Man Linux: Main Page and Category List


       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 "" 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
       <> 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
       example 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
       interaction 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
       example is <>.

       news - Newsgroup or News message


       A  newsgroup-name  is  a  period-delimited  hierarchical  name, such as
       "comp.infosystems.www.misc".   If  <newsgroup-name>  is  "*"   (as   in
       <news:*>),  it  is  used  to  refer to "all available news groups".  An
       example is <news:comp.lang.ada>.

       A message-id corresponds to the Message-ID of  IETF  RFC 1036,  without
       the  enclosing  "<" and ">"; it takes the form unique@full_domain_name.
       A message identifier may be distinguished from a news group name by the
       presence of the "@" character.

       telnet - Telnet login


       The  Telnet  URL  scheme is used to designate interactive text services
       that may be accessed by the Telnet protocol.  The final  "/"  character
       may   be   omitted.    The   default   port   is  23.   An  example  is

       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   noncritical

       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 3.24 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at