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man:uri

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

NAME

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

SYNOPSIS

     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 ]

DESCRIPTION

     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-
     able.
     URIs are the standard way to name hypertext link destinations for tools
     such as web browsers.  The string "http://www.kernel.org" is a URL (and
     thus it is also a URI).  Many people use the term URL loosely as a syn-
     onym 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
     resource.
 Usage
     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 com-
     puter, either its name as determined by DNS or an IP  address  (numbers
     separated  by  periods).   Thus the URI <http://fred:fredpassword@exam-
     ple.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
     http://ip_server/path
     http://ip_server/path?query
     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 <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 inter-
     action mechanism (called POST) which does not include the data  in  the
     URI.  See the Common Gateway Interface specification at for more infor-
     mation.
     ftp - File Transfer Protocol (FTP)
     ftp://ip_server/path
     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
     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 <mailto:dwheeler@dwheeler.com>.
     news - Newsgroup or News message
     news:newsgroup-name
     news:message-id
     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
     telnet://ip_server/
     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-
     net://melvyl.ucop.edu/>.
     file - Normal file
     file://ip_server/path_segments
     file:path_segments
     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
     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
     man:command-name
     man:command-name(section)
     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
     info:virtual-filename
     info:virtual-filename#nodename
     info:(virtual-filename)
     info:(virtual-filename)nodename
     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 spa-
     ces  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
     GCC>.
     whatis - Documentation search
     whatis:string
     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
     ghelp:name-of-application
     This  loads  GNOME  help for the given application.  Note that not much
     documentation currently exists in this format.
     ldap - Lightweight Directory Access Protocol
     ldap://hostport
     ldap://hostport/
     ldap://hostport/dn
     ldap://hostport/dn?attributes
     ldap://hostport/dn?attributes?scope
     ldap://hostport/dn?attributes?scope?filter
     ldap://hostport/dn?attributes?scope?filter?extensions
     This scheme supports queries to the Lightweight Directory Access Proto-
     col (LDAP), a protocol for querying a set of servers for hierarchically
     organized information (such as people and  computing  resources).   See
     RFC 2255  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 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 sup-
                 ported  to  be  valid),   otherwise   it   is   noncritical
                 (optional).
     LDAP  queries  are  easiest to explain by example.  Here's a query that
     asks ldap.itd.umich.edu for information about the University of  Michi-
     gan in the U.S.:
     ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US
     To just get its postal address attribute, request:
     ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US?postalAddress
     To  ask  a  host.com at port 6666 for information about the person with
     common name (cn) "Babs Jensen" at University of Michigan, request:
     ldap://host.com:6666/o=University%20of%20Michigan,c=US??sub?(cn=Babs%20Jensen)
     wais - Wide Area Information Servers
     wais://hostport/database
     wais://hostport/database?search
     wais://hostport/database/wtype/wpath
     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  inter-
     nal  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., pros-
     pero).  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:
  1. _ . ! ~ * ' ( )
     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.,
     "http://www.kernel.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 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
     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., <www.w3.org/Addressing>).  Such references are  primar-
     ily  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 pre-
     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.

CONFORMING TO

     (IETF RFC 2396) (HTML 4.0)

NOTES

     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.
 Security
     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.
     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 dam-
     aging remote operation to occur.  The  unsafe  URL  is  typically  con-
     structed  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  per-
     formed.
     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-
     lic.

BUGS

     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.

SEE ALSO

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

COLOPHON

     This  page  is  part of release 4.16 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
     https://www.kernel.org/doc/man-pages/.

Linux 2017-09-15 URI(7)

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