In the vast landscape of computing, wildcards serve as powerful tools for pattern matching, allowing users to select multiple files or navigate complex directory structures with concise commands. Among these, the double-asterisk () wildcard stands out as an exceptionally versatile and potent symbol. Unlike its single-asterisk counterpart, which typically matches characters within a single directory level, the double-asterisk extends its reach, enabling recursive matching across multiple subdirectories. Understanding what the double-asterisk () wildcard means is crucial for anyone looking to optimize their workflow in shell scripting, programming, or even just efficient file management. This guide will delve into its functionality, applications, and best practices, empowering you to harness its full potential in various technical contexts.

Unlocking the Power of the Double-Asterisk Wildcard

The double-asterisk wildcard, often referred to as a “recursive glob” or “globstar,” is a pattern-matching construct that signifies zero or more directories and their contents. While a single asterisk () typically matches any sequence of characters within the current directory level, goes deeper. It allows you to match files and directories at any depth within a specified path, making it incredibly powerful for tasks that involve traversing complex file systems.

Historically, this functionality wasn’t standard across all Unix-like shells. Modern shells like Zsh and Bash (with the globstar option enabled) have adopted it, significantly enhancing their capabilities for file system operations. This recursive matching capability means that a pattern like src//.js wouldn’t just find JavaScript files directly in the src folder, but also in src/components/.js, src/utils/helpers/.js, and so on. This dramatically simplifies commands that would otherwise require complex loops or the find utility.

Consider a scenario where you need to locate all image files within a project directory, regardless of how many subfolders deep they reside. Without the double-asterisk, you might have to run multiple commands or write a script to iterate through each directory. With ``, a single command can achieve this, streamlining your workflow and reducing the potential for error. This recursive wildcard is a testament to the evolution of shell functionalities, catering to the growing complexity of modern software projects.

Double-Asterisk in Shell Scripting and File System Navigation

In the realm of shell scripting, particularly with Bash and Zsh, the double-asterisk (``) wildcard provides an indispensable tool for efficient file system navigation and manipulation. For Bash users, it’s important to remember that this feature is not enabled by default. You must activate it using the shopt -s globstar command. Once enabled, you can perform powerful recursive file matching operations that significantly simplify complex tasks.

Imagine you’re managing a large codebase and need to find all configuration files with a .yml extension, no matter where they are located within the project directory. Instead of using the slower find . -name ".yml" command, you can simply use ls /.yml (after enabling globstar in Bash). This command will list all matching files, providing a more intuitive and often faster alternative for many common scenarios. This efficiency is particularly noticeable in large projects with deep directory structures.

The utility extends beyond just listing files. You can use with commands like `rm`, `cp`, or `mv` to perform operations on entire sets of recursively matched files. However, caution is advised, especially with destructive commands like `rm`, as a broad match could inadvertently delete critical files. Always double-check your patterns and consider using interactive modes (e.g., `rm -i`) when dealing with recursive deletions. According to a [Linux Journal article](https://www.linuxjournal.com/content/globbing-and-wildcards), understanding advanced globbing features like is a hallmark of an efficient shell user.

1. Open your terminal: Launch your preferred shell (Bash or Zsh).
2. Enable globstar (Bash only): If you are using Bash, type shopt -s globstar and press Enter. This command needs to be run once per session, or added to your .bashrc for persistence. Zsh has this enabled by default.
3. Construct your recursive pattern: Use `` to represent zero or more directories. For example, to find all .log files: ls /.log.
4. Execute the command: Observe the output, which will include files matching the pattern across all subdirectories.

Beyond the Shell: in Programming Languages and Build Tools

The concept of the double-asterisk wildcard isn’t confined solely to shell environments; it has permeated various programming languages, build systems, and configuration files, offering a consistent and powerful way to define recursive file patterns. This widespread adoption underscores its effectiveness for tasks like including source files, defining watch patterns, or specifying files to ignore.

In Python, for instance, the glob module provides the glob.glob('/.py', recursive=True) function, which allows developers to find Python files nested deep within a project directory. Similarly, JavaScript ecosystems leverage libraries like glob (via npm) for tasks in Node.js applications or front-end build processes. These tools enable complex file matching for tasks such as bundling assets with Webpack, linting code across an entire repository, or setting up file watchers for live reloading during development.

Many build tools and version control systems also embrace the `` wildcard. Git’s .gitignore files frequently use it to exclude entire directories or specific file types recursively. For example, /node_modules/ will ignore all node_modules directories at any level, and /.log will ignore all log files. This widespread adoption illustrates how the double-asterisk has become a standard, intuitive symbol for recursive pattern matching, making it a valuable skill for any developer or system administrator. For more insights into advanced pattern matching, you might find resources on Bash’s official documentation helpful.

You’re most likely seeing a special feature of some shells that allow wildcard filename patterns to match across directory boundaries, as opposed to a single *, which is a wildcard that matches only within a directory.

If you do not have such a shell, ** will likely be equivalent to *, because “matching zero or more characters followed by zero or more characters” is the same as just “matching zero or more characters”.
But if you do have such a shell, ** will match all files and directories in the current directory and subdirectories, whereas * only matches files and directories in the current directory. (In both cases “dot files”, those with names starting with ., are not matched).
**’s real power comes when you use it in more specific patterns. For example, you can specify all .txt files no matter what subdirectory they are in with **/*.txt, whereas *.txt only matches those in the current directory.
You should look at the wildcard matching rules for your shell to know for sure what your shell is doing. For example, the bash manual says:

*

Matches any string, including the null string. When the ‘globstar’ shell option is enabled, and ‘*’ is used in a filename expansion context, two adjacent ‘*’s used as a single pattern will match all files and zero or more directories and subdirectories. If followed by a ‘/’, two adjacent ‘*’s will match only directories and subdirectories.

In recent versions of bash the ‘globstar’ shell option is disabled by default. Enabled via:
shopt -s globstar 
I believe zsh also supports this syntax.
It’s important to keep in mind that wildcards are expanded by the shell, not by the ls command. If you type ls **, or ls *.txt, the ls command itself never sees the * characters; it only sees an expanded list of files matching the pattern, just as if you had typed the entire list on the command line.