Why GNU parallel used with exported Bash function with an argument enclosed in <> leads to “No such file or directory”?

If you’re reading this, chances are you’ve stumbled upon a frustrating issue while trying to use GNU parallel with an exported Bash function that takes an argument enclosed in <>. You’ve carefully crafted your command, but instead of the expected output, you’re greeted with the cryptic error message “No such file or directory”. Fear not, dear reader, for we’re about to dive into the depths of this problem and emerge victorious on the other side!

The Basics: GNU parallel and Exported Bash Functions

Before we tackle the issue at hand, let’s quickly cover the basics. GNU parallel is a powerful tool for parallelizing commands, allowing you to take advantage of multiple CPU cores and speed up processing time. It’s often used in conjunction with exported Bash functions, which enable you to wrap complex commands or scripts in a reusable package.

export -f my_function
parallel my_function ::: input1 input2 input3

In this example, `my_function` is an exported Bash function that takes an argument, and GNU parallel is used to execute it in parallel on the inputs `input1`, `input2`, and `input3`.

The Problem: <> and File Paths

Now, let’s introduce the culprit: the <> symbols. When you enclose an argument in <>, Bash performs word splitting and pathname expansion on the contents. This means that if your argument contains file paths or special characters, Bash will attempt to expand them, leading to unexpected results.

my_function() {
    echo "Processing $1"
}

export -f my_function
parallel my_function ::: "" "" ""

In this example, the argument `` is enclosed in <>. When GNU parallel tries to execute the function, Bash expands the argument to `file1.txt`, which is then passed to the function. However, since `file1.txt` is not a valid file path, you’ll receive the “No such file or directory” error.

The Culprit: Word Splitting and Pathname Expansion

Bash’s word splitting and pathname expansion can be both a blessing and a curse. When you enclose an argument in <>, Bash performs the following operations:

• Word splitting: Bash splits the argument into individual words based on the `IFS` (Internal Field Separator) variable.
• Pathname expansion: Bash expands the resulting words into file paths, using glob patterns and wildcards.

In the context of GNU parallel, this means that Bash will attempt to expand the argument `` into a valid file path, which is not what you intended.

The Solution: Proper Quoting and Escaping

So, how do you avoid this pitfall? The answer lies in proper quoting and escaping. When working with GNU parallel and exported Bash functions, it’s essential to quote and escape your arguments correctly:

my_function() {
    echo "Processing $1"
}

export -f my_function
parallel my_function ::: \'\ \'\ \'\

In this revised example, we’ve added single quotes around the argument, and escaped the <> symbols using backslashes. This ensures that Bash treats the argument as a single string, without attempting to expand it.

Additional Tips and Tricks

To further avoid issues with word splitting and pathname expansion:

• Use single quotes instead of double quotes, as they prevent variable expansion.
• Avoid using special characters in your arguments, or escape them properly.
• Verify that your file paths are correct and existent.
• Test your command without GNU parallel to ensure the output is as expected.

Conclusion

In conclusion, the “No such file or directory” error when using GNU parallel with exported Bash functions and arguments enclosed in <> is often caused by Bash’s word splitting and pathname expansion. By understanding the underlying mechanisms and applying proper quoting and escaping, you can avoid this pitfall and unlock the full potential of GNU parallel.

Remember, with great power comes great responsibility. Harness the might of GNU parallel, but tame the beast with careful argument handling and quoting. Happy parallelizing!

Solution	Example
Proper Quoting and Escaping	parallel my_function ::: \'\ \'\ \'\
Single Quotes instead of Double Quotes	parallel my_function ::: '' '' ''
Escaping Special Characters	parallel my_function ::: ''

By following these guidelines and examples, you’ll be well on your way to mastering GNU parallel and exported Bash functions. Happy coding!

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