Add client-side shell syntax highlighter for Aliki theme

Add a simple shell/bash syntax highlighter for documentation code blocks.
The highlighter is focused on command-line documentation rather than full
bash scripts.

Highlights:
- $ prompts (gray)
- Commands/executables - first word on line (blue)
- Options like -f, --flag, --option=value (cyan)
- Single and double quoted strings (green)
- Comments starting with # (gray, italic)

The highlighter targets code blocks with language classes: bash, sh, shell,
console.

Files added:
- lib/rdoc/generator/template/aliki/js/bash_highlighter.js
- test/rdoc/generator/aliki/highlight_bash_test.rb

Author: st0012

lib/rdoc/generator/template/aliki/_head.rhtml

@@ -140,6 +140,11 @@
   defer
 ></script>
 
+<script
+  src="<%= h asset_rel_prefix %>/js/bash_highlighter.js?v=<%= h RDoc::VERSION %>"
+  defer
+></script>
+
 <script
   src="<%= h asset_rel_prefix %>/js/aliki.js?v=<%= h RDoc::VERSION %>"
   defer

lib/rdoc/generator/template/aliki/css/rdoc.css

@@ -1059,6 +1059,18 @@ main h6 a:hover {
   font-style: italic;
 }
 
+/* Shell Syntax Highlighting */
+.sh-prompt { color: var(--code-gray); }
+.sh-command { color: var(--code-blue); }
+.sh-option { color: var(--code-cyan); }
+.sh-string { color: var(--code-green); }
+.sh-envvar { color: var(--code-purple); }
+
+.sh-comment {
+  color: var(--code-gray);
+  font-style: italic;
+}
+
 /* Emphasis */
 em {
   text-decoration-color: var(--color-emphasis-decoration);

