urn(7) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       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         top

       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 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 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
       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 informa‐

       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 ad‐
       dress.  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 delim‐
       iting "/" is also optional and the gophertype defaults to "1".

       selector is the Gopher selector string.  In the Gopher protocol, Go‐
       pher 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 ex‐
       ample 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
       "@" 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 <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 in‐
       terpreted".  If the path is to a directory, the viewer should display
       the directory's contents with links to each containee; not all view‐
       ers 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 ex‐
       ample, <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 loca‐
       tion," 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 com‐
       mand 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

       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 writ‐
       ing, 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 node‐
       names 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 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 Pro‐
       tocol (LDAP), a protocol for querying a set of servers for hierarchi‐
       cally organized information (such as people and computing resources).
       See RFC 2255 ⟨http://www.ietf.org/rfc/rfc2255.txt⟩ for more informa‐
       tion on the LDAP URL scheme.  The components of this URL are:

       hostport    the LDAP server to query, written as a hostname option‐
                   ally followed by a colon and the port number.  The de‐
                   fault 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 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 re‐
                   turn).  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

       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 data‐
       base 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 sup‐
       port 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 hierar‐
       chical 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 charac‐
       ters 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 al‐
       ways has the reserved purpose of being the escape indicator, it must
       be escaped as "%25".  It is common practice to escape space charac‐
       ters as the plus symbol (+) in query text; this practice isn't uni‐
       formly 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.,
       "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 prac‐
       tice 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 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 pri‐
       marily intended for human interpretation rather than machine, with
       the assumption that context-based heuristics are sufficient to com‐
       plete 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, abbrevi‐
       ated 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 docu‐
       ment; use the standard format as described here.

CONFORMING TO         top

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

NOTES         top

       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 performed.

       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.

BUGS         top

       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.

SEE ALSO         top

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

       IETF RFC 2255 ⟨http://www.ietf.org/rfc/rfc2255.txt

COLOPHON         top

       This page is part of release 5.09 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                            2020-08-13                           URI(7)