Extended attributes

From xattr(7): "Extended attributes are name:value pairs associated permanently with files and directories". There are four extended attribute classes: security, system, trusted and user.

Warning: By default, extended attributes are not preserved by cp, rsync, and other similar programs, see #Preserving extended attributes.

Extended attributes are also used to set Capabilities.

User extended attributes

User extended attributes can be used to store arbitrary information about a file. To create one:

$ setfattr -n user.checksum -v "3baf9ebce4c664ca8d9e5f6314fb47fb" file.txt

Use getfattr to display extended attributes:

$ getfattr -e text -d file.txt

# file: file.txt
user.checksum="3baf9ebce4c664ca8d9e5f6314fb47fb"

To remove an extended attribute:

$ setfattr -x user.checksum file.txt

To find files with certain extended attributes use rawhide^AUR:

$ rh /path/to/dir '"XATTR_REGEX".reea'

Some other user extended attributes include:

user.mime_type: helps programs know mimetype and set it with less guesswork.
user.charset: used by the Apache httpd module mod_mime_xattr.
user.creator: The name of the application that created the file.

XDG also proposes a set of standardized extended attributes to be used by programs:

user.xdg.comment: supported by Dolphin and other file managers.
user.xdg.origin.url: for files downloaded from a url.
user.xdg.robots.index: "true" if a file is included in indexing, "false" otherwise
user.xdg.robots.backup: "true" if a file is included in backup, "false" otherwise
user.xdg.language
user.xdg.creator
user.xdg.publisher
user.xdg.origin.email.subject
user.xdg.origin.email.from
user.xdg.origin.email.message-id

Preserving extended attributes

Command	Preserves by default?/Required flag
`cp`	`--archive`/`--preserve=all`/`--preserve=xattr`
`mv`	Yes¹
`tar`	`--xattrs` for creation and `--xattrs-include='*'` for extraction
`bsdtar`	`-p` for extraction
rsync	`-X, --xattrs`
cpio	No
gzip	No
pax	No
syncthing	by enabling syncXattrs

mv silently discards extended attributes when the target file system does not support them.

To preserve extended attributes with text editors you need to configure them to truncate files on saving instead of using rename(2). [1]

Support

File systems

All major Linux file systems including Ext4, Btrfs, ZFS, and XFS support extended attributes. The kernel allows to have extended attribute names of up to 255 bytes and values of up to 64 KiB, but Ext4 and Btrfs might impose smaller limits, requiring extended attributes to be within a "filesystem block".

NTFS uses Alternative Data Streams to store user. The mount option user_xattr or streams_interface=xattr should be used by default. However, it might not be supported if mount option streams_interface=windows is used. ntfs-3g supports mapping Alternative Data Streams to extended attributes in FUSE.

NFS does not currently support extended attributes.

Software

Application	Supported extended attributes	Notes
baloo	user.xdg.tags user.baloo.rating
caja	Yes	Supported by caja-xattr-tags and caja.eiciel extensions.
Chromium	No	Used to support referrer and url, but was disabled due to privacy and security concerns. See also CVE-2018-20483.
brave^AUR	No
CURL	user.xdg.origin.url user.xdg.referrer.url	Enabled with `--xattrs` flag.
dolphin	user.baloo.rating user.xdg.comment user.xdg.tags	[2]
Dropbox	`user.com.dropbox.attributes`
emacs-vm	No	Resets mbox xattrs.
Epiphany	No
Exiftool	No
fd	No
findutils	No
Firefox	No
gallery-dl^AUR	No
Gwenview	`user.xdg.tags`
kfind	No
konqueror	No
kio	[3]
Nautilus	No
recoll	`user.xdg.tags`
tagspaces^AUR	No
tmsu^AUR	No
Thunar	No
youtube-dl^AUR, yt-dlp	user.xdg.referrer.url user.dublincore.title user.dublincore.date user.dublincore.description user.dublincore.contributor user.dublincore.format	Enabled with `--xattrs` flag.
Wget	user.xdg.origin.url user.xdg.referrer.url	Enabled with `--xattrs` flag.
Webkit	No

Other tagging systems

It might not be possible to use extended attributes due to lack of support of either the file system or software. For this reason, many media formats store metadata included in the file format that can be viewed using programs like Exiftool or more specified ones like id3^AUR for audio.

For all files: Exiftool
For audio: Audio tag editors
For video: ffprobe(1) from ffmpeg

gvfs

Another filesystem-independent workaround is Gnome virtual filesystem: gvfs which is used to store metadata (gvfsd-metadata). For example, Firefox stores metadata this way and can be viewed with:

$ gio info --attributes=metadata:: downloaded.html

Other programs that use this approach include:

Thunar: to save file color highlights.