R: Download File from the Internet

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"),
              headers = NULL, ...)

Arguments

`url`	a `character` string (or longer vector for the `"libcurl"` method) naming the URL of a resource to be downloaded.
`destfile`	a character string (or vector, see the `url` argument) with the file path where the downloaded file is to be saved. Tilde-expansion is performed.
`method`	Method to be used for downloading files. Current download methods are `"internal"`, `"libcurl"`, `"wget"`, `"curl"` and `"wininet"` (Windows only), 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"`. Not used for methods `"wget"` and `"curl"`. See also ‘Details’, notably about using `"wb"` for Windows.
`cacheOK`	logical. Is a server-side cached value acceptable?
`extra`	character vector of additional command-line arguments for the `"wget"` and `"curl"` methods.
`headers`	named character vector of additional HTTP headers to use in HTTP[S] requests. It is ignored for non-HTTP[S] URLs. The `User-Agent` header taken from the `HTTPUserAgent` option (see `options`) is automatically used as the first header.
`...`	allow additional arguments to be passed, unused.

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://⁠’ or ‘⁠file://⁠’. Which methods support which schemes varies by R version, but method = "auto" will try to find a method which supports the scheme.

For method = "auto" (the default) currently the "internal" method is used for ‘⁠file://⁠’ URLs and "libcurl" for all others.

Support for method "libcurl" was optional on Windows prior to R 4.2.0: use capabilities("libcurl") to see if it is supported on an earlier version. It uses an external library of that name (https://curl.se/libcurl/) against which R can be compiled.

When method "libcurl" is used, 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.

Nowadays the "internal" method only supports the ‘⁠file://⁠’ scheme (for which it is the default). On Windows the "wininet" method currently supports ‘⁠file://⁠’ and (but deprecated with a warning) ‘⁠http://⁠’ and ‘⁠https://⁠’ schemes.

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. (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.)

See url for how ‘⁠file://⁠’ URLs are interpreted, especially on Windows. The "internal" and "wininet" methods do not percent-decode, 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 "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. This is often insufficient for downloads of large files (50MB or more) and so should be increased when download.file is used in packages to do so. Note that the user can set the default timeout by the environment variable R_DEFAULT_INTERNET_TIMEOUT in recent versions of R, so to ensure that this is not decreased packages should use something like

    options(timeout = max(300, getOption("timeout")))

(It is unrealistic to require download times of less than 1s/MB.)

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

A progress bar tracks the transfer platform-specifically:

On Windows: If the file length is known, the full width of the bar is the known length. Otherwise the initial width represents 100 Kbytes and is doubled whenever the current width is exceeded. (In non-interactive use this uses a text version. If the file length is known, an equals sign represents 2% of the transfer completed: otherwise a dot represents 10Kb.)
On a Unix-alike: If the file length is known, an equals sign represents 2% of the transfer completed: otherwise a dot represents 10Kb.

The choice of binary transfer (mode = "wb" or "ab") is important on Windows, since unlike Unix-alikes it does distinguish between text and binary files and for text transfers changes ‘⁠\n⁠’ line endings to ‘⁠\r\n⁠’ (aka ‘CRLF’).

On Windows, if mode is not supplied (missing()) and url ends in one of ‘⁠.gz⁠’, ‘⁠.bz2⁠’, ‘⁠.xz⁠’, ‘⁠.tgz⁠’, ‘⁠.zip⁠’, ‘⁠.jar⁠’, ‘⁠.rda⁠’, ‘⁠.rds⁠’, ‘⁠.RData⁠’ or ‘⁠.pdf⁠’, mode = "wb" is set so that a binary transfer is done to help unwary users.

Code written to download binary files must use mode = "wb" (or "ab"), 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. When simultaneously downloading two or more files (see the url argument) and download of at least one file succeeds, 0 is returned with attribute retvals, which provides an integer vector of the same length as url with a result code for each file (0 for success and non-zero for failure).

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 the URL is unavailable except when mode specifies appending when the file should be unchanged.

Setting Proxies

For the Windows-only method "wininet", the ‘Internet Options’ of the system are used to choose proxies and so on; these are set in the Control Panel and are those used for system browsers.

For the "libcurl" and "curl" methods, proxies can be set via the environment variables http_proxy or ftp_proxy. See https://curl.se/libcurl/c/libcurl-tutorial.html for further details.

Secure URLs

Methods which access ‘⁠https://⁠’ and (where supported) ‘⁠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 https://curl.se/docs/sslcerts.html.

On Windows with method = "libcurl", the CA root certificates are provided by the OS when R was linked with libcurl with Schannel enabled, which is the current default in Rtools. This can be verified by checking that libcurlVersion() returns a version string containing ‘⁠"Schannel"⁠’. If it does not, for verification to be on the environment variable CURL_CA_BUNDLE must be set to a path to a certificate bundle file, usually named ‘ca-bundle.crt’ or ‘curl-ca-bundle.crt’. (This is normally done automatically 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 https://curl.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.

On Windows with method = "libcurl", when R was linked with libcurl with Schannel enabled, the connection fails if it cannot be established that the certificate has not been revoked. Some MITM proxies present particularly in corporate environments do not work with this behavior. It can be changed by setting environment variable R_LIBCURL_SSL_REVOKE_BEST_EFFORT to TRUE, with the consequence of reducing security.

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).

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.

The supported methods do change: method libcurl was introduced in R 3.2.0 and was optional on Windows until R 4.2.0 – use capabilities("libcurl") in a program to see if it is available.

‘⁠ftp://⁠’ URLs

Most modern browsers do not support such URLs, and ‘⁠https://⁠’ ones are much preferred for use in R. ‘⁠ftps://⁠’ URLs have always been rare, and are nowadays even less supported.

It is intended that R will continue to allow such URLs for as long as libcurl does, but as they become rarer this is increasingly untested. What ‘protocols’ the version of libcurl being used supports can be seen by calling libcurlVersion().

These 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 "libcurl" method uses passive mode which was almost universally used by browsers before they dropped support altogether.

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 (https://www.gnu.org/software/wget/) is commonly installed on Unix-alikes (but not macOS). Windows binaries are available from MSYS2 and elsewhere.

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