feat: argparse and dry-run.

README.md

@@ -1,9 +1,70 @@
 # bilibili-merge
 
-Merge bilibili downloaded videos.
+Merge bilibili downloaded videos with audio files.
 
 ## Install
 
-```
+```bash
 cargo install bilibili-merge
 ```
+
+## Usage
+
+```bash
+bilibili-merge [OPTIONS] [PATH]
+```
+
+### Arguments
+
+- `PATH` - Path to the directory containing video and audio files (default: current directory)
+
+### Options
+
+- `-n, --dry-run` - Dry run mode: show what would be done without actually doing it
+- `-f, --force` - Force overwrite existing files
+- `-h, --help` - Print help information
+
+## How It Works
+
+1. **Video File Selection**: The tool automatically finds the **largest file** (by file size) in the specified directory and uses it as the video file. This means:
+   - If there's only one video file, it will be selected
+   - If there are multiple files, the largest one (by size) will be selected
+   - Only files (not directories) are considered
+   - The selection is based on file size, not file name
+
+2. **Audio File Matching**: The tool looks for an audio file with the same base name as the video file, but with `.m4a` extension. For example:
+   - Video: `video.mp4` → Audio: `video.m4a`
+   - Video: `episode_01.flv` → Audio: `episode_01.m4a`
+
+3. **Processing Steps**:
+   - Converts the audio file from M4A to AAC format
+   - Renames the original video file to `original.<extension>` as a backup
+   - Merges the video and audio into a single file
+   - The merged file is saved as `<original_name>.mp4` (always MP4 format, regardless of input format)
+
+## Examples
+
+```bash
+# Merge video and audio in current directory
+bilibili-merge
+
+# Merge video and audio in a specific directory
+bilibili-merge /path/to/video/directory
+
+# Dry run to see what would be done
+bilibili-merge -n
+
+# Force overwrite existing files
+bilibili-merge -f
+
+# Combine options
+bilibili-merge -n -f /path/to/directory
+```
+
+## Notes
+
+- The tool requires `ffmpeg` to be installed and available in your PATH
+- By default, the tool will **not** overwrite existing files (use `-f` to force)
+- The original video file is automatically backed up as `original.<extension>`
+- The output file is always in MP4 format, regardless of the input video format
+- In dry-run mode, you'll get a shell script that you can review or execute manually