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

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

       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 unavailable.

       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
       synonym for URI (though technically URLs are a subset of URIs).

       URIs can be absolute or relative.  An absolute identifier refers to a
       resource independent of context, while a relative identifier refers to a
       resource by describing the difference from the current context.  Within a
       relative path reference, the complete path segments "." and ".." have
       special meanings: "the current hierarchy level" and "the level above this
       hierarchy level", respectively, just like they do in UNIX-like systems.
       A path segment which contains a colon character can't be used as the
       first segment of a relative URI path (e.g., "this:that"), because it
       would be mistaken for a scheme name; precede such segments with ./ (e.g.,
       "./this:that").  Note that descendants of MS-DOS (e.g., Microsoft
       Windows) replace devicename colons with the vertical bar ("|") in URIs,
       so "C:" becomes "C|".

       A fragment identifier, if included, refers to a particular named portion
       (fragment) of a resource; text after a '#' identifies the fragment.  A
       URI beginning with '#' refers to that fragment in the current resource.

       There are many different URI schemes, each with specific additional rules
       and meanings, but they are intentionally made to be as similar as
       possible.  For example, many URL schemes permit the authority to be the
       following format, called here an ip_server (square brackets show what's

       ip_server = [user [ : password ] @ ] host [ : port]

       This format allows you to optionally insert a username, a user plus
       password, and/or a port number.  The host is the name of the host
       computer, either its name as determined by DNS or an IP address (numbers
       separated by periods).  Thus the URI
       <http://fred:fredpassword@example.com:8080/> logs into a web server on
       host example.com as fred (using fredpassword) using port 8080.  Avoid
       including a password in a URI if possible because of the many security
       risks of having a password written down.  If the URL supplies a username
       but no password, and the remote server requests a password, the program
       interpreting the URL should request one from the user.

       Here are some of the most common schemes in use on UNIX-like systems that
       are understood by many tools.  Note that many tools using URIs also have
       internal schemes or specialized schemes; see those tools' documentation
       for information on those schemes.

       http - Web (HTTP) server


       This is a URL accessing a web (HTTP) server.  The default port is 80.  If
       the path refers to a directory, the web server will choose what to
       return; usually if there is a file named "index.html" or "index.htm" its
       content is returned, otherwise, a list of the files in the current
       directory (with appropriate links) is generated and returned.  An example
       is <http://lwn.net>.

       A query can be given in the archaic "isindex" format, consisting of a
       word or phrase and not including an equal sign (=).  A query can also be
       in the longer "GET" format, which has one or more query entries of the
       form key=value separated by the ampersand character (&).  Note that key
       can be repeated more than once, though it's up to the web server and its
       application programs to determine if there's any meaning to that.  There
       is an unfortunate interaction with HTML/XML/SGML and the GET query
       format; when such URIs with more than one key are embedded in SGML/XML
       documents (including HTML), the ampersand (&) has to be rewritten as
       &amp;.  Note that not all queries use this format; larger forms may be
       too long to store as a URI, so they use a different interaction mechanism
       (called POST) which does not include the data in the URI.  See the Common
       Gateway Interface specification at ⟨http://www.w3.org/CGI⟩ for more

       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 <mailto:dwheeler@dwheeler.com>.

       news - Newsgroup or News message


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

       A message-id corresponds to the Message-ID of IETF RFC 1036,
       ⟨http://www.ietf.org/rfc/rfc1036.txt⟩ without the enclosing "<" and ">";
       it takes the form unique@full_domain_name.  A message identifier may be
       distinguished from a news group name by the presence of the "@"

       telnet - Telnet login


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

       file - Normal file


       This represents a file or directory accessible locally.  As a special
       case, ip_server can be the string "localhost" or the empty string; this
       is interpreted as "the machine from which the URL is being interpreted".
       If the path is to a directory, the viewer should display the directory's
       contents with links to each containee; not all viewers currently do this.
       KDE supports generated files through the URL <file:/cgi-bin>.  If the
       given file isn't found, browser writers may want to try to expand the
       filename via filename globbing (see glob(7) and glob(3)).

