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 "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

       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:fredpassword@exam-
       ple.com:8080/> logs into a web server on host example.com as fred  (us-
       ing 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 <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
       <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 in-
       cluded, 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 ad-
       dress.  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 ex-
       ample 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  <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
       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  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 un-
       derscores for spaces in nodenames.  In both GNOME and KDE, if the  form
       without  the nodename is used the nodename is assumed to be "Top".  Ex-
       amples 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


       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 <http://www.ietf.org/rfc/rfc2255.txt> for more information  on
       the LDAP URL scheme.  The components of this URL are:

       hostport    the  LDAP server to query, written as a hostname optionally
                   followed by a colon and the port number.  The default  LDAP
                   port  is  TCP  port  389.   If empty, the client determines
                   which the LDAP server to use.

       dn          the LDAP Distinguished Name, which identifies the base  ob-
                   ject  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 sup-
                   ported to be  valid),  otherwise  it  is  noncritical  (op-

       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.:


       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  inter-
       nal  information,  and  the  GNOME help browser has the toc: scheme for
       various starting locations).  There are many schemes that have been de-
       fined  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 up-
       percase 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 re-
       served 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 ex-
       ample, "%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,           <http://www.fwi.uva.nl/~mes/jargon/h
       /HackerWritingStyle.html>, for more information).  Older documents sug-
       gested 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 as-
       sumption  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.

       (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 (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  au-
       thor of this man page prefers the KDE approach here, though a more reg-
       ular 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  pre-
       fers 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  lo-
       cate 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 en-
       tity associated with the resource, will in fact cause a possibly damag-
       ing 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  per-

       It is clearly unwise to use a URI that contains a password which is in-
       tended 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  ar-
       bitrary  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.  Al-
       ternatively, a future version of the filesystem specification may spec-
       ify  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 <http://www.ietf.org/rfc/rfc2255.txt>

       This  page  is  part of release 5.05 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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