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RedHat 9 (Linux i386) - man page for url (redhat section 7)

URI(7)				    Linux Programmer's Manual				   URI(7)

       uri, url, urn - uniform resource identifier (URI), including a URL or URN

       URI = [ absoluteURI | relativeURI ] [ "#" fragment ]

       absoluteURI = scheme ":" ( hierarchical_part | opaque_part )

       relativeURI = ( net_path | absolute_path | relative_path ) [ "?" query ]

       scheme = "http" | "ftp" | "gopher" | "mailto" | "news" | "telnet" | "file" | "man" | "info" | "whatis" | "ldap" | "wais" | ...

       hierarchical_part = ( net_path | absolute_path ) [ "?" query ]

       net_path = "//" authority [ absolute_path ]

       absolute_path = "/"  path_segments

       relative_path = relative_segment [ absolute_path ]

       A  Uniform  Resource  Identifier  (URI)	is  a  short  string of characters identifying an
       abstract or physical resource (for example, a web page).  A Uniform Resource Locator (URL)
       is  a  URI that identifies a resource through its primary access mechanism (e.g., its net-
       work "location"), rather than by name or some other attribute of that resource.	A Uniform
       Resource Name (URN) is a URI that must remain globally unique and persistent even when the
       resource ceases to exist or becomes unavailable.

       URIs are the standard way to name hypertext  link  destinations	for  tools  such  as  web
       browsers.   The	string "http://www.kernelnotes.org" is a URL (and thus it's a URI).  Many
       people use the term URL loosely as a synonym for URI (though technically URLs are a subset
       of URIs).

       URIs can be absolute or relative.  An absolute identifier refers to a resource independent
       of context, while a relative identifier refers to a resource by describing the  difference
       from  the  current  context.  Within a relative path reference, the complete path segments
       "." and ".." have special meanings: "the current hierarchy level"  and  "the  level  above
       this  hierarchy level", respectively, just like they do in Unix-like systems.  A path seg-
       ment which contains a colon character can't be used as the first segment of a relative URI
       path  (e.g.,  "this:that"),  because  it would be mistaken for a scheme name; precede such
       segments with ./ (e.g., "./this:that").	Note that descendents of MS-DOS (e.g.,	Microsoft
       Windows)  replace  devicename  colons with the vertical bar ("|") in URIs, so "C:" becomes

       A fragment identifier, if included, refers to a particular named portion (fragment)  of	a
       resource;  text	after  a '#' identifies the fragment.  A URI beginning with '#' refers to
       that fragment in the current resource.

       There are many different URI schemes, each with specific additional  rules  and	meanings,
       but  they  are  intentionally  made  to	be as similar as possible.  For example, many URL
       schemes permit the authority to be the following format, called here an ip_server  (square
       brackets show what's optional):

       ip_server = [user [ : password ] @ ] host [ : port]

       This  format  allows  you to optionally insert a user name, a user plus password, and/or a
       port number.  The host is the name of the host computer, either its name as determined  by
       DNS  or an IP address (numbers separated by periods).  Thus the URI <http://fred:fredpass-
       word@xyz.com:8080/> logs into a web server on host xyz.com as  fred  (using  fredpassword)
       using  port  8080.   Avoid  including  a password in a URI if possible because of the many
       security risks of having a password written down.  If the URL supplies a user name but  no
       password,  and  the  remote  server  requests a password, the program interpreting the URL
       should request one from the user.

       Here are some of the most common schemes in use on Unix-like systems that  are  understood
       by  many tools.	Note that many tools using URIs also have internal schemes or specialized
       schemes; see those tools' documentation for information on those schemes.

   http - Web (HTTP) server

       This is a URL accessing a web (HTTP) server.  The default port is 80.  If the path  refers
       to  a  directory,  the  web  server will choose what to return; usually if there is a file
       named "index.html" or "index.htm" its content is returned, otherwise, a list of the  files
       in  the	current directory (with appropriate links) is generated and returned.  An example
       is <http://lwn.net>.

