# Environment Variables

All configuration values can be overridden from the command line by setting the corresponding environment variable. Because many operating systems restrict environment variables to be alphanumeric characters or _, the configuration key needs to be formatted slightly differently to the normal foo.bar.baz form.

Variables starting with MDBOOK_ are used for configuration. The key is created by removing the MDBOOK_ prefix and turning the resulting string into kebab-case. Double underscores (__) separate nested keys, while a single underscore (_) is replaced with a dash (-).

For example:

• MDBOOK_foo -> foo
• MDBOOK_FOO -> foo
• MDBOOK_FOO__BAR -> foo.bar
• MDBOOK_FOO_BAR -> foo-bar
• MDBOOK_FOO_bar__baz -> foo-bar.baz

So by setting the MDBOOK_BOOK__TITLE environment variable you can override the book's title without needing to touch your book.toml.

Note: To facilitate setting more complex config items, the value of an environment variable is first parsed as JSON, falling back to a string if the parse fails.

This means, if you so desired, you could override all book metadata when building the book with something like

$export MDBOOK_BOOK="{'title': 'My Awesome Book', authors: ['Michael-F-Bryan']}"$ mdbook build


The latter case may be useful in situations where mdbook is invoked from a script or CI, where it sometimes isn't possible to update the book.toml before building.