Sidecar files

Sidecar files, also known as buddy files or connected files, are supplementary files that provide extra information about the main file.

In Libiry, we can have two sidecar files per book file: a cover image file and a markdown file with book data in it.

In Libiry, the relationship between the source file and the sidecar file is based on the file name; sidecar files have the same base name as the source file, but with a different extension.

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OPF files

What they are

OPF (Open Packaging Format) files are XML files that store metadata alongside an e-book. So, an OPF file is a [sidecar file](Sidecar files).

How Libiry uses OPF files

Libiry sees OPF as old data, inherited from Calibre.

1. When reading metadata for an e-book, Libiry checks if an OPF file exists
2. If the metadata cannot be read from the e-book itself, they are read from the OPF file instead
3. When writing metadata, Libiry stores it either in the e-book itself or in a [markdown](markdown files) [sidecar](Sidecar files), depending on your user settings. If an OPF file still exists at that point, it is cleaned up

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Markdown files

Markdown files are plain text files with a bit of formatting in them.

For example:

**bold** → bold *italic* → italic # Heading → a heading - item → a bullet point [link](https://example.com) → a clickable link

They can be read by both humans and machines.

Markdown files are commonly used for documents, README files, tools like Obsidian and Logseq and websites. Wikipedia, for example, consists entirely of markdown pages. A markdown file can start with so-called YAML frontmatter, but it doesn’t have to.

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YAML

YAML frontmatter is used in 8 Glossary/Markdown files to define variables and their values. It is an important element of Obsidian too. It is defined at the top of a note and is contained within --- and ---.

It looks like this:

---
cover:
booktitle: The Handmaid's Tale
author: Margaret Atwood
author_sort: Atwood, Margaret
isbn: 9781551994963
rating:
publisher: McClelland & Stewart
publication_date: 1999-05-08
language: en
pages:
series:
series_index:
translator:
illustrator:
tags:
- read
description:
notes:
---

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