Add implementation plan for most commonly used commands.

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+# DOS Command Implementation Plan
+
+## Overview
+This document outlines the plan to implement the most commonly used DOS commands that are currently missing from the dosage shell implementation.
+
+## Implementation Priority
+
+### Phase 1: Core File/Directory Operations (High Priority)
+
+#### 1. CD/CHDIR - Change Directory
+- **Status**: Not implemented
+- **Priority**: High (Essential for navigation)
+- **Implementation**:
+  - Update parser to recognize `CD` and `CHDIR` commands
+  - Add `Chdir` variant handling in eval.zig
+  - Use `std.fs.cwd().setDir()` or equivalent
+  - Handle relative and absolute paths
+  - Support `CD..` (parent directory)
+  - Support `CD\` (root directory)
+  - Update current working directory tracking in main loop
+
+#### 2. COPY - Copy Files
+- **Status**: Not implemented  
+- **Priority**: High (Most common file operation)
+- **Implementation**:
+  - Add parsing for `COPY source dest` syntax
+  - Handle FileSpec sources and destinations (files, CON, devices)
+  - Implement file-to-file copying with `std.fs.copyFile()`
+  - Support wildcard copying (*.txt, etc.)
+  - Handle overwrite prompts and `/Y` switch
+  - Support copying to/from devices like CON
+
+#### 3. DEL/ERASE - Delete Files
+- **Status**: Not implemented
+- **Priority**: High (Essential file operation)
+- **Implementation**:
+  - Add parsing for `DEL` and `ERASE` commands
+  - Support wildcard deletion (*.bak, etc.)
+  - Implement confirmation prompts
+  - Use `std.fs.deleteFile()` for single files
+  - Use directory iteration for wildcards
+  - Handle access denied and file not found errors
+
+#### 4. MD/MKDIR - Create Directory
+- **Status**: Not implemented
+- **Priority**: High (Essential for file organization)
+- **Implementation**:
+  - Add parsing for `MD` and `MKDIR` commands
+  - Use `std.fs.cwd().makeDir()` 
+  - Handle nested directory creation
+  - Provide appropriate error messages for existing directories
+
+#### 5. SET - Environment Variables
+- **Status**: Not implemented
+- **Priority**: Medium (Important for shell functionality)
+- **Implementation**:
+  - Add global environment variable storage (HashMap)
+  - Support `SET` (display all variables)
+  - Support `SET VAR=value` (set variable)
+  - Support `SET VAR` (display single variable)
+  - Integrate with external command execution
+  - Handle special variables like PATH, PROMPT
+
+### Phase 2: Extended File Operations (Medium Priority)
+
+#### 6. RD/RMDIR - Remove Directory
+- **Status**: Not implemented
+- **Priority**: Medium
+- **Implementation**:
+  - Add parsing for `RD` and `RMDIR` commands
+  - Use `std.fs.cwd().deleteDir()`
+  - Handle non-empty directory errors
+  - Support `/S` switch for recursive deletion
+
+#### 7. REN/RENAME - Rename Files/Directories  
+- **Status**: Not implemented
+- **Priority**: Medium
+- **Implementation**:
+  - Add parsing for `REN oldname newname` syntax
+  - Use `std.fs.cwd().rename()`
+  - Support wildcard renaming
+  - Handle cross-drive moves (convert to copy+delete)
+
+#### 8. MOVE - Move Files/Directories
+- **Status**: Not implemented  
+- **Priority**: Medium
+- **Implementation**:
+  - Add parsing for `MOVE source dest` syntax
+  - Try rename first, fallback to copy+delete for cross-filesystem moves
+  - Support directory moving
+  - Handle overwrite confirmation
+
+#### 9. PATH - Command Search Path
+- **Status**: Not implemented
+- **Priority**: Medium
+- **Implementation**:
+  - Support `PATH` (display current path)
+  - Support `PATH=directory;directory2` (set path)
+  - Integrate with external command resolution
+  - Store in environment variables
+
+### Phase 3: Utility Commands (Low Priority)
+
+#### 10. PROMPT - Command Prompt Customization
+- **Status**: Not implemented
+- **Priority**: Low
+- **Implementation**:
+  - Support `PROMPT` (display current prompt)
+  - Support `PROMPT text$G` (set prompt format)
+  - Implement prompt codes ($P=path, $G=>, $N=drive, etc.)
+  - Integrate with main shell loop
+
+#### 11. MORE - Paged Output Display
+- **Status**: Not implemented
+- **Priority**: Low  
+- **Implementation**:
+  - Implement paging with screen height detection
+  - Support "Press any key to continue" prompts
+  - Handle pipe input for `command | MORE`
+  - Add /S switch for squashing blank lines
+
+#### 12. FIND - Text Search in Files
+- **Status**: Not implemented
+- **Priority**: Low
+- **Implementation**:
+  - Add parsing for `FIND "text" filename` syntax
+  - Support /I switch for case-insensitive search
+  - Support /V switch for non-matching lines
+  - Support /N switch for line numbers
+  - Handle multiple files and wildcards
+
+#### 13. TREE - Directory Tree Display
+- **Status**: Not implemented
+- **Priority**: Low
+- **Implementation**:
+  - Implement recursive directory traversal
+  - Create tree-like ASCII art display
+  - Support /F switch for showing files
+  - Support /A switch for ASCII characters only
+
+#### 14. ATTRIB - File Attributes
+- **Status**: Not implemented
+- **Priority**: Low
+- **Implementation**:
+  - Display file attributes (R/H/S/A)
+  - Support setting/clearing attributes
+  - Use platform-specific attribute APIs
+  - Handle wildcards for multiple files
+
+#### 15. VOL - Volume Information
+- **Status**: Not implemented  
+- **Priority**: Low
+- **Implementation**:
+  - Display volume label and serial number
+  - Use filesystem stat APIs
+  - Handle different drive specifications
+
+## Implementation Guidelines
+
+### Code Organization
+- Add command parsing to `src/syntax.zig` in the `parseBuiltinCommand()` function
+- Add command execution to `src/eval.zig` in the builtin command switch
+- Update `BuiltinCommand` enum variants as needed
+- Maintain DOS-authentic error messages and behavior
+
+### Error Handling
+- Use DOS-style error messages ("File not found", "Access denied", etc.)
+- Return appropriate exit codes (0 for success, 1+ for errors)
+- Handle edge cases gracefully (missing arguments, invalid paths, etc.)
+
+### Testing Strategy
+- Test each command with valid and invalid arguments
+- Test with various file types and permissions
+- Test error conditions and edge cases
+- Verify DOS-authentic behavior and output format
+
+### Cross-Platform Considerations
+- Use Zig standard library APIs for maximum portability
+- Handle path separators correctly (DOS uses backslash)
+- Consider case sensitivity differences between platforms
+- Test on both Unix-like and Windows systems
+
+## Dependencies and Prerequisites
+- Current codebase with working parser and evaluator
+- Zig standard library filesystem APIs
+- Basic I/O redirection support (already implemented)
+- Error handling infrastructure (already in place)
+
+## Estimated Implementation Effort
+- Phase 1 (High Priority): ~3-4 days of development
+- Phase 2 (Medium Priority): ~2-3 days of development  
+- Phase 3 (Low Priority): ~3-4 days of development
+- **Total**: ~8-11 days for complete implementation
+
+This plan provides a roadmap for transforming the dosage shell from a basic command interpreter into a fully functional DOS-style environment with comprehensive file and directory management capabilities.
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