fflags

File flags, often abbreviated as fflags, are a small set of boolean attributes attached to a file’s metadata at the filesystem level. They are separate from POSIX permissions and access control lists, and they influence how the kernel treats the file. Flags can affect whether a file can be modified, whether it can be backed up or dumped, or how it behaves during certain filesystem operations. They are intended as lightweight controls that complement traditional permissions rather than replace them.

The exact meaning and storage of fflags vary by filesystem and operating system. In Linux, file flags are manipulated with the chattr utility and observed with tools like lsattr. Commonly used flags include immutable (the file cannot be modified, renamed, or deleted) and append-only (data can be appended but not overwritten). Other flags can affect backup behavior or synchronization, but support depends on the filesystem.

In BSD-derived systems and macOS, file flags are managed with chflags. Flags are organized into user and system categories, and examples include those that prevent modification or deletion, or influence backup behavior. The specific textual names and effects of flags differ by OS, so users should consult the local documentation for exact semantics.

Portability considerations are important: flags are not universally preserved across all copies, backups, or transfers, and some tools or filesystems may ignore certain flags. Administrative actions and security policies can also interact with file flags, potentially overriding or clearing them.

In summary, fflags provide a filesystem-level mechanism to enforce lightweight behavioral constraints on files, standing alongside traditional permissions and access controls. See also: chmod, chflags, chattr, lsattr, and extended attributes for related mechanisms.