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

       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  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

       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 com-
       puter, either its name as determined by DNS or an IP  address  (numbers
       separated   by   periods).    Thus   the   URI   <http://fred:fredpass-> logs into a web server  on  host  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 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 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 <>.

       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 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 "@"

       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, 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 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).  More information on the LDAP URL scheme  is  available  in
       RFC 2255.  <> 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
                   <> 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
                   <> 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  non-critical

       LDAP queries are easiest to explain by example.  Here's  a  query  that
       asks  for  information  about  the  University  of
       Michigan 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

       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  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 representing 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 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
       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,,  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.,   <>).    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 only be used 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.

CONFORMING TO           (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
       (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 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 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   file  system).   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 file-system
       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.

       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at

Linux                             2000-03-14                            URI(7)

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Created with the man page lookup class by Andrew Collington,