How to Simultaneously Process Multiple Commands into Variables in Bash

Discover how to execute two commands simultaneously in Bash, improving the efficiency of your scripts by using process substitution.
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Solving Command Execution Timing Issues in Bash

When writing scripts in Bash, performance is key, especially when you're looking to gather system information quickly. In this guide, we'll tackle a common problem faced by many Linux users, especially those working with motd (Message of the Day) scripts: How can you execute two commands at the same time and store their outputs in separate variables efficiently?

The Problem at Hand

As described in a recent inquiry, the current setup in a motd script involves two commands that sequentially retrieve system information:

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This configuration has a drawback: both commands run one after the other, which can lead to longer execution times—up to 2 seconds in this case. The user wishes to run these commands concurrently to cut down on the time taken to gather the required information.

The Solution: Using Process Substitution

To achieve simultaneous command execution in Bash, we can utilize process substitution combined with file descriptors. This technique allows us to execute and read two commands at the same time without the need for temporary files. Here’s how you can implement this:

Step-by-Step Breakdown

Start Both Commands in the Background:
Use process substitution to run both commands in the background. This will allow them to execute concurrently.

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exec {fd} creates a new file descriptor. We’ll use these for reading the outputs.

< <(command) runs the command and provides output for reading.

Read Outputs into Variables:
Next, you'll read the outputs from the created file descriptors into your variables.

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Here, read -r reads a single line from the file descriptor.

Close the File Descriptors:
After you've read the necessary data, it's good practice to close the file descriptors to free up system resources.

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Putting It All Together

Here’s how the full implementation would look:

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Conclusion

By utilizing process substitution and file descriptors, you can effectively run multiple commands in parallel and significantly reduce execution time in your Bash scripts. This method is compatible with newer versions of Bash (4.x and above), ensuring that you remain within the default tools of Debian or Raspberry Pi environments.

This enhanced approach ensures your scripts remain efficient, allowing for quicker system administration and monitoring tasks. If you're looking to enhance your bash scripting skills further, implementing such techniques can be a significant step forward.

Thank you for reading, and happy scripting!