       A query can be given in the archaic "isindex" format, consisting of a word or  phrase  and
       not  including  an  equal sign (=).  A query can also be in the longer "GET" format, which
       has one or more query entries of the form key=value separated by the  ampersand	character
       (&).   Note  that key can be repeated more than once, though it's up to the web server and
       its application programs to determine if there's any meaning to that.  There is an  unfor-
       tunate  interaction  with HTML/XML/SGML and the GET query format; when such URIs with more
       than one key are embedded in SGML/XML documents (including HTML), the ampersand (&) has to
       be rewritten as &amp;.  Note that not all queries use this format; larger forms may be too
       long to store as a URI, so they use a different interaction mechanism (called POST)  which
       does  not  include the data in the URI.	See the Common Gateway Interface specification at
       <http://www.w3.org/CGI> for more information.

   ftp - File Transfer Protocol (FTP)

       This is a URL accessing a file through the file transfer protocol (FTP).  The default port
       (for  control)  is 21.  If no username is included, the user name "anonymous" is supplied,
       and in that case many clients provide as  the  password	the  requestor's  Internet  email
       address.  An example is <ftp://ftp.is.co.za/rfc/rfc1808.txt>.

   gopher - Gopher server
       gopher://ip_server/gophertype selector
       gopher://ip_server/gophertype selector%09search
       gopher://ip_server/gophertype selector%09search%09gopher+_string

       The  default  gopher  port  is  70.   gophertype is a single-character field to denote the
       Gopher type of the resource to which the URL refers.  The entire path may also  be  empty,
       in which case the delimiting "/" is also optional and the gophertype defaults to "1".

       selector  is  the Gopher selector string.  In the Gopher protocol, Gopher selector strings
       are a sequence of octets which may contain any octets except 09 hexadecimal  (US-ASCII  HT
       or tab), 0A hexadecimal (US-ASCII character LF), and 0D (US-ASCII character CR).

   mailto - Email address

       This  is  an  email  address, usually of the form name@hostname.  See mailaddr(7) for more
       information on the correct format of an email address.  Note that any % character must  be
       rewritten as %25.  An example is <mailto:dwheeler@dwheeler.com>.

   news - Newsgroup or News message

       A   newsgroup-name  is  a  period-delimited  hierarchical  name,  such  as  "comp.infosys-
       tems.www.misc".	If <newsgroup-name> is "*" (as in <news:*>), it is used to refer to  "all
       available news groups".	An example is <news:comp.lang.ada>.

       A     message-id     corresponds     to	  the	 Message-ID    of    IETF    RFC    1036,
       <http://www.ietf.org/rfc/rfc1036.txt> without the enclosing "<" and ">"; it takes the form
       unique@full_domain_name.  A message identifier may be distinguished from a news group name
       by the presence of the "@" character.

   telnet - Telnet login

       The Telnet URL scheme is used to designate interactive text services that may be  accessed
       by  the Telnet protocol.  The final "/" character may be omitted.  The default port is 23.
       An example is <telnet://melvyl.ucop.edu/>.

   file - Normal file

       This represents a file or directory accessible locally.	As a special case,  host  can  be
       the string "localhost" or the empty string; this is interpreted as `the machine from which
       the URL is being interpreted'.  If the path is to a directory, the viewer  should  display
       the  directory's contents with links to each containee; not all viewers currently do this.
       KDE supports generated files through the URL <file:/cgi-bin>.  If  the  given  file  isn't
       found,  browser	writers may want to try to expand the filename via filename globbing (see
       glob(7) and glob(3)).

       The second format (e.g., <file:/etc/passwd>) is a correct format for referring to a  local
       file.  However, older standards did not permit this format, and some programs don't recog-
       nize this as a URI.  A more portable syntax is to use an empty string as the server  name,
       e.g., <file:///etc/passwd>; this form does the same thing and is easily recognized by pat-
       tern matchers and older programs as a URI.  Note that if you really  mean  to  say  "start
       from  the  current location," don't specify the scheme at all; use a relative address like
       <../test.txt>, which has the side-effect of being scheme-independent.  An example of  this
       scheme is <file:///etc/passwd>.

   man - Man page documentation

       This refers to local online manual (man) reference pages.  The command name can optionally
       be followed by a parenthesis and section number; see man(7) for more  information  on  the
       meaning	of  the section numbers.  This URI scheme is unique to Unix-like systems (such as
       Linux) and is not currently registered by the IETF.  An example is <man:ls(1)>.

