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Explainer: Text formats
textutil, and Textovert its wrapper, convert between nine different formats, most of them in widespread use for documents that are largely based on text. This article explains a little about each of them, and its sequel tomorrow looks at how PDF differs. In each case, I give an example file size for a document containing the wordsThis is a test.
in a total of 15 characters.
Plain text
Conventionally, plain text files in macOS are most usually encoded using Unicode UTF-8, requiring just 15 bytes for the hex bytes
54 68 69 73 20 69 73 20 61 20 74 65 73 74 2e. Of course that contains no font or layout information, just the raw content.
Rich Text (RTF)
This was introduced and its specification developed by Microsoft during the late 1980s and 90s, for cross-platform interchange, primarily between its own products. Support for this in Mac OS X came in Cocoa and its rich text editor TextEdit, inherited from NeXTSTEP. The format contains two main groups of features, styled text with fonts, and simple layout that has been extended to include the embedding of images and other non-text content.
RTF files consist of text, originally ASCII but now with Unicode support. Although not actually a mark-up language, its source code appears similar.
Each RTF file opens with the ‘magic’ characters {\rtf introducing information about conformity of the code. Following that is a preamble that is likely to contain platform-specific information, a font table and colour tables. The latter should include an expanded colour table for macOS. Then follows content, typically setting the font and size, with the paragraph content. For the example file, size is 378 bytes.
RTFD
RTF has several shortcomings, particularly in handling embedded images, so NeXTSTEP extended it to a bundle format, Rich Text Format Directory, RTFD, that transferred to Mac OS X. RTF content of a document is stored in a file named TXT.rtf, alongside separate files containing scalable images that can include PDF, and the whole directory is treated as if it was a single file. Although this works well in macOS, it never caught on in Windows, so hasn’t achieved the popularity it deserved. As the example file doesn’t have any images, its size as RTFD is also 378 bytes.
Microsoft Word
From its inception in 1983 until it switched to docx, Microsoft Word’s native file format has had the extension .doc. This is a binary format that has been successfully reversed for OpenOffice and LibreOffice open source, so incorporated into many products, including Cocoa and macOS.
From 2002, Microsoft Word has used a series of XML-based formats, since 2006 conforming to standards published first by Ecma then ISO/IEC, using the extension .docx, and known as Office Open XML. Support has been incorporated into macOS.
The .doc version of the example file requires 19 KB, while the .docx version takes only 4 KB.
HTML
This has evolved through a series of versions since its release in 1993, and is the markup language that dominates the web. Its structure should be well-known, and consists of an opening document type declaration followed by tagged elements containing metadata and content. Support for writing HTML is built into the Cocoa HTML Writer in macOS. This uses CSS to define styles in the header that are then applied to sections of the content, for example<body>
<p class="p1">This is a test.</p>
</body>
The example requires only 538 bytes of HTML.
webarchive
This format is proprietary to Apple and its Safari browser, and when viewed in a capable text editor such as BBEdit, is shown as consisting of the serialised contents of a displayed web page, in XML format. In fact, as fds corrects me below, “a .webarchive is better described as a collection of web resources serialized via NSKeyedArchiver into binary plist format, bundled together into a single file in yet another property list, also saved in the binary property list format.”
When viewed in an editor such as BBEdit, after its opening XML and document type declaration as a property list, this consists of a dictionary of key-value pairs, themselves including sub-dictionaries of Web Resources. The content of each, its WebResourceData, is encoded in Base-64, making it impossible to read in a text editor. Although these can be large, for the example only 778 bytes of storage is required, showing the efficiency of the binary property list format.
WordML
Between the original .doc and the Ecma .docx formats, Microsoft Word used an intermediate WordProcessingML (or WordML) format in XML. After a standard XML header, this declares
<?mso-application progid=”Word.Document”?>
followed by a list of schemas. Although of largely historical interest now, some old Word documents may remain in this format. The example file requires 1 KB of storage.
ODT
This is OpenDocument Text, another XML-based format that was developed around the same time as WordML, and supported by many free apps and ‘office’ suites. Its opening structure is similar to that of WordML, but references oasis and OpenDocument sources. The example here requires 2 KB of storage.
Pages
One significant omission from the list of text formats supported by textutil is that used by Apple’s own Pages. This proprietary format changed significantly in 2009. Currently, a .pages document is a Zipped bundle containing thumbnail JPEG previews of the document, and two folders of files. Content appears to be saved in Apple iWork Archive files with the .iwa extension, and quite unlike RTFD.
