How to Use the parallel Command to Combine Elements of Two Arrays in Bash Scripts

Learn how to effectively use the `parallel` command in Bash to combine elements from two arrays, ensuring efficient parallel processing in your shell scripts.
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How to Use the parallel Command to Combine Elements of Two Arrays in Bash Scripts

When working with multiple arrays in a Bash script, you might find yourself needing to process elements of those arrays in parallel. This is particularly useful for optimizing execution time when running commands, such as Python scripts, on numerous input files. However, passing elements of two arrays to a parallel command can sometimes lead to unexpected behavior if not done correctly. In this guide, we will explore the correct way to achieve this and ensure that your scripts perform as expected.

The Problem: Incorrect Execution of the parallel Command

In your initial script, you had defined two arrays, arr_1 and arr_2, containing log and JSON files respectively. You wanted to print out combinations of elements from these two arrays in pairs. However, instead of getting the expected output where each log file matched with its corresponding JSON file, you encountered an issue where the command executed far more times than anticipated.

Your Initial Script

Here’s a snippet of your original Bash script where the problem occurred:

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

Expected Output

You expected the following pairs to be printed:

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

Actual Output

Instead, the output repeated entries, leading to confusion about why the command executed 16 times instead of just 4. This suggests that the elements from both arrays weren't paired correctly.

The Solution: Using :::+ for Correct Array Pairing

The issue lies in the way you're trying to pass the two arrays to the parallel command. To achieve the desired effect, you should be using the :::+ syntax. This was introduced in version 20160422 of GNU Parallel, and it allows you to combine elements of the arrays in a one-to-one pairing correctly.

Updated Command

Here’s the corrected command that will provide the expected output:

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

Explanation of the Command

parallel: The command itself, which allows you to execute tasks in parallel.

--halt 2: An option that stops the execution after encountering two failures.

echo "{1} {2}": The command you want to run; {1} and {2} are placeholders for the current items from arr_1 and arr_2.

:::: Indicates that the next argument will be treated as a series of input items.

:::+ : This is the key to correctly combining the two arrays, ensuring they are paired in a one-to-one fashion.

Why It Works

The :::+ syntax tells the parallel command that the elements from arr_1 and arr_2 should be processed as corresponding pairs. This prevents the excessive repetition you experienced earlier and allows your command to execute precisely as intended.

Conclusion

Using the parallel command for handling multiple arrays in a Bash script can significantly improve efficiency, especially when processing numerous files. By simply switching to the :::+ option when you want to pass multiple arrays, you ensure that your commands run correctly, enhancing both functionality and readability in your scripts. Happy scripting!

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