   info - Info page documentation

       This scheme refers to online info reference pages (generated from texinfo files), a  docu-
       mentation  format  used	by  programs such as the GNU tools.  This URI scheme is unique to
       Unix-like systems (such as Linux) and is not currently registered by the IETF.  As of this
       writing,  GNOME	and  KDE differ in their URI syntax and do not accept the other's syntax.
       The first two formats are the GNOME format; in nodenames all spaces are written as  under-
       scores.	The second two formats are the KDE format; spaces in nodenames must be written as
       spaces, even though this is forbidden by the URI standards.  It's hoped that in the future
       most  tools  will  understand  all of these formats and will always accept underscores for
       spaces in nodenames.  In both GNOME and KDE, if the form without the nodename is used  the
       nodename  is  assumed  to  be  "Top".   Examples  of  the  GNOME format are <info:gcc> and
       <info:gcc#G++_and_GCC>.	Examples of the KDE format are	<info:(gcc)>  and  <info:(gcc)G++
       and GCC>.

   whatis - Documentation search

       This scheme searches the database of short (one-line) descriptions of commands and returns
       a list of descriptions containing that string.  Only complete word matches  are	returned.
       See  whatis(1).	This URI scheme is unique to Unix-like systems (such as Linux) and is not
       currently registered by the IETF.

   ghelp - GNOME help documentation

       This loads GNOME help for the given application.  Note that not	much  documentation  cur-
       rently exists in this format.

   ldap - Lightweight Directory Access Protocol

       This scheme supports queries to the Lightweight Directory Access Protocol (LDAP), a proto-
       col for querying a set of servers for hierarchically-organized information (such as people
       and  computing  resources).   More  information on the LDAP URL scheme is available in RFC
       2255.  <http://www.ietf.org/rfc/rfc2255.txt> The components of this URL are:

       hostport    the LDAP server to query, written as a hostname optionally followed by a colon
		   and	the  port  number.  The default LDAP port is TCP port 389.  If empty, the
		   client determines which the LDAP server to use.

       dn	   the LDAP Distinguished Name, which identifies the  base  object  of	the  LDAP
		   search (see RFC 2253 <http://www.ietf.org/rfc/rfc2253.txt> section 3).

       attributes  a  comma-separated  list  of  attributes  to be returned; see RFC 2251 section
		   4.1.5.  If omitted, all attributes should be returned.

       scope	   specifies the scope of the search, which can be one	of  "base"  (for  a  base
		   object  search),  "one"  (for  a  one-level	search),  or "sub" (for a subtree
		   search).  If scope is omitted, "base" is assumed.

       filter	   specifies the search filter (subset of entries to  return).	If  omitted,  all
		   entries	  should       be	returned.	 See	   RFC	     2254
		   <http://www.ietf.org/rfc/rfc2254.txt> section 4.

       extensions  a comma-separated list of type=value pairs, where the =value  portion  may  be
		   omitted  for  options  not  requiring it.  An extension prefixed with a '!' is
		   critical  (must  be	supported  to  be  valid),  otherwise  it's  non-critical

       LDAP   queries	are   easiest	to   explain  by  example.   Here's  a	query  that  asks
       ldap.itd.umich.edu for information about the University of Michigan in the U.S.:

       To just get its postal address attribute, request:

       To ask a host.com at port 6666 for information about the  person  with  common  name  (cn)
       "Babs Jensen" at University of Michigan, request:

   wais - Wide Area Information Servers

       This  scheme  designates  a  WAIS  database,  search,  or  document  (see  IETF	RFC  1625
       <http://www.ietf.org/rfc/rfc1625.txt> for more information  on  WAIS).	Hostport  is  the
       hostname, optionally followed by a colon and port number (the default port number is 210).

       The  first  form  designates  a WAIS database for searching.  The second form designates a
       particular search of the WAIS database database.  The third form designates  a  particular
       document  within  a  WAIS  database to be retrieved.  wtype is the WAIS designation of the
       type of the object and wpath is the WAIS document-id.

   other schemes
       There are many other URI schemes.  Most tools that accept URIs support a set  of  internal
       URIs  (e.g.,  Mozilla  has  the about: scheme for internal information, and the GNOME help
       browser has the toc: scheme for various starting locations).  There are many schemes  that
       have  been  defined  but are not as widely used at the current time (e.g., prospero).  The
       nntp: scheme is deprecated in favor of the news: scheme.  URNs are to be supported by  the
       urn:  scheme, with a hierarchical name space (e.g., urn:ietf:... would identify IETF docu-
       ments); at this time URNs are not widely implemented.  Not all tools support all schemes.

       URIs use a limited number of characters so that they can be typed in and used in a variety
       of situations.