lib/rdoc/generator/template/aliki/js/bash_highlighter.js

@@ -0,0 +1,243 @@
+/**
+ * Client-side shell syntax highlighter for RDoc
+ * Focused on command-line documentation (not full bash scripts)
+ *
+ * Highlights: $ prompts, commands (first word), options (--flag), strings, env vars (VAR=), comments (#)
+ */
+
+(function() {
+  'use strict';
+
+  /**
+   * Escape HTML special characters
+   */
+  function escapeHtml(text) {
+    return text
+      .replace(/&/g, '&')
+      .replace(/</g, '&lt;')
+      .replace(/>/g, '&gt;')
+      .replace(/"/g, '&quot;')
+      .replace(/'/g, '&#39;');
+  }
+
+  /**
+   * Highlight a single line of shell code
+   */
+  function highlightLine(line) {
+    if (line.trim() === '') {
+      return escapeHtml(line);
+    }
+
+    const tokens = [];
+    let i = 0;
+    const len = line.length;
+
+    // Skip leading whitespace
+    while (i < len && (line[i] === ' ' || line[i] === '\t')) {
+      tokens.push(escapeHtml(line[i]));
+      i++;
+    }
+
+    // Check for $ prompt at line start (after whitespace)
+    if (i < len && line[i] === '$') {
+      const nextChar = line[i + 1];
+      // $ followed by space or end of line = prompt
+      if (nextChar === ' ' || nextChar === undefined || i + 1 >= len) {
+        tokens.push('<span class="sh-prompt">', escapeHtml('$'), '</span>');
+        i++;
+      }
+    }
+
+    // Check for # comment at line start (after whitespace)
+    if (i < len && line[i] === '#') {
+      // Entire rest of line is a comment
+      tokens.push('<span class="sh-comment">', escapeHtml(line.substring(i)), '</span>');
+      return tokens.join('');
+    }
+
+    // Track if we're at a word boundary (after whitespace)
+    let afterWhitespace = true;
+    // Track if we've seen the command (first word) on this line
+    let seenCommand = false;
+
+    // Process rest of line
+    while (i < len) {
+      const char = line[i];
+
+      // Whitespace
+      if (char === ' ' || char === '\t') {
+        tokens.push(escapeHtml(char));
+        i++;
+        afterWhitespace = true;
+        continue;
+      }
+
+      // Comment (# in middle of line, must be after whitespace)
+      if (char === '#' && afterWhitespace) {
+        tokens.push('<span class="sh-comment">', escapeHtml(line.substring(i)), '</span>');
+        break;
+      }
+
+      // Double-quoted string
+      if (char === '"') {
+        let end = i + 1;
+        while (end < len && line[end] !== '"') {
+          if (line[end] === '\\' && end + 1 < len) {
+            end += 2;
+          } else {
+            end++;
+          }
+        }
+        if (end < len) end++; // Include closing quote
+        const str = line.substring(i, end);
+        tokens.push('<span class="sh-string">', escapeHtml(str), '</span>');
+        i = end;
+        afterWhitespace = false;
+        continue;
+      }
+
+      // Single-quoted string
+      if (char === "'") {
+        let end = i + 1;
+        while (end < len && line[end] !== "'") {
+          end++;
+        }
+        if (end < len) end++; // Include closing quote
+        const str = line.substring(i, end);
+        tokens.push('<span class="sh-string">', escapeHtml(str), '</span>');
+        i = end;
+        afterWhitespace = false;
+        continue;
+      }
+
+      // Environment variable (ALLCAPS=value, must be after whitespace)
+      if (afterWhitespace && /[A-Z]/.test(char)) {
+        // Look ahead to see if this is an env var pattern
+        let end = i + 1;
+        while (end < len && /[A-Z0-9_]/.test(line[end])) {
+          end++;
+        }
+        if (end < len && line[end] === '=') {
+          // It's an environment variable
+          const envName = line.substring(i, end + 1); // Include =
+          tokens.push('<span class="sh-envvar">', escapeHtml(envName), '</span>');
+          i = end + 1;
+          // Read value (until space, unless quoted)
+          if (i < len && (line[i] === '"' || line[i] === "'")) {
+            // Value is quoted, will be handled by string parsing
+          } else {
+            // Unquoted value
+            let valueEnd = i;
+            while (valueEnd < len && line[valueEnd] !== ' ' && line[valueEnd] !== '\t') {
+              valueEnd++;
+            }
+            if (valueEnd > i) {
+              const value = line.substring(i, valueEnd);
+              tokens.push(escapeHtml(value));
+              i = valueEnd;
+            }
+          }
+          afterWhitespace = false;
+          continue;
+        }
+        // Not an env var, fall through to command/word handling
+      }
+
+      // Option (starts with -, must be after whitespace)
+      if (char === '-' && afterWhitespace) {
+        let end = i + 1;
+        // Handle -- long options
+        if (end < len && line[end] === '-') {
+          end++;
+        }
+        // Read option name (letters, numbers, dashes, underscores)
+        while (end < len && /[a-zA-Z0-9_-]/.test(line[end])) {
+          end++;
+        }
+        // Handle --option=value
+        if (end < len && line[end] === '=') {
+          end++;
+          // Read value (until space or end)
+          while (end < len && line[end] !== ' ' && line[end] !== '\t') {
+            end++;
+          }
+        }
+        const option = line.substring(i, end);
+        tokens.push('<span class="sh-option">', escapeHtml(option), '</span>');
+        i = end;
+        afterWhitespace = false;
+        continue;
+      }
+
+      // Command (first word on line, starts with letter/number/@/~/. for paths)
+      // Handles: command, ./script, ../script, ~/bin/cmd
+      const isPathStart = char === '.' && i + 1 < len && (line[i + 1] === '/' || (line[i + 1] === '.' && i + 2 < len && line[i + 2] === '/'));
+      if (!seenCommand && afterWhitespace && (/[a-zA-Z0-9@~]/.test(char) || isPathStart)) {
+        let end = i + 1;
+        // Read until whitespace or end
+        while (end < len && line[end] !== ' ' && line[end] !== '\t') {
+          end++;
+        }
+        const cmd = line.substring(i, end);
+        tokens.push('<span class="sh-command">', escapeHtml(cmd), '</span>');
+        i = end;
+        afterWhitespace = false;
+        seenCommand = true;
+        continue;
+      }
+
+      // Everything else (arguments, operators, punctuation)
+      tokens.push(escapeHtml(char));
+      i++;
+      afterWhitespace = false;
+    }
+
+    return tokens.join('');
+  }
+
+  /**
+   * Highlight shell source code
+   */
+  function highlightShell(code) {
+    const lines = code.split('\n');
+    const highlighted = lines.map(highlightLine);
+    return highlighted.join('\n');
+  }
+
+  /**
+   * Initialize shell syntax highlighting on page load
+   */
+  function initHighlighting() {
+    // Target code blocks with shell-related language classes
+    const selectors = [
+      'pre.bash',
+      'pre.sh',
+      'pre.shell',
+      'pre.console',
+      'pre[data-language="bash"]',
+      'pre[data-language="sh"]',
+      'pre[data-language="shell"]',
+      'pre[data-language="console"]'
+    ];
+
+    const codeBlocks = document.querySelectorAll(selectors.join(', '));
+
+    codeBlocks.forEach(block => {
+      if (block.getAttribute('data-highlighted') === 'true') {
+        return;
+      }
+
+      const code = block.textContent;
+      const highlighted = highlightShell(code);
+
+      block.innerHTML = highlighted;
+      block.setAttribute('data-highlighted', 'true');
+    });
+  }
+
+  if (document.readyState === 'loading') {
+    document.addEventListener('DOMContentLoaded', initHighlighting);
+  } else {
+    initHighlighting();
+  }
+})();