       The second format (e.g., <file:/etc/passwd>) is a correct format for
       referring to a local file.  However, older standards did not permit this
       format, and some programs don't recognize this as a URI.  A more portable
       syntax is to use an empty string as the server name, for example,
       <file:///etc/passwd>; this form does the same thing and is easily
       recognized by pattern matchers and older programs as a URI.  Note that if
       you really mean to say "start from the current location", don't specify
       the scheme at all; use a relative address like <../test.txt>, which has
       the side-effect of being scheme-independent.  An example of this scheme
       is <file:///etc/passwd>.

       man - Man page documentation


       This refers to local online manual (man) reference pages.  The command
       name can optionally be followed by a parenthesis and section number; see
       man(7) for more information on the meaning of the section numbers.  This
       URI scheme is unique to UNIX-like systems (such as Linux) and is not
       currently registered by the IETF.  An example is <man:ls(1)>.

       info - Info page documentation


       This scheme refers to online info reference pages (generated from texinfo
       files), a documentation format used by programs such as the GNU tools.
       This URI scheme is unique to UNIX-like systems (such as Linux) and is not
       currently registered by the IETF.  As of this writing, GNOME and KDE
       differ in their URI syntax and do not accept the other's syntax.  The
       first two formats are the GNOME format; in nodenames all spaces are
       written as underscores.  The second two formats are the KDE format;
       spaces in nodenames must be written as spaces, even though this is
       forbidden by the URI standards.  It's hoped that in the future most tools
       will understand all of these formats and will always accept underscores
       for spaces in nodenames.  In both GNOME and KDE, if the form without the
       nodename is used the nodename is assumed to be "Top".  Examples of the
       GNOME format are <info:gcc> and <info:gcc#G++_and_GCC>.  Examples of the
       KDE format are <info:(gcc)> and <info:(gcc)G++ and GCC>.

       whatis - Documentation search


       This scheme searches the database of short (one-line) descriptions of
       commands and returns a list of descriptions containing that string.  Only
       complete word matches are returned.  See whatis(1).  This URI scheme is
       unique to UNIX-like systems (such as Linux) and is not currently
       registered by the IETF.

       ghelp - GNOME help documentation


       This loads GNOME help for the given application.  Note that not much
       documentation currently exists in this format.

       ldap - Lightweight Directory Access Protocol


       This scheme supports queries to the Lightweight Directory Access Protocol
       (LDAP), a protocol for querying a set of servers for hierarchically
       organized information (such as people and computing resources).  See
       RFC 2255 ⟨http://www.ietf.org/rfc/rfc2255.txt⟩ for more information on
       the LDAP URL scheme.  The components of this URL 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 ⟨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

       scope       specifies the scope of the search, which can be one of "base"
                   (for a base object search), "one" (for a one-level search),
                   or "sub" (for a subtree search).  If scope is omitted, "base"
                   is assumed.

       filter      specifies the search filter (subset of entries to return).
                   If omitted, all entries should be returned.  See RFC 2254
                   ⟨http://www.ietf.org/rfc/rfc2254.txt⟩ section 4.

       extensions  a comma-separated list of type=value pairs, where the =value
                   portion may be omitted for options not requiring it.  An
                   extension prefixed with a '!' is critical (must be supported
                   to be valid), otherwise it is noncritical (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 Michigan in
       the U.S.:


       To just get its postal address attribute, request:


       To ask a host.com at port 6666 for information about the person with
       common name (cn) "Babs Jensen" at University of Michigan, request:


       wais - Wide Area Information Servers


       This scheme designates a WAIS database, search, or document (see IETF
       RFC 1625 ⟨http://www.ietf.org/rfc/rfc1625.txt⟩ for more information on
       WAIS).  Hostport is the hostname, optionally followed by a colon and port
       number (the default port number is 210).

