src/library/utils/man/download.file.Rd

% File src/library/utils/man/download.file.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2022 R Core Team
% Distributed under GPL 2 or later

\name{download.file}
\alias{download.file}
\concept{proxy}
\concept{ftp}
\concept{http}
\title{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, \dots)
}
\arguments{
  \item{url}{a \code{\link{character}} string (or longer vector e.g.,
    for the \code{"libcurl"} method) naming the URL of a resource to be
    downloaded.}

  \item{destfile}{a character string (or vector, see the \code{url}
    argument) with the file path where the downloaded file is to be
    saved.  Tilde-expansion is performed.}

  \item{method}{Method to be used for downloading files.  Current
    download methods are \code{"internal"}, \code{"wininet"} (Windows
    only) \code{"libcurl"}, \code{"wget"} and \code{"curl"}, and there
    is a value \code{"auto"}: see \sQuote{Details} and \sQuote{Note}.

    The method can also be set through the option
    \code{"download.file.method"}: see \code{\link{options}()}.
  }

  \item{quiet}{If \code{TRUE}, suppress status messages (if any), and
    the progress bar.}

  \item{mode}{character.  The mode with which to write the file.  Useful
    values are \code{"w"}, \code{"wb"} (binary), \code{"a"} (append) and
    \code{"ab"}.  Not used for methods \code{"wget"} and \code{"curl"}.
    See also \sQuote{Details}, notably about using \code{"wb"} for Windows.
  }
  \item{cacheOK}{logical.  Is a server-side cached value acceptable?}

  \item{extra}{character vector of additional command-line arguments for
    the \code{"wget"} and \code{"curl"} methods.}

  \item{headers}{named character vector of HTTP headers to use in HTTP
    requests.  It is ignored for non-HTTP URLs.  The \code{User-Agent}
    header, coming from the \code{HTTPUserAgent} option (see
    \code{\link{options}}) is used as the first header, automatically.}

  \item{\dots}{allow additional arguments to be passed, unused.}
}
\details{
  The function \code{download.file} can be used to download a single
  file as described by \code{url} from the internet and store it in
  \code{destfile}.

  The \code{url} must start with a scheme such as \samp{http://},
  \samp{https://}, \samp{ftp://} or \samp{file://}.  Which methods
  support which schemes varies by \R version, but \code{method = "auto"}
  will try to find a method which supports the scheme.

  For \code{method = "auto"} (the default) the \code{"internal"} method
  is used for \samp{file://} URLs, and \code{"libcurl"} for all others.

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

  When method \code{"libcurl"} is used, it provides
  (non-blocking) access to \samp{https://} and (usually) \samp{ftps://}
  URLs.  There is support for simultaneous downloads, so \code{url} and
  \code{destfile} can be character vectors of the same length greater
  than one (but the method has to be specified explicitly and not
  \emph{via} \code{"auto"}).  For a single URL and \code{quiet = FALSE}
  a progress bar is shown in interactive use.

  For methods \code{"wget"} and \code{"curl"} a system call is made to
  the tool given by \code{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.

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

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

  Support for \samp{http://} URLs in the \code{"internal"} method and
  \samp{ftp://} URLs in the \code{"internal"} and \code{"wininet"}
  methods was deprecated in \R 4.1.1 and removed in \R 4.2.0.

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

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

  The remaining details apply to the \code{"wininet"} and
  \code{"libcurl"} methods only.

  The timeout for many parts of the transfer can be set by the option
  \code{timeout} which defaults to 60 seconds.  This is often
  insufficient for downloads of large files (50MB or more) and
  so should be increased when \code{download.file} is used in packages
  to do so.  Note that the user can set the default timeout by the
  environment variable \env{R_DEFAULT_INTERNET_TIMEOUT} in recent
  versions of \R, so to ensure that this is not decreased packages should
  use something like
  \preformatted{
    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
  \code{quiet} argument and the \code{internet.info} option: the details
  depend on the platform and scheme.  For the \code{"internal"} method
  setting option \code{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
  \code{"libcurl"} method values of the option less than 2 give verbose
  output.

