Скачать или смотреть How to Escape an Array of Parameters for Use in eval with flock

Learn how to properly quote an array of arguments in Bash to use with `eval` and `flock`, even when they contain spaces or special syntax.
---
This video is based on the question https://stackoverflow.com/q/64009011/ asked by the user 'ivan_pozdeev' ( https://stackoverflow.com/u/648265/ ) and on the answer https://stackoverflow.com/a/64009012/ provided by the user 'ivan_pozdeev' ( https://stackoverflow.com/u/648265/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Escape an array of parameters for use in `eval` for `flock`

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Escape an Array of Parameters for Use in eval with flock

If you've ever struggled with passing arrays to scripts in Bash, especially when trying to use them in conjunction with eval and tools like flock, you're not alone. This common challenge can lead to frustrating errors, particularly when your arguments contain spaces or special characters. In this guide, we'll explore the problem and provide a clear, effective solution using Bash's parameter transformation feature.

The Problem

You need to insert an array of arguments into an eval string that will be executed via bash -c. However, this becomes tricky when using flock because it does not accept separate arguments when running a script using the -c option. Here’s an example of what you might be trying to accomplish:

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

The challenge is making sure that the arguments are properly quoted so that they can be parsed correctly into separate command-line arguments, ensuring that spaces and special characters do not cause issues.

The Solution

Using Parameter Transformation with the Q Operator

Fortunately, Bash version 4.4 introduced a handy new feature: the Q operator for parameter transformation. This operator is designed specifically for quoting strings in a way that can be reused as input while preserving the integrity of the original strings. Here's how to use it effectively:

Define Your Array: First, create an array to store your arguments. For example:

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

Quote the Arguments: When using the Q operator, it’s important to use the * operator instead of @ when embedding them in a command. The * operator will treat all elements in the array as a single argument. Here's the correct syntax:

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

The output will look like this:

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

This correctly handles the surrounding quotes and special characters, allowing them to be passed safely to the command.

Why Use * Instead of @

Using the @ operator can lead to unexpected behavior in certain cases where you require the arguments to remain as a single entity. For example:

Using @ :

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

This results in improper argument parsing and output.

Using *:

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

This produces the desired output, ensuring that arguments containing spaces are handled properly, as you can see below:

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

Utilizing $* Syntax

You can also apply this quoting technique to the $* syntax for even more flexibility. For example:

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

This can further streamline passing arguments if needed.

Conclusion

Handling arrays in Bash can be daunting, particularly when you need to pass them to a command-line utility like flock via eval. However, with the introduction of the Q operator in Bash 4.4, you can quote your parameters accurately and avoid the pitfalls of incorrect parsing. Remember to use the * operator when embedding your arrays, and you'll be on your way to mastering argument passing in Bash scripts.

By following these guidelines, you'll streamline your scripting process and minimize frustration with special characters and spacing. Happy scripting!