xattr

xattr refers to extended attributes, a filesystem feature that lets applications associate arbitrary metadata with files and directories beyond standard attributes (such as size, timestamps, and permissions). Extended attributes store small name/value pairs and are commonly implemented on Unix-like systems, including Linux (ext2/3/4, XFS, Btrfs), macOS (HFS+, APFS) and other filesystems that support metadata extensions.

Attribute names are typically namespaced and use a domain-like form (for example, user.comment or com.apple.quarantine). Namespaces convey intended use and access controls; common namespaces include user.* for general-purpose data, system.* or com.* for OS-managed metadata, and security.* for labels used by mechanisms like SELinux. Some namespaces are restricted to privileged processes.

Usage scenarios include storing application-specific metadata, filesystem-level security labels, access control information, resource forks and Finder tags on macOS, and quarantine flags for downloaded content. Tools for manipulating xattrs vary by platform: Linux users often employ getfattr/setfattr or library interfaces (libattr), macOS provides an xattr command and system APIs, and various language bindings (for example, Python modules) expose access to extended attributes.

Limitations and considerations include per-attribute and total-size limits imposed by filesystems, inconsistent propagation across network protocols and archive formats, and potential privacy or security implications when metadata is transferred unintentionally. Extended attributes are not part of POSIX file semantics and behavior can differ between systems and tools.