Extended attributes implement the ability for a user to attach
name:value pairs to objects within the XFS filesystem.
They could be used to store meta-information about the file.
For example "character-set=kanji" could tell a document browser to
use the Kanji character set when displaying that document
and "thumbnail=..." could provide a reduced resolution overview of a
high resolution graphic image.
This document describes the
attr
command, which is mostly compatible with the IRIX command of the same name.
It is thus aimed specifically at users of the XFS filesystem - for
filesystem independent extended attribute manipulation, consult the
getfattr(1)
and
setfattr(1)
documentation.
In the XFS filesystem, the
names
can be up to 256 bytes in length, terminated by the first 0 byte.
The intent is that they be printable ASCII (or other character set)
names for the attribute.
The
values
can be up to 64KB of arbitrary binary data.
Attributes can be attached to all types of XFS inodes:
regular files, directories, symbolic links, device nodes, etc.
XFS uses 2 disjoint attribute name spaces associated with every
filesystem object.
They are the
root
and
user
address spaces.
The
root
address space is accessable only to the superuser,
and then only by specifying a flag argument to the function call.
Other users will not see or be able to modify attributes in the
root
address space.
The
user
address space is protected by the normal file permissions mechanism,
so the owner of the file can decide who is able to see and/or modify
the value of attributes on any particular file.
DESCRIPTION
The
attr
utility allows the manipulation of extended attributes associated with
filesystem objects from within shell scripts.
There are four main operations that
attr
can perform:
GET
The
-g attrname
option tells
attr
to search the named object and print (to stdout) the value
associated with that attribute name.
With the
-q
flag, stdout will be exactly and only the value of the attribute,
suitable for storage directly into a file or processing via a piped command.
LIST
The
-l
option tells
attr
to list the names of all the attributes that are associated with the object,
and the number of bytes in the value of each of those attributes.
With the
-q
flag, stdout will be a simple list of only the attribute names,
one per line, suitable for input into a script.
REMOVE
The
-r attrname
option tells
attr
to remove an attribute with the given name from the object if the
attribute exists.
There is no output on sucessful completion.
SET/CREATE
The
-s attrname
option tells
attr
to set the named attribute of the object to the value read from stdin.
If an attribute with that name already exists,
its value will be replaced with this one.
If an attribute with that name does not already exist,
one will be created with this value.
With the
-V attrvalue
flag, the attribute will be set to have a value of
attrvalue
and stdin will not be read.
With the
-q
flag, stdout will not be used.
Without the
-q
flag, a message showing the attribute name and the entire value
will be printed.
When the
-L
option is given and the named object is a symbolic link,
operate on the attributes of the object referenced by the symbolic link.
Without this option, operate on the attributes of the symbolic link itself.
When the
-R
option is given and the process has appropriate privileges,
operate in the
root
attribute namespace rather that the
USER
attribute namespace.
The
-S
option is similar, except it specifies use of the
security
attribute namespace.
When the
-q
option is given
attr
will try to keep quiet.
It will output error messages (to stderr)
but will not print status messages (to stdout).
NOTES
The standard file interchange/archive programs
tar(1),
and
cpio(1)
will not archive or restore extended attributes,
while the
xfsdump(8)
program will.
CAVEATS
The list option present in the IRIX version of this command is not supported.
getfattr
provides a mechanism to retrieve all of the attribute names.
