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ATTR
Section: XFS Compatibility API (1)Updated: Extended Attributes
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NAME
attr - extended attributes on XFS filesystem objectsSYNOPSIS
attr [ -LRSq ] -s attrname [ -V attrvalue ] pathname attr [ -LRSq ] -g attrname pathname attr [ -LRSq ] -r attrname pathname attr [ -LRSq ] -l pathname
OVERVIEW
Extended attributes implement the ability for a user to attach name:value pairs to objects within the XFS filesystem.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.
Extended attributes can 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.
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 accessible 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 successful 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.SEE ALSO
getfattr(1), setfattr(1), attr_get(3), attr_set(3), attr_multi(3), attr_remove(3), attr(5), and xfsdump(8).
Index
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Time: 04:45:20 GMT, September 16, 2022 Content-type: text/html; charset=UTF-8
ATTR
Section: File Formats (5)Index Return to Main Contents
NAME
attr - Extended attributesDESCRIPTION
Extended attributes are name:value pairs associated permanently with files and directories, similar to the environment strings associated with a process. An attribute may be defined or undefined. If it is defined, its value may be empty or non-empty.Extended attributes are extensions to the normal attributes which are associated with all inodes in the system (i.e. the stat(2) data). They are often used to provide additional functionality to a filesystem - for example, additional security features such as Access Control Lists (ACLs) may be implemented using extended attributes.
Users with search access to a file or directory may retrieve a list of attribute names defined for that file or directory.
Extended attributes are accessed as atomic objects. Reading retrieves the whole value of an attribute and stores it in a buffer. Writing replaces any previous value with the new value.
Space consumed for extended attributes is counted towards the disk quotas of the file owner and file group.
Currently, support for extended attributes is implemented on Linux by the ext2, ext3, ext4, XFS, JFS and reiserfs filesystems.
EXTENDED ATTRIBUTE NAMESPACES
Attribute names are zero-terminated strings. The attribute name is always specified in the fully qualified namespace.attribute form, eg. user.mime_type, trusted.md5sum, system.posix_acl_access, or security.selinux.The namespace mechanism is used to define different classes of extended attributes. These different classes exist for several reasons, e.g. the permissions and capabilities required for manipulating extended attributes of one namespace may differ to another.
Currently the security, system, trusted, and user extended attribute classes are defined as described below. Additional classes may be added in the future.
Extended security attributes
The security attribute namespace is used by kernel security modules, such as Security Enhanced Linux. Read and write access permissions to security attributes depend on the policy implemented for each security attribute by the security module. When no security module is loaded, all processes have read access to extended security attributes, and write access is limited to processes that have the CAP_SYS_ADMIN capability.Extended system attributes
Extended system attributes are used by the kernel to store system objects such as Access Control Lists and Capabilities. Read and write access permissions to system attributes depend on the policy implemented for each system attribute implemented by filesystems in the kernel.Trusted extended attributes
Trusted extended attributes are visible and accessible only to processes that have the CAP_SYS_ADMIN capability (the super user usually has this capability). Attributes in this class are used to implement mechanisms in user space (i.e., outside the kernel) which keep information in extended attributes to which ordinary processes should not have access.Extended user attributes
Extended user attributes may be assigned to files and directories for storing arbitrary additional information such as the mime type, character set or encoding of a file. The access permissions for user attributes are defined by the file permission bits.The file permission bits of regular files and directories are interpreted differently from the file permission bits of special files and symbolic links. For regular files and directories the file permission bits define access to the file's contents, while for device special files they define access to the device described by the special file. The file permissions of symbolic links are not used in access checks. These differences would allow users to consume filesystem resources in a way not controllable by disk quotas for group or world writable special files and directories.
For this reason, extended user attributes are only allowed for regular files and directories, and access to extended user attributes is restricted to the owner and to users with appropriate capabilities for directories with the sticky bit set (see the chmod(1) manual page for an explanation of Sticky Directories).
FILESYSTEM DIFFERENCES
The kernel and the filesystem may place limits on the maximum number and size of extended attributes that can be associated with a file. Some file systems, such as ext2/3 and reiserfs, require the filesystem to be mounted with the user_xattr mount option in order for extended user attributes to be used.In the current ext2, ext3 and ext4 filesystem implementations, each extended attribute must fit on a single filesystem block (1024, 2048 or 4096 bytes, depending on the block size specified when the filesystem was created).
In the XFS and reiserfs filesystem implementations, there is no practical limit on the number or size of extended attributes associated with a file, and the algorithms used to store extended attribute information on disk are scalable.
In the JFS filesystem implementation, names can be up to 255 bytes and values up to 65,535 bytes.
ADDITIONAL NOTES
Since the filesystems on which extended attributes are stored might also be used on architectures with a different byte order and machine word size, care should be taken to store attribute values in an architecture independent format.AUTHORS
Andreas Gruenbacher, <a.gruenbacher@bestbits.at> and the SGI XFS development team, <linux-xfs@oss.sgi.com>.SEE ALSO
getfattr(1), setfattr(1).
Index
- NAME
- DESCRIPTION
- EXTENDED ATTRIBUTE NAMESPACES
- FILESYSTEM DIFFERENCES
- ADDITIONAL NOTES
- AUTHORS
- SEE ALSO
This document was created by man2html, using the manual pages.
Time: 04:45:54 GMT, September 16, 2022
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