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Extended file attributes are user-defined or application-defined metadata that provide additional information about a file or its contents. They are not always supported by all file systems or operating systems and can be used to classify files, store information for network administrators, and provide security and access restrictions. There is no formal standard for their implementation, but they are often organized as attribute-value pairs.
Extended file attributes are information that can be attached to a computer file to include additional data about the file or its contents. Unlike system attributes, such as the file extension or whether the file can be modified, many extended file attributes are actually user-defined or application-defined metadata, most of the time they have no meaningful meaning to the operating system. Examples of extended file attributes include the author of a file, the name of a department related to the contents of the file, or, in some situations, data that the operating system can use if any, such as the location of the display icon for the file. Not all file systems, storage devices, or operating systems support extended file attributes, which means they may not always be transferred with a file and may not always be recoverable from a system outside where it was created. The exact implementation of extended file attributes varies from one file system to another, with some versions using a single file system to store metadata and others storing the information in the file header itself.
There is no formal standard that dictates how extended file attributes should be implemented or how an operating system or file system should handle them. Most often, extended attributes are organized as metadata in attribute-value pairs, a basic data structure consisting of the name of an attribute followed by its value. This gives extended attributes flexibility in their use, because a program or user can give an attribute any name and value needed, although the length of the value is usually limited to a certain number of characters.
Extended file attributes are often used to help classify files in some way. This can be done with attributes such as a file’s author, some kind of brief description of the file’s contents, or a human-readable description of the application that created the file. In some operating systems, extended attributes can be used in file searches so that you can quickly assemble a bunch of files with some common extended attributes.
In networked file systems, extended file attributes can be used by network administrators to store information that can be read and used by special applications. This can include security and access restrictions beyond the basic ones provided by the operating system, or it can be information that helps determine network storage locations for files on a large system. One complication that can occur when using extended attributes over networks is that the target system may not support attributes and may strip information from the file, meaning that extended data may be missing if the file is returned across the same network.
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