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

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://, https://, ftp:// or file://.

If method = "auto" is chosen (the default), on a Unix-alike method "libcurl" is used except "internal" for file:// URLs.

Method "libcurl" uses the library of that name (http://curl.haxx.se/libcurl/). It provides (non-blocking) access to https:// and (usually) ftps:// URLs. There is support for simultaneous downloads, so url and destfile can be character vectors of the same length greater than one (but the method has to be specified explicitly and not via "auto"). For a single URL and quiet = FALSE a progress bar is shown in interactive use.

For methods "wget" and "curl" 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. They will block all other activity on the R process until they complete: this may make a GUI unresponsive.

cacheOK = FALSE is useful for http:// and https:// URLs: it will attempt to get a copy directly from the site rather than from an intermediate cache. It is used by available.packages.

The "libcurl" and "wget" methods follow http:// and https:// redirections to any scheme they support: the "internal" method follows http:// to http:// redirections only. (For method "curl" use argument extra = "-L". To disable redirection in wget, use extra = "--max-redirect=0".) The "wininet" method supports some redirections but not all. (For method "libcurl", messages will quote the endpoint of redirections.)

Note that https:// URLs are not supported by the "internal" method but are supported by the "libcurl" method and the "wininet" method on Windows.

See url for how file:// URLs are interpreted, especially on Windows. The "internal" and "wininet" methods do not percent-decode file:// URLs, but the "libcurl" and "curl" methods do: method "wget" does not support them.

Most methods do not percent-encode special characters such as spaces in URLs (see URLencode), but it seems the "wininet" method does.

The remaining details apply to the "internal", "wininet" and "libcurl" methods only.

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. For the "internal" method setting option 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. For the "libcurl" method values of the option less than 2 give verbose output.

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.

Value

An (invisible) integer code, 0 for success and non-zero for failure. For the "wget" and "curl" 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.

What happens to the destination file(s) in the case of error depends on the method and R version. Currently the "internal", "wininet" and "libcurl" methods will remove the file if there the URL is unavailable except when mode specifies appending when the file should be unchanged.

Setting Proxies

The next two paragraphs apply 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.

Much the same scheme is supported by method = "libcurl", including no_proxy, http_proxy and ftp_proxy, and for the last two a contents of [user:password@]machine[:port] where the parts in brackets are optional. See http://curl.haxx.se/libcurl/c/libcurl-tutorial.html for details.

Secure URLs

Methods which access https:// and ftps:// URLs should try to verify the site certificates. This is usually done using the CA root certificates installed by the OS (although we have seen instances in which these got removed rather than updated). For further information see http://curl.haxx.se/docs/sslcerts.html.

This is an issue for method = "libcurl" on Windows, where the OS does not provide a suitable CA certificate bundle, so by default on Windows certificates are not verified. To turn verification on, set environment variable CURL_CA_BUNDLE to the path to a certificate bundle file, usually named ‘ca-bundle.crt’ or ‘curl-ca-bundle.crt’. (This is normally done for a binary installation of R, which installs ‘R_HOME/etc/curl-ca-bundle.crt’ and sets CURL_CA_BUNDLE to point to it if that environment variable is not already set.) For an updated certificate bundle, see http://curl.haxx.se/docs/sslcerts.html. Currently one can download a copy from https://raw.githubusercontent.com/bagder/ca-bundle/master/ca-bundle.crt and set CURL_CA_BUNDLE to the full path to the downloaded file.

Note that the root certificates used by R may or may not be the same as used in a browser, and indeed different browsers may use different certificate bundles (there is typically a build option to choose either their own or the system ones).

FTP sites

ftp: URLs are accessed using the FTP protocol which has a number of variants. One distinction is between ‘active’ and ‘(extended) passive’ modes: which is used is chosen by the client. The "internal" and "libcurl" methods use passive mode, and that is almost universally used by browsers. Prior to R 3.2.3 the "wininet" method used active mode: nowadays it first tries passive and then active.

Good practice

Setting the method should be left to the end user. Neither of the wget nor curl commands is widely available: you can check if one is available via Sys.which, and should do so in a package or script.

If you use download.file in a package or script, you must check the return value, since it is possible that the download will fail with a non-zero status but not an R error. (This was more likely prior to R 3.4.0.)

The supported methods do change: method libcurl was introduced in R 3.2.0 and is still optional on Windows – use capabilities("libcurl") in a program to see if it is available.

Note

Files of more than 2GB are supported on 64-bit builds of R; they may be truncated on some 32-bit builds.

Methods "wget" and "curl" are mainly for historical compatibility but provide may provide capabilities not supported by the "libcurl" or "wininet" methods.

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

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

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

See Also

options to set the HTTPUserAgent, timeout and internet.info options used by some of the methods.

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.