       The first form designates a WAIS database for searching.  The second form
       designates a particular search of the WAIS database database.  The third
       form designates a particular document within a WAIS database to be
       retrieved.  wtype is the WAIS designation of the type of the object and
       wpath is the WAIS document-id.

       other schemes

       There are many other URI schemes.  Most tools that accept URIs support a
       set of internal URIs (e.g., Mozilla has the about: scheme for internal
       information, and the GNOME help browser has the toc: scheme for various
       starting locations).  There are many schemes that have been defined but
       are not as widely used at the current time (e.g., prospero).  The nntp:
       scheme is deprecated in favor of the news: scheme.  URNs are to be
       supported by the urn: scheme, with a hierarchical name space (e.g.,
       urn:ietf:... would identify IETF documents); at this time URNs are not
       widely implemented.  Not all tools support all schemes.

   Character encoding
       URIs use a limited number of characters so that they can be typed in and
       used in a variety of situations.

       The following characters are reserved, that is, they may appear in a URI
       but their use is limited to their reserved purpose (conflicting data must
       be escaped before forming the URI):

                 ; / ? : @ & = + $ ,

       Unreserved characters may be included in a URI.  Unreserved characters
       include uppercase and lowercase English letters, decimal digits, and the
       following limited set of punctuation marks and symbols:

               - _ . ! ~ * ' ( )

       All other characters must be escaped.  An escaped octet is encoded as a
       character triplet, consisting of the percent character "%" followed by
       the two hexadecimal digits representing the octet code (you can use
       uppercase or lowercase letters for the hexadecimal digits).  For example,
       a blank space must be escaped as "%20", a tab character as "%09", and the
       "&" as "%26".  Because the percent "%" character always has the reserved
       purpose of being the escape indicator, it must be escaped as "%25".  It
       is common practice to escape space characters as the plus symbol (+) in
       query text; this practice isn't uniformly defined in the relevant RFCs
       (which recommend %20 instead) but any tool accepting URIs with query text
       should be prepared for them.  A URI is always shown in its "escaped"

       Unreserved characters can be escaped without changing the semantics of
       the URI, but this should not be done unless the URI is being used in a
       context that does not allow the unescaped character to appear.  For
       example, "%7e" is sometimes used instead of "~" in an HTTP URL path, but
       the two are equivalent for an HTTP URL.

       For URIs which must handle characters outside the US ASCII character set,
       the HTML 4.01 specification (section B.2) and IETF RFC 2718 (section
       2.2.5) recommend the following approach:

       1.  translate the character sequences into UTF-8 (IETF RFC 2279)—see
           utf-8(7)—and then

       2.  use the URI escaping mechanism, that is, use the %HH encoding for
           unsafe octets.

   Writing a URI
       When written, URIs should be placed inside double quotes (e.g.,
       "http://www.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 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 in
       various European languages.  Older documents suggested inserting the
       prefix "URL:" just before the URI, but this form has never caught on.

       The URI syntax was designed to be unambiguous.  However, as URIs have
       become commonplace, traditional media (television, radio, newspapers,
       billboards, etc.) have increasingly used abbreviated URI references
       consisting of only the authority and path portions of the identified
       resource (e.g., <www.w3.org/Addressing>).  Such references are primarily
       intended for human interpretation rather than machine, with the
       assumption that context-based heuristics are sufficient to complete the
       URI (e.g., hostnames beginning with "www" are likely to have a URI prefix
       of "http://" and hostnames beginning with "ftp" likely to have a prefix
       of "ftp://").  Many client implementations heuristically resolve these
       references.  Such heuristics may change over time, particularly when new
       schemes are introduced.  Since an abbreviated URI has the same syntax as
       a relative URL path, abbreviated URI references cannot be used where
       relative URIs are permitted, and can be used only when there is no
       defined base (such as in dialog boxes).  Don't use abbreviated URIs as
       hypertext links inside a document; use the standard format as described

       (IETF RFC 2396) ⟨http://www.ietf.org/rfc/rfc2396.txt⟩, (HTML 4.0)

       Any tool accepting URIs (e.g., a web browser) on a Linux system should be
       able to handle (directly or indirectly) all of the schemes described
       here, including the man: and info: schemes.  Handling them by invoking
       some other program is fine and in fact encouraged.

       Technically the fragment isn't part of the URI.

       For information on how to embed URIs (including URLs) in a data format,
       see documentation on that format.  HTML uses the format <A HREF="uri">
       text </A>.  Texinfo files use the format @uref{uri}.  Man and mdoc have
       the recently added UR macro, or just include the URI in the text (viewers
       should be able to detect :// as part of a URI).

