A Uniform Resource Locator (URL) is a Uniform Resource Identifier (URI) that specifies where a particular resource is available – and the mechanism for retrieving it. They are often informally referred to as web addresses.

A Uniform Resource Locator (URL) is a specific type of Uniform Resource Identifier (URI) that specifies where an identified resource is available – and the mechanism for retrieving it. In popular usage, URLs are often incorrectly regarded as being the same as URIs; the other type of URI is the less common Uniform Resource Name (URN).

The best-known example of a URL is for the addresses of web pages on the World Wide Web, such as http://www.example.com/ and URLs are commonly informally referred to as web addresses.

The structure of all URIs (both locators and names) is:

scheme:[//[user:password@]host[:port]][/]path[?query][#fragment]
  1. The scheme specifies the mechanism (or protocol) for retrieving the resource, e.g., ftp or https
  2. The authority part consists of an optional authentication section (username and password), followed by a host (which may be expressed as a hostname or IP address) with an optional port number if non-standard ports are used.
  3. The path is similar to a filesystem path (and may map to one).
  4. There may be an optional query string of data (usually specified as name-value pairs separated by an ampersand, &).
  5. The final part (after #) is called the fragment; this can be used to specify a secondary resource within a document, e.g., a section heading.

On Unix systems, the command is commonly used to work with URLs.

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