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NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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.
Usage
There are many different URI schemes, each with specific additional
rules and meanings, but they are intentionally made to be as similar
as possible. For example, many URL schemes permit the authority to
be the following format, called here an ip_server (square brackets
show what's optional):
ip_server = [user [ : password ] @ ] host [ : port]
This format allows you to optionally insert a username, a user plus
password, and/or a port number. The host is the name of the host
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
http://ip_server/path
http://ip_server/path?query
This is a URL accessing a web (HTTP) server. The default port is 80.
If the path refers to a directory, the web server will choose what to
return; usually if there is a file named "index.html" or "index.htm"
its content is returned, otherwise, a list of the files in the
current directory (with appropriate links) is generated and returned.
An 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 &. 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‐
tion.
ftp - File Transfer Protocol (FTP)
ftp://ip_server/path
This is a URL accessing a file through the file transfer protocol
(FTP). The default port (for control) is 21. If no username is
included, the username "anonymous" is supplied, and in that case many
clients provide as the password the requestor's Internet email
address. An example is <ftp://ftp.is.co.za/rfc/rfc1808.txt>.
gopher - Gopher server
gopher://ip_server/gophertype selector
gopher://ip_server/gophertype selector%09search
gopher://ip_server/gophertype selector%09search%09gopher+_string
The default gopher port is 70. gophertype is a single-character
field to denote the Gopher type of the resource to which the URL
refers. The entire path may also be empty, in which case the delim‐
iting "/" is also optional and the gophertype defaults to "1".
selector is the Gopher selector string. In the Gopher protocol,
Gopher selector strings are a sequence of octets which may contain
any octets except 09 hexadecimal (US-ASCII HT or tab), 0A hexadecimal
(US-ASCII character LF), and 0D (US-ASCII character CR).
mailto - Email address
mailto:email-address
This is an email address, usually of the form name@hostname. See
mailaddr(7) for more information on the correct format of an email
address. Note that any % character must be rewritten as %25. An
example is <mailto:dwheeler@dwheeler.com>.
news - Newsgroup or News message
news:newsgroup-name
news:message-id
A newsgroup-name is a period-delimited hierarchical name, such as
"comp.infosystems.www.misc". If <newsgroup-name> is "*" (as in
<news:*>), it is used to refer to "all available news groups". An
example is <news:comp.lang.ada>.
A message-id corresponds to the Message-ID of IETF RFC 1036,
⟨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
telnet://ip_server/
The Telnet URL scheme is used to designate interactive text services
that may be accessed by the Telnet protocol. The final "/" character
may be omitted. The default port is 23. An example is <tel‐
net://melvyl.ucop.edu/>.
file - Normal file
file://ip_server/path_segments
file:path_segments
This represents a file or directory accessible locally. As a special
case, ip_server can be the string "localhost" or the empty string;
this is interpreted as "the machine from which the URL is being
interpreted". If the path is to a directory, the viewer should dis‐
play 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 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
man:command-name
man:command-name(section)
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
<man:ls(1)>.
info - Info page documentation
info:virtual-filename
info:virtual-filename#nodename
info:(virtual-filename)
info:(virtual-filename)nodename
This scheme refers to online info reference pages (generated from
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
whatis:string
This scheme searches the database of short (one-line) descriptions of
commands and returns a list of descriptions containing that string.
Only complete word matches are returned. See whatis(1). This URI
scheme is unique to UNIX-like systems (such as Linux) and is not cur‐
rently registered by the IETF.
ghelp - GNOME help documentation
ghelp:name-of-application
This loads GNOME help for the given application. Note that not much
documentation currently exists in this format.
ldap - Lightweight Directory Access Protocol
ldap://hostport
ldap://hostport/
ldap://hostport/dn
ldap://hostport/dn?attributes
ldap://hostport/dn?attributes?scope
ldap://hostport/dn?attributes?scope?filter
ldap://hostport/dn?attributes?scope?filter?extensions
This scheme supports queries to the Lightweight Directory Access 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
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 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⟩ sec‐
tion 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.:
ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US
To just get its postal address attribute, request:
ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US?postalAddress
To ask a host.com at port 6666 for information about the person with
common name (cn) "Babs Jensen" at University of Michigan, request:
ldap://host.com:6666/o=University%20of%20Michigan,c=US??sub?(cn=Babs%20Jensen)
wais - Wide Area Information Servers
wais://hostport/database
wais://hostport/database?search
wais://hostport/database/wtype/wpath
This scheme designates a WAIS database, search, or document (see IETF
RFC 1625 ⟨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
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.,
"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.
(IETF RFC 2396) ⟨http://www.ietf.org/rfc/rfc2396.txt⟩, (HTML 4.0)
⟨http://www.w3.org/TR/REC-html40⟩.
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.
Security
A URI does not in itself pose a security threat. There is no general
guarantee that a URL, which at one time located a given resource,
will continue to do so. Nor is there any guarantee that a URL will
not locate a different resource at some later point in time; such a
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.
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.txt⟩
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-12-12 URI(7)
Pages that refer to this page: systemd.unit(5)