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Extended file attributes

Extended file attributes are file system features that enable users to associate computer files with metadata not interpreted by the filesystem, whereas regular attributes have a purpose strictly defined by the filesystem. Unlike forks, which can usually be as large as the maximum file size, extended attributes are usually limited in size to a value significantly smaller than the maximum file size. Typical uses include storing the author of a document, the character encoding of a plain-text document, or a checksum, cryptographic hash or digital certificate, and discretionary access control information.

Implementations
AIX In AIX, the JFS2 v2 filesystem supports extended attributes, which are accessible using the command. Haiku's "Mail" service stores all message content and metadata in extended file attributes, and the MIME types of files are stored in their attributes. Extended file attributes can be viewed and edited in Be-like systems' GUI through the file-manager, often Tracker or derivatives thereof. FreeBSD In FreeBSD 5.0 and later, the UFS1, UFS2, and ZFS filesystems support extended attributes, using the family of system calls. Any regular file may have a list of extended attributes. Each attribute consists of a name and the associated data. The name must be a null-terminated string, and exists in a namespace identified by a small-integer namespace identifier. Currently, two namespaces exist: user and system. The user namespace has no restrictions with regard to naming or contents. The system namespace is primarily used by the kernel for access control lists and mandatory access control. Linux In Linux, the ext2, ext3, ext4, JFS, Squashfs, UBIFS, Yaffs2, ReiserFS, Reiser4, XFS, Btrfs, OrangeFS, Lustre, OCFS2 1.6, ZFS, and F2FS in 1997 was added to Linux around 2002. Extended attributes can be accessed and modified using the getfattr and setfattr commands from the attr package on most distributions. getting, setting, and removing extended attributes from files or directories using a Linux-like API. From the command line, these abilities are exposed through the xattr utility. Since macOS 10.5, files downloaded from the World Wide Web are marked with com.apple.quarantine via extended file attributes. In some older versions of macOS (such as Mac OS X 10.6), user space extended attributes were not preserved on save in common Cocoa applications (TextEdit, Preview etc.). OpenBSD Support for extended file attributes was removed from the OpenBSD source code in 2005 due to a lack of interest in Access Control Lists. OS/2 In OS/2 version 1.2 and later, the High Performance File System was designed with extended attributes in mind, but support for them was also retro-fitted on the FAT filesystem of DOS. For compatibility with other operating systems using a FAT partition, OS/2 attributes are stored inside a single file "" located in the root directory. This file is normally inaccessible when an operating system supporting extended attributes manages the disk, but can be freely manipulated under, for example, DOS. Files and directories having extended attributes use one or more clusters inside this file. The logical cluster number of the first used cluster is stored inside the owning file's or directory's directory entry. and their ownership and permissions can differ from those of the parent file. Version 4 of the Network File System supports extended attributes in much the same way as Solaris. Windows NT On Windows NT, extended attributes with names of up to 255 ASCII characters and values of up to 65535 bytes are supported by FAT, NTFS-3G supports mapping ADS to extended attributes in FUSE; it also maps file attributes that way. ==See also==
Source: Wikipedia ↗
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