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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 is also 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 descendants 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 username, 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 AT 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 username  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 username "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 AT 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, ip_server  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,
       for  example, <file:///etc/passwd>; this form does the same thing and is easily recognized
       by pattern 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).   See  RFC 2255 ⟨http://www.ietf.org/rfc/rfc2255.txt⟩ for more
       information on the LDAP URL scheme.  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  is  noncritical

       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.

   Character encoding
       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 uppercase
       and lowercase English letters, decimal digits, and the following limited set  of  punctua‐
       tion 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 uppercase or lowercase letters for the hexadecimal dig‐
       its).  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.

   Writing a URI
       When written, URIs should  be  placed  inside  double  quotes  (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 be used only 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.

       (IETF  RFC 2396)  ⟨http://www.ietf.org/rfc/rfc2396.txt⟩,  (HTML 4.0) ⟨http://www.w3.org/TR

       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 be obtained only 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
       recommended  against except in those rare cases where the "password" parameter is intended
       to be public.

       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.

       lynx(1), man2html(1), mailaddr(7), utf-8(7)

       IETF RFC 2255 ⟨http://www.ietf.org/rfc/rfc2255.txt⟩

       This page is part of release 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2014-03-18                                     URI(7)

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