       The  following characters are reserved, that is, they may appear in a URI but their use is
       limited to their reserved purpose (conflicting data must be  escaped  before  forming  the

		 ; / ? : @ & = + $ ,

       Unreserved  characters  may  be	included in a URI.  Unreserved characters include include
       upper and lower case English letters, decimal digits, and the  following  limited  set  of
       punctuation marks and symbols:

	       - _ . ! ~ * ' ( )

       All other characters must be escaped.  An escaped octet is encoded as a character triplet,
       consisting of the percent character "%" followed by the two hexadecimal digits  represent-
       ing  the  octet code (you can use upper or lower case letters for the hexadecimal digits).
       For example, a blank space must be escaped as "%20", a tab character as "%09", and the "&"
       as  "%26".  Because the percent "%" character always has the reserved purpose of being the
       escape indicator, it must be escaped as "%25".  It is  common  practice	to  escape  space
       characters  as the plus symbol (+) in query text; this practice isn't uniformly defined in
       the relevant RFCs (which recommend %20 instead) but any tool  accepting	URIs  with  query
       text should be prepared for them.  A URI is always shown in its "escaped" form.

       Unreserved  characters  can be escaped without changing the semantics of the URI, but this
       should not be done unless the URI is being used in a  context  that  does  not  allow  the
       unescaped  character to appear.	For example, "%7e" is sometimes used instead of "~" in an
       http URL path, but the two are equivalent for an http URL.

       For URIs which must handle characters outside the US ASCII character set,  the  HTML  4.01
       specification  (section	B.2)  and  IETF  RFC 2718 (section 2.2.5) recommend the following

       1.  translate the character sequences into UTF-8 (IETF RFC 2279) - see utf-8(7) - and then

       2.  use the URI escaping mechanism, that is, use the %HH encoding for unsafe octets.

       When written,  URIs  should  be	placed	inside	doublequotes  (e.g.,  "http://www.kernel-
       notes.org"),  enclosed  in angle brackets (e.g., <http://lwn.net>), or placed on a line by
       themselves.  A warning for those who use double-quotes: never move extraneous  punctuation
       (such  as  the  period  ending a sentence or the comma in a list) inside a URI, since this
       will change the value of the URI.  Instead, use angle brackets instead,	or  switch  to	a
       quoting	system	that  never  includes extraneous characters inside quotation marks.  This
       latter system, called the 'new' or 'logical' quoting system  by	"Hart's  Rules"  and  the
       "Oxford	Dictionary  for  Writers and Editors", is preferred practice in Great Britain and
       hackers	worldwide  (see   the	Jargon	 File's   section   on	 Hacker   Writing   Style
       <http://www.fwi.uva.nl/~mes/jargon/h/HackerWritingStyle.html>   for   more   information).
       Older documents suggested inserting the prefix "URL:" just before the URI, but  this  form
       has never caught on.

       The  URI syntax was designed to be unambiguous.	However, as URIs have become commonplace,
       traditional media (television, radio, newspapers, billboards, etc.) have increasingly used
       abbreviated URI references consisting of only the authority and path portions of the iden-
       tified resource (e.g., <www.w3.org/Addressing>).  Such references are  primarily  intended
       for  human  interpretation  rather  than  machine,  with the assumption that context-based
       heuristics are sufficient to complete the URI (e.g., hostnames beginning  with  "www"  are
       likely to have a URI prefix of "http://" and hostnames beginning with "ftp" likely to have
       a prefix of "ftp://").  Many client implementations  heuristically  resolve  these  refer-
       ences.	Such  heuristics  may  change over time, particularly when new schemes are intro-
       duced.  Since an abbreviated URI has the same syntax as a relative URL  path,  abbreviated
       URI references cannot be used where relative URIs are permitted, and can only be used when
       there is no defined base (such as in dialog boxes).  Don't use abbreviated URIs as  hyper-
       text links inside a document; use the standard format as described here.

       Any  tool  accepting URIs (e.g., a web browser) on a Linux system should be able to handle
       (directly or indirectly) all of the schemes described here, including the man:  and  info:
       schemes.  Handling them by invoking some other program is fine and in fact encouraged.

       Technically the fragment isn't part of the URI.

       For  information on how to embed URIs (including URLs) in a data format, see documentation
       on that format.	HTML uses the format <A HREF="uri"> text </A>.	 Texinfo  files  use  the
       format @uref{uri}.  Man and mdoc have the recently-added UR macro, or just include the URI
       in the text (viewers should be able to detect :// as part of a URI).

