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


   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
   Uniform  Resource  Name (URN) is a URI that must remain globally unique
   and persistent even when  the  resource  ceases  to  exist  or  becomes

   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 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
   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

   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:fredpassword@example.com:8080/> logs into a web server  on
   host  example.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 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
   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@dwheeler.com>.

   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,
   ⟨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 "@"

   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

   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 recognize 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 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
   syntax.   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
   currently 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


   This  scheme  supports  queries  to  the  Lightweight  Directory Access
   Protocol  (LDAP),  a  protocol  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

   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
   documents); 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 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
   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 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.,
   "http://www.kernelnotes.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  identified
   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  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)   ⟨http://www.ietf.org/rfc/rfc2396.txt⟩,  (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
   (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 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
   documentation 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
   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 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

   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


   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
   documentation 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-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
   automatically  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 ⟨http://www.ietf.org/rfc/rfc2255.txt


   This  page  is  part of release 4.09 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


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