Extended attributes

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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 --name=user.checksum --value="3baf9ebce4c664ca8d9e5f6314fb47fb" file.txt

Use getfattr to display extended attributes: 

$ getfattr --encoding=text --dump file.txt
# file: file.txt
user.checksum="3baf9ebce4c664ca8d9e5f6314fb47fb"

To remove an extended attribute: 

$ setfattr --remove=user.checksum file.txt

To find files with certain extended attributes use rawhideAUR: 

$ 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

user.xdg.tags is not part of the official standard, but it has become a "de facto" standard as several popular programs have implemented support for it (see #Software). It is implemented as a CSV list of user-specified tags for each file.

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
  1. 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]

Just like you should do for any other data you do not want to lose, you should make regular backups of your extended attributes. To make a full backup of the extended attributes of all files in the current directory (recursively): 

$ getfattr --dump --recursive . > backup.txt

To restore a backup: 

$ setfattr --restore=backup.txt

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 supports extended attributes since Linux 5.9.

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-binAUR No
CURL 
user.xdg.origin.url
user.xdg.referrer.url
Enabled with --xattrs flag.
Dolphin 
user.baloo.rating
user.xdg.comment
user.xdg.tags
Dolphin provides comprehensive support for file tagging, including the ability to add tags to files through the context menu, and support for searching by file tags. Tags are stored as CSV in the user.xdg.tags attribute. See also Dolphin#File tagging.

[2], [3]

Dropbox user.com.dropbox.attributes
emacs-vm No Resets mbox xattrs.
Epiphany No
Exiftool No
fd No
findutils No
Firefox No
gallery-dlAUR No
Gwenview user.xdg.tags
kfind No
konqueror No
kio [4]
Nautilus No Full extended file attributes support can be added by the Nautilus extension from eicielAUR.
rclone No
recoll user.xdg.tags
tagspacesAUR No
tmsuAUR No
Thunar No
youtube-dlAUR, 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 id3AUR for audio.

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.

See also