  A progress bar tracks the transfer platform-specifically:
  \describe{
    \item{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.)}
    \item{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 (\code{mode = "wb"} or \code{"ab"}) is
  important on Windows, since unlike Unix-alikes it does distinguish
  between text and binary files and for text transfers changes \code{\\n}
  line endings to \code{\\r\\n} (aka \file{CRLF}).

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

  Code written to download binary files must use \code{mode = "wb"} (or
  \code{"ab"}), but the problems incurred by a text transfer will only
  be seen on Windows.
}
\note{
  Files of more than 2GB are supported on 64-bit builds of \R; they
  may be truncated on some 32-bit builds.

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

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

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

  \command{curl} (\url{https://curl.se/}) is installed on macOS and
  increasingly commonly on Unix-alikes.  Windows binaries are available
  at that URL.
}
\section{Setting Proxies}{
  For the Windows-only method \code{"wininet"}, the \sQuote{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 \code{"libcurl"} and \code{"curl"} methods, proxies can be set
  \emph{via} the environment variables \env{http_proxy} or
  \env{ftp_proxy}.  See
  \url{https://curl.se/libcurl/c/libcurl-tutorial.html} for further
  details.
}
\section{Secure URLs}{
  Methods which access \samp{https://} and \samp{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 \url{https://curl.se/docs/sslcerts.html}.

  On Windows, with \code{method = "libcurl"}, the CA root certificates are
  provided by the OS when \R was linked with \code{libcurl} with
  \code{Schannel} enabled, which is the default in Rtools.  This can be
  verified by that \code{libcurlVersion()} returns a version string
  containing "Schannel".  If it does not, for verification to be on, the
  environment variable \env{CURL_CA_BUNDLE} must be set to a path to a
  certificate bundle file, usually named \file{ca-bundle.crt} or
  \file{curl-ca-bundle.crt}.  (This is normally done automatically for a
  binary installation of \R, which installs
  \file{\var{R_HOME}/etc/curl-ca-bundle.crt} and sets \env{CURL_CA_BUNDLE}
  to point to it if that environment variable is not already set.) For an
  updated certificate bundle, see \url{https://curl.se/docs/sslcerts.html}. 
  Currently one can download a copy from
  \url{https://raw.githubusercontent.com/bagder/ca-bundle/master/ca-bundle.crt}
  and set \env{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).
}
\section{FTP sites}{
  \samp{ftp:} URLs are accessed using the FTP protocol which has a
  number of variants.  One distinction is between \sQuote{active} and
  \sQuote{(extended) passive} modes: which is used is chosen by the
  client.  The  \code{"libcurl"} methods uses passive
  mode, and that is almost universally used by browsers.
}
\section{Good practice}{
  Setting the \code{method} should be left to the end user.  Neither of
  the \command{wget} nor \command{curl} commands is widely available:
  you can check if one is available \emph{via} \code{\link{Sys.which}},
  and should do so in a package or script.

  If you use \code{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 \code{method}s do change: method \code{libcurl} was
  introduced in \R 3.2.0 and was optional on Windows until \R 4.2.0 --
  use \code{\link{capabilities}("libcurl")} in a program to see if it is
  available.
}
\value{
  An (invisible) integer code, \code{0} for success and non-zero for
  failure.  For the \code{"wget"} and \code{"curl"} methods this is the
  status code returned by the external program.  The \code{"internal"}
  method can return \code{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 \code{"internal"},
  \code{"wininet"} and \code{"libcurl"} methods will remove the file if
  there the URL is unavailable except when \code{mode} specifies
  appending when the file should be unchanged.
}
\seealso{
  \code{\link{options}} to set the \code{HTTPUserAgent}, \code{timeout}
  and \code{internet.info} options used by some of the methods.

  \code{\link{url}} for a finer-grained way to read data from URLs.

  \code{\link{url.show}}, \code{\link{available.packages}},
  \code{\link{download.packages}} for applications.

  Contributed packages \CRANpkg{RCurl} and \CRANpkg{curl} provide more
  comprehensive facilities to download from URLs.
}
\keyword{utilities}