution: Using Temporary Files

To overcome this limitation, we can redirect the output of each version command to temporary files, wait for all processes to finish, and then read from those files. Here’s how you can do it step by step:

Step-by-step Instructions

Run Commands in the Background and Redirect Output.
Use the > operator to direct the output of each command into separate temporary files.

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Read the Values Back into Variables.
After the wait command, read the values from the temporary files into your variables.

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Clean Up Temporary Files.
It's good practice to remove any temporary files you created to keep your system tidy.

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Final Script

Here’s how your final script would look:

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Conclusion

By redirecting output to temporary files and reading them back, you can successfully capture the software versions in a clean and efficient manner without running into issues with empty variable assignments. This technique not only saves time by utilizing parallel processing but also keeps your shell script organized and functional.

Don't hesitate to put this technique into practice in your own shell scripts! Now you can confidently tackle parallel assignments without fear of running into empty variables.