       The GNOME and KDE desktop environments currently vary in the URIs they
       accept, in particular in their respective help browsers.  To list man
       pages, GNOME uses <toc:man> while KDE uses <man:(index)>, and to list
       info pages, GNOME uses <toc:info> while KDE uses <info:(dir)> (the author
       of this man page prefers the KDE approach here, though a more regular
       format would be even better).  In general, KDE uses <file:/cgi-bin/> as a
       prefix to a set of generated files.  KDE prefers documentation in HTML,
       accessed via the <file:/cgi-bin/helpindex>.  GNOME prefers the ghelp
       scheme to store and find documentation.  Neither browser handles file:
       references to directories at the time of this writing, making it
       difficult to refer to an entire directory with a browsable URI.  As noted
       above, these environments differ in how they handle the info: scheme,
       probably the most important variation.  It is expected that GNOME and KDE
       will converge to common URI formats, and a future version of this man
       page will describe the converged result.  Efforts to aid this convergence
       are encouraged.

       A URI does not in itself pose a security threat.  There is no general
       guarantee that a URL, which at one time located a given resource, will
       continue to do so.  Nor is there any guarantee that a URL will not locate
       a different resource at some later point in time; such a guarantee can be
       obtained only from the person(s) controlling that namespace and the
       resource in question.

       It is sometimes possible to construct a URL such that an attempt to
       perform a seemingly harmless operation, such as the retrieval of an
       entity associated with the resource, will in fact cause a possibly
       damaging remote operation to occur.  The unsafe URL is typically
       constructed by specifying a port number other than that reserved for the
       network protocol in question.  The client unwittingly contacts a site
       that is in fact running a different protocol.  The content of the URL
       contains instructions that, when interpreted according to this other
       protocol, cause an unexpected operation.  An example has been the use of
       a gopher URL to cause an unintended or impersonating message to be sent
       via a SMTP server.

       Caution should be used when using any URL that specifies a port number
       other than the default for the protocol, especially when it is a number
       within the reserved space.

       Care should be taken when a URI contains escaped delimiters for a given
       protocol (for example, CR and LF characters for telnet protocols) that
       these are not unescaped before transmission.  This might violate the
       protocol, but avoids the potential for such characters to be used to
       simulate an extra operation or parameter in that protocol, which might
       lead to an unexpected and possibly harmful remote operation to be

       It is clearly unwise to use a URI that contains a password which is
       intended to be secret.  In particular, the use of a password within the
       "userinfo" component of a URI is strongly recommended against except in
       those rare cases where the "password" parameter is intended to be public.

       Documentation may be placed in a variety of locations, so there currently
       isn't a good URI scheme for general online documentation in arbitrary
       formats.  References of the form <file:///usr/doc/ZZZ> don't work because
       different distributions and local installation requirements may place the
       files in different directories (it may be in /usr/doc, or /usr/local/doc,
       or /usr/share, or somewhere else).  Also, the directory ZZZ usually
       changes when a version changes (though filename globbing could partially
       overcome this).  Finally, using the file: scheme doesn't easily support
       people who dynamically load documentation from the Internet (instead of
       loading the files onto a local filesystem).  A future URI scheme may be
       added (e.g., "userdoc:") to permit programs to include cross-references
       to more detailed documentation without having to know the exact location
       of that documentation.  Alternatively, a future version of the filesystem
       specification may specify file locations sufficiently so that the file:
       scheme will be able to locate documentation.

       Many programs and file formats don't include a way to incorporate or
       implement links using URIs.

       Many programs can't handle all of these different URI formats; there
       should be a standard mechanism to load an arbitrary URI that
       automatically detects the users' environment (e.g., text or graphics,
       desktop environment, local user preferences, and currently executing
       tools) and invokes the right tool for any URI.

       lynx(1), man2html(1), mailaddr(7), utf-8(7)

       IETF RFC 2255 ⟨http://www.ietf.org/rfc/rfc2255.txtCOLOPHON
       This page is part of release 5.11 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

Linux                              2021-03-22                             URI(7)