# The watch command

The watch command is useful when you want your book to be rendered on every file change. You could repeatedly issue mdbook build every time a file is changed. But using mdbook watch once will watch your files and will trigger a build automatically whenever you modify a file.

#### Specify a directory

The watch command can take a directory as an argument to use as the book’s root instead of the current working directory.

mdbook watch path/to/book


#### --open

When you use the --open (-o) option, mdbook will open the rendered book in your default web browser.

#### --dest-dir

The --dest-dir (-d) option allows you to change the output directory for the book. Relative paths are interpreted relative to the book’s root directory. If not specified it will default to the value of the build.build-dir key in book.toml, or to ./book.