       The GNOME and KDE desktop environments currently vary in the URIs they accept, in particu-
       lar  in their respective help browsers.	To list man pages, GNOME uses <toc:man> while KDE
       uses <man:(index)>, and	to  list  info	pages,	GNOME  uses  <toc:info>  while	KDE  uses
       <info:(dir)>  (the  author  of  this man page prefers the KDE approach here, though a more
       regular format would be even better).  In general, KDE uses <file:/cgi-bin/> as	a  prefix
       to  a  set  of  generated  files.   KDE	prefers  documentation	in HTML, accessed via the
       <file:/cgi-bin/helpindex>.  GNOME prefers the ghelp scheme to store  and  find  documenta-
       tion.   Neither	browser handles file: references to directories at the time of this writ-
       ing, making it difficult to refer to an entire directory with a browsable URI.	As  noted
       above,  these  environments  differ in how they handle the info: scheme, probably the most
       important variation.  It is expected that GNOME and KDE will converge to common	URI  for-
       mats,  and  a future version of this man page will describe the converged result.  Efforts
       to aid this convergence are encouraged.

       A URI does not in itself pose a security threat.  There is no  general  guarantee  that	a
       URL, which at one time located a given resource, will continue to do so.  Nor is there any
       guarantee that a URL will not locate a different resource at some  later  point	in  time;
       such  a	guarantee  can only be obtained from the person(s) controlling that namespace and
       the resource in question.

       It is sometimes possible to construct a URL such that an attempt to  perform  a	seemingly
       harmless  operation, such as the retrieval of an entity associated with the resource, will
       in fact cause a possibly damaging remote operation to occur.  The unsafe URL is	typically
       constructed  by specifying a port number other than that reserved for the network protocol
       in question.  The client unwittingly contacts a site that is in fact running  a	different
       protocol.   The	content of the URL contains instructions that, when interpreted according
       to this other protocol, cause an unexpected operation.  An example has been the use  of	a
       gopher URL to cause an unintended or impersonating message to be sent via a SMTP server.

       Caution	should	be  used  when	using any URL that specifies a port number other than the
       default for the protocol, especially when it is a number within the reserved space.

       Care should be taken when a URI contains escaped delimiters  for  a  given  protocol  (for
       example,  CR  and  LF characters for telnet protocols) that these are not unescaped before
       transmission.  This might violate the protocol, but avoids the potential for such  charac-
       ters  to be used to simulate an extra operation or parameter in that protocol, which might
       lead to an unexpected and possibly harmful remote operation to be performed.

       It is clearly unwise to use a URI that contains a password which is intended to be secret.
       In  particular, the use of a password within the 'userinfo' component of a URI is strongly
       disrecommended except in those rare cases where the 'password' parameter is intended to be

       IETF RFC 2396, <http://www.ietf.org/rfc/rfc2396.txt> HTML 4.0.  <http://www.w3.org/TR/REC-

       Documentation may be placed in a variety of locations, so there currently isn't a good URI
       scheme  for  general  online  documentation  in arbitrary formats.  References of the form
       <file:///usr/doc/ZZZ> don't work because different distributions  and  local  installation
       requirements  may  place  the  files  in  different directories (it may be in /usr/doc, or
       /usr/local/doc, or /usr/share, or  somewhere  else).   Also,  the  directory  ZZZ  usually
       changes	when  a version changes (though filename globbing could partially overcome this).
       Finally, using the file: scheme doesn't easily support people who dynamically  load  docu-
       mentation  from	the  Internet  (instead of loading the files onto a local filesystem).	A
       future URI scheme may be added (e.g., "userdoc:") to permit programs to include cross-ref-
       erences	to  more detailed documentation without having to know the exact location of that
       documentation.  Alternatively, a future version of the filesystem specification may  spec-
       ify file locations sufficiently so that the file: scheme will be able to locate documenta-

       Many programs and file formats don't include a way to incorporate or implement links using

       Many  programs can't handle all of these different URI formats; there should be a standard
       mechanism to load an arbitrary URI  that  automatically	detects  the  users'  environment
       (e.g.,  text  or graphics, desktop environment, local user preferences, and currently-exe-
       cuting tools) and invokes the right tool for any URI.

       David A. Wheeler (dwheeler@dwheeler.com) wrote this man page.

       lynx(1),      mailaddr(7),      utf-8(7),      man2html(1),	IETF	  RFC	    2255.

Linux					    2000-03-14					   URI(7)

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