Property List Files

Content and Structure of Property Lists

Property lists are a technical tool for storing structured data in a standardized form that can be “understood” by every application in the Apple world. The following types of data can be stored in a property list in any order and any number:

• Strings, consisting of a sequence of characters in the Unicode standard
• Integer numbers
• Floating-point numbers
• Logical values (yes/no)
• Absolute time and date specifications
• Any raw data, stored as a sequence of bytes
• Lists (Arrays), formed from the above types of data
• Association tables (Dictionaries), which pair unique values (“keys”) with other values, both using the above types of data.

As the name suggests, an example of a dictionary is a wordbook or vocabulary list: unique terms in a particular language are paired with possible translations into another language. Here is a dictionary that associates four strings with four other strings:

Haus: house
Hund: dog
Katze: cat 
Maus: mouse 

In addition, the following rules apply:

• The mentioned data types can be mixed in lists and dictionaries in any fashion.
• By using arrays and dictionaries, the data can be nested arbitrarily. For example, a plist file could contain a list of dictionaries that map strings to arrays containing whole numbers. So we would have an array of dictionaries with arrays of integers.

Variations of the property list file format

macOS currently knows three different ways a property list can be physically represented in a file:

• a text file containing XML data, according to Apple’s document type definition (DTD) for property lists. The DTD can be downloaded from
  https://www.apple.com/DTDs/PropertyList–1.0.dtd
  (Internet link).
• a binary file containing the property list, encoded in a processor-independent way. Binary files are usually smaller than XML files with the same content.
• a text file encoding the structure of property lists by different types of parentheses. This file format was used by predecessor versions of macOS, for example by NeXT OPENSTEP for Mach.

The last file type is considered deprecated. It should not be used to create new plist files. For this reason, macOS and PropEdit can read but will never write such files.

Today’s property list files should usually have names with the file name extension .plist, no matter what format variant was used to store the list. However, PropEdit will also accept files with any other name extensions. The application keeps customized file name extensions when saving changes.

The current property list file format variations can be used fully interchangeably when storing preference settings and most other property lists. Mac OS X preferred XML lists to store preference settings in operating system versions prior to Tiger (10.4). As of Mac OS X 10.4 and later, the system has begun to store preference files in binary format, which saves storage space.

If property list files are used for other purposes, it will depend on the specific case which file format is permitted. Please note that macOS won’t accept property list files in binary format for some use cases. For certain purposes, usually documented by Apple in the reference manuals, XML has to be used. An example would be the process startup management of macOS via the launchd program.