download.file {utils}R Documentation

Download File from the Internet

Description

This function can be used to download a file from the Internet.

Usage

download.file(url, destfile, method, quiet = FALSE, mode = "w",
              cacheOK = TRUE,
              extra = getOption("download.file.extra"))

Arguments

url

A character string naming the URL of a resource to be downloaded.

destfile

A character string with the name where the downloaded file is saved. Tilde-expansion is performed.

method

Method to be used for downloading files. Currently download methods "internal", "wget", "curl" and "lynx" are available, and there is a value "auto": see ‘Details’ and ‘Note’.

The method can also be set through the option "download.file.method": see options().

quiet

If TRUE, suppress status messages (if any), and the progress bar.

mode

character. The mode with which to write the file. Useful values are "w", "wb" (binary), "a" (append) and "ab". Only used for the "internal" method.

cacheOK

logical. Is a server-side cached value acceptable? Implemented for the "internal", "wget" and "curl" methods.

extra

character vector of additional command-line arguments for the "wget", "curl" and "lynx" methods.

Details

The function download.file can be used to download a single file as described by url from the internet and store it in destfile. The url must start with a scheme such as http://, ftp:// or file://.

If method = "auto" is chosen (the default), the internal method is chosen for file:// URLs, and for the others provided capabilities("http/ftp") is true (which it almost always is). Otherwise methods "wget", "curl" and "lynx" are tried in turn.

cacheOK = FALSE is useful for http:// URLs, and will attempt to get a copy directly from the site rather than from an intermediate cache. (Not all platforms support it.) It is used by available.packages.

The remaining details apply to method "internal" only.

Note that https:// URLs are not supported by the internal method.

See url for how file:// URLs are interpreted, especially on Windows. This function does decode encoded URLs.

The timeout for many parts of the transfer can be set by the option timeout which defaults to 60 seconds.

The level of detail provided during transfer can be set by the quiet argument and the internet.info option. The details depend on the platform and scheme, but setting internet.info to 0 gives all available details, including all server responses. Using 2 (the default) gives only serious messages, and 3 or more suppresses all messages.

A progress bar tracks the transfer. If the file length is known, an equals sign represents 2% of the transfer completed: otherwise a dot represents 10Kb.

Code written to download binary files must use mode = "wb", but the problems incurred by a text transfer will only be seen on Windows.

Method "wget" can be used with proxy firewalls which require user/password authentication if proper values are stored in the configuration file for wget.

Value

An (invisible) integer code, 0 for success and non-zero for failure. For the "wget" and "lynx" methods this is the status code returned by the external program. The "internal" method can return 1, but will in most cases throw an error.

Setting Proxies

This applies to the internal code only.

Proxies can be specified via environment variables. Setting "no_proxy" to "*" stops any proxy being tried. Otherwise the setting of "http_proxy" or "ftp_proxy" (or failing that, the all upper-case version) is consulted and if non-empty used as a proxy site. For FTP transfers, the username and password on the proxy can be specified by "ftp_proxy_user" and "ftp_proxy_password". The form of "http_proxy" should be "http://proxy.dom.com/" or "http://proxy.dom.com:8080/" where the port defaults to 80 and the trailing slash may be omitted. For "ftp_proxy" use the form "ftp://proxy.dom.com:3128/" where the default port is 21. These environment variables must be set before the download code is first used: they cannot be altered later by calling Sys.setenv.

Usernames and passwords can be set for HTTP proxy transfers via environment variable http_proxy_user in the form user:passwd. Alternatively, http_proxy can be of the form "http://user:pass@proxy.dom.com:8080/" for compatibility with wget. Only the HTTP/1.0 basic authentication scheme is supported.

Note

Methods "wget" and "lynx" are mainly for historical compatibility, but they and "curl" can be used for URLs (e.g., https:// URLs or those that use cookies) which the internal method does not support. They will block all other activity on the R process.

For methods "wget", "curl" and "lynx" a system call is made to the tool given by method, and the respective program must be installed on your system and be in the search path for executables.

wget (http://www.gnu.org/software/wget/) is commonly installed on Unix-alikes (but not OS X). Windows binaries are available from Cygwin, gnuwin32 and elsewhere.

curl (http://curl.haxx.se/) is installed on OS X and commonly on Unix-alikes. Windows binaries are available at that URL.

Use of lynx (http://lynx.browser.org/ is of historical interest and now deprecated.

See Also

options to set the HTTPUserAgent, timeout and internet.info options.

url for a finer-grained way to read data from URLs.

url.show, available.packages, download.packages for applications.

Contributed package RCurl provides more comprehensive facilities to download from URLs.


[Package utils version 3.2.0 Index]