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           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 network "location"),
           rather than by name or some other attribute of that resource.   A  Uni-
           form  Resource Name (URN) is a URI that must remain globally unique and
           persistent even when the resource ceases to exist or  becomes  unavail-
           URIs are the standard way to name hypertext link destinations for tools
           such as web browsers.  The string "" 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 segment 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 "C|".
           A  fragment  identifier, if included, refers to a particular named por-
           tion (fragment) of a resource; text after a '#'  identifies  the  frag-
           ment.   A URI beginning with '#' refers to that fragment in the current
           (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 exam-
           ple is <>.
           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 unfortunate 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 inter-
           action mechanism (called POST) which does not include the data  in  the
           URI.  See the Common Gateway Interface specification at for more infor-
           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 <>.
           gopher - Gopher server
           gopher://ip_server/gophertype selector
           gopher://ip_server/gophertype selector%09search
           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 exam-
           ple is <>.
           news - Newsgroup or News message
           A  newsgroup-name  is  a  period-delimited  hierarchical  name, such as
           "comp.infosystems.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,  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 <tel-
           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  inter-
           preted".   If the path is to a directory, the viewer should display the
           directory's contents with links to each containee; not all viewers cur-
           rently   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 recognize this as a URI.   A  more
           portable syntax is to use an empty string as the server name, for exam-
           ple, <file:///etc/passwd>; this form does the same thing and is  easily
           recognized  by pattern matchers and older programs as a URI.  Note that
           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 tex-
           info files), a documentation 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 syn-
           tax.  The first two formats are the  GNOME  format;  in  nodenames  all
           spaces  are written as underscores.  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
           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  cur-
           rently registered by the IETF.
           ghelp - GNOME help documentation
           This  loads  GNOME  help for the given application.  Note that not much
           documentation currently exists in this format.
           ldap - Lightweight Directory Access Protocol
           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
                       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 sup-
                       ported  to  be  valid),   otherwise   it   is   noncritical
           LDAP  queries  are  easiest to explain by example.  Here's a query that
           asks for information about the University of  Michi-
           gan in the U.S.:
           To just get its postal address attribute, request:
           To  ask  a 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  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
           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 URI):
                     ; / ? : @ & = + $ ,
           Unreserved characters may be included in a URI.  Unreserved  characters
           include  uppercase  and  lowercase 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 representing the octet  code  (you  can  use
           uppercase  or lowercase letters for the hexadecimal digits).  For exam-
           ple, 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  (sec-
           tion 2.2.5) recommend the following approach:
           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.,
           ""),  enclosed  in  angle   brackets   (e.g.,
           <>),  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 brack-
           ets instead, or switch to a quoting system that never includes extrane-
           ous characters inside quotation marks.  This latter system, called  the
           'new'  or  'logical'  quoting  system by "Hart's Rules" and the "Oxford
           fix of "http://" and hostnames beginning with "ftp" likely  to  have  a
           prefix of "ftp://").  Many client implementations heuristically resolve
           these references.  Such heuristics may change over  time,  particularly
           when new schemes are introduced.  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  hypertext links inside a document; use the standard format as
           described here.


           (IETF RFC 2396) (HTML 4.0)


           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 (view-
           ers should be able to detect :// as part of a URI).
           The  GNOME and KDE desktop environments currently vary in the URIs they
           accept, in particular 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 documenta-
           tion  in  HTML,  accessed  via  the  <file:/cgi-bin/helpindex>.   GNOME
           prefers  the  ghelp  scheme  to  store and find documentation.  Neither
           browser handles file: references to directories at  the  time  of  this
           writing,  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 formats, 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 guaran-
           tee can be obtained only from the person(s) controlling that  namespace
           and the resource in question.
           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 characters 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  per-
           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  pub-


           Documentation  may  be  placed in a variety of locations, so there cur-
           rently 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  require-
           ments  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 file-
           name globbing could partially overcome this).  Finally, using the file:
           scheme doesn't easily support people who dynamically load documentation
           from the Internet (instead of loading the files onto a  local  filesys-
           tem).   A  future  URI scheme may be added (e.g., "userdoc:") to permit
           programs to include cross-references  to  more  detailed  documentation
           without  having  to  know  the  exact  location  of that documentation.
           Alternatively, a future version of  the  filesystem  specification  may
           specify  file  locations  sufficiently so that the file: scheme will be
           able to locate documentation.
           Many programs and file formats don't include a way  to  incorporate  or
           implement links using URIs.
           Many  programs  can't  handle all of these different URI formats; there
           should be a standard mechanism to load an arbitrary URI that  automati-
           cally  detects  the users' environment (e.g., text or graphics, desktop
           environment, local user preferences, and currently executing tools) and
           invokes the right tool for any URI.


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

    Linux 2014-03-18 URI(7)


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