defaults program allows users to read, write, and delete Mac OS X user preferences from the command line. OS X applications use the defaults system to record user preferences and other information that must be maintained when the applications aren't running. Setting preferences is usually done (more easily) from the GUI, but some options are not available in the GUI.Property lists
User preferences and application settings are stored in files called property lists (with an extension of .plist). These files can store binary values, so they can sometimes look meaningless if you view the raw file. Aside from the Preferences dialogs, property list files can be updated with the Property List Editor application (part of the developer tools), or with the defaults command line program.
User defaults belong to domains, which typically correspond to individual applications. For example, the domain for Firefox is org.mozilla.firefox with the settings saved in a file called org.mozilla.firefox.plist. Each domain has a dictionary of keys and values representing its defaults; for example, "Default Font" = "Helvetica". Keys are always strings, but values can be complex data structures comprising arrays, dictionaries, strings, and binary data.
User plist files are saved in ~/Library/Preferences/, while system wide plist files are stored in /Library/Preferences/.
When you update a setting using defaults, it only affects the current user.
View preferences
To see all user preferences, use:defaults read
The output list will be very long because every setting for every application will be printed.
To see all Firefox user preferences, use:defaults read org.mozilla.firefox
Note that Firefox stores most of it's settings in Firefox specific files, and not in Apple's defaults system. The same is true in Windows where most of the Firefox settings are not in the Windows registry.
Update or delete a preference
To update a key use:defaults write domain { key 'value' }
Of course, you have to know the name of the key and the acceptable values you can use ahead of time.
To delete a key use:defaults delete domain key
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