Brace expansion tree
The article demonstrates a Bash brace expansion one-liner that generates 30 English words (such as "when," "where," "then," "there," "hence," "henceforth," etc.) from a single nested brace pattern. The post then explains how the brace expansion tree works to produce those words.
Background
- **Brace expansion** is a feature in bash (the standard command-line shell on most Linux/macOS systems) that lets you generate multiple strings from a compact pattern. For example, `a{b,c}d` expands to `abd acd`. It is often used to shorten repetitive commands like `mkdir {2023,2024,2025}`.
- **John D. Cook** is a well-known applied mathematician and blogger who writes about math, computing, and programming curiosities. His posts often dissect clever or surprising code snippets.
- **Peter Krumins** (also known as "pkrumins") runs a blog (catonmat.net) that famously catalogues shell tricks, including the "Bash Brace Expansion" series that this one-liner comes from.
- The core idea here is that a single, carefully nested brace expression can encode a tree of word fragments — essentially a miniature grammar. The example generates 30 real English words (all variants of "wh-", "th-", "h-" adverbs and pronouns like "where", "whence", "hence", "therewithal") by expanding paths through that tree. This is a demonstration of how brace expansion is more powerful than it first appears: it is not just a list shortcut but a compact way to generate Cartesian products of string parts.