% File src/library/utils/man/download.file.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2015 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"))
}
\arguments{
  \item{url}{A character string naming the URL of a resource to be
    downloaded.}

  \item{destfile}{A character string with the name where the downloaded
    file is 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"}.  Only used for the \code{"internal"} method.
#ifdef windows
    (See also \sQuote{Details}.)
#endif
  }
  \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.}
}
\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{ftp://} or \samp{file://}.

#ifdef unix
  If \code{method = "auto"} is chosen (the default), the internal method
  is chosen for \samp{file://} URLs, and for the others provided
  \code{\link{capabilities}("http/ftp")} is true (which it almost always
  is).  Otherwise methods \code{"wget"} and \code{"curl"}.
#endif
#ifdef windows
  If \code{method = "auto"} is chosen (the default), the internal method
  is used on Windows.
#endif
  
  Support for method \code{"libcurl"} is optional: use
  \code{\link{capabilities}("libcurl")} to see if it is supported on
  your build.  It provides (non-blocking) access to \samp{https://} and
  \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.  For a single URL and \code{quiet = FALSE}
  there 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://} URLs, and 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 remaining details apply to method \code{"internal"} only.

  Note that \samp{https://} URLs are
#ifdef unix
  not supported by the internal method.
#endif
#ifdef windows
  only supported if \option{--internet2} or environment variable
  \env{R_WIN_INTERNET2} was set or \code{\link{setInternet2}(TRUE)} was
  used (to make use of Windows internal functions), and then only if the
  certificate is considered to be valid.
#endif

  See \code{\link{url}} for how \samp{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
  \code{timeout} which defaults to 60 seconds.

  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, but setting
  \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.

#ifdef windows
  A progress bar tracks the transfer.  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.)
  

  If \code{mode} is not supplied and \code{url} ends in one of
  \code{.gz}, \code{.bz2}, \code{.xz}, \code{.tgz}, \code{.zip},
  \code{.rda} or \code{.RData} a binary transfer is done.  Since Windows
  (unlike Unix-alikes) does distinguish between text and binary files,
  care is needed that other binary file types are transferred with
  \code{mode = "wb"}.

  There is an alternative method \code{"wininet"}. (This selected for
  \code{method = "internal"} if the command line flag
  \option{--internet2}, was used or \code{\link{setInternet2}(TRUE)}
  called.)  Then 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 Internet Explorer.  That version does not support
  \code{cacheOK = FALSE}.
#endif
#ifdef unix
  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 \code{mode = "wb"}, but
  the problems incurred by a text transfer will only be seen on Windows.
#endif
}
\note{
   Files of more than 2GB are supported on 64-bit builds of \R; they
  may be truncated on some 32-bit builds.
  
  Method \code{"wget"} is mainly for historical compatibility, but it
  and \code{"curl"} can be used for URLs (e.g., \samp{https://} URLs or
  those that use cookies) which the internal method does not support.

  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{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.

  \command{curl} (\url{http://curl.haxx.se/}) is installed on OS X and
  commonly on Unix-alikes.  Windows binaries are available at that URL.
}
\section{Setting Proxies}{
  This applies to the internal code only.

  Proxies can be specified via environment variables.
  Setting \env{no_proxy} to \code{*} stops any proxy being tried.
  Otherwise the setting of \env{http_proxy} or \env{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 \env{ftp_proxy_user}
  and \env{ftp_proxy_password}.  The form of \env{http_proxy}
  should be \code{http://proxy.dom.com/} or
  \code{http://proxy.dom.com:8080/} where the port defaults to
  \code{80} and the trailing slash may be omitted. For
  \env{ftp_proxy} use the form \code{ftp://proxy.dom.com:3128/}
  where the default port is \code{21}.  These environment variables
  must be set before the download code is first used: they cannot be
  altered later by calling \code{\link{Sys.setenv}}.

  Usernames and passwords can be set for HTTP proxy transfers via
  environment variable \env{http_proxy_user} in the form
  \code{user:passwd}.  Alternatively, \env{http_proxy} can be of the
  form \code{http://user:pass@proxy.dom.com:8080/} for compatibility
  with \code{wget}.  Only the HTTP/1.0 basic authentication scheme is
  supported.
#ifdef windows
  Under Windows, if \env{http_proxy_user} is set to \code{ask} then
  a dialog box will come up for the user to enter the username and
  password.  \bold{NB:} you will be given only one opportunity to enter this,
  but if proxy authentication is required and fails there will be one
  further prompt per download.
#endif

  Much the same scheme is supported by \code{method = "libcurl"}, including
  \env{no_proxy}, \env{http_proxy} and \env{ftp_proxy}, and for the last
  two a contents of \code{[user:password@]machine[:port]} where the
  parts in brackets are optional.  See
  \url{http://curl.haxx.se/libcurl/c/libcurl-tutorial.html} for details.
}
\section{Secure URLs}{
  Methods which access \samp{https://} and \samp{ftps://} URLs usually
  try to verify their 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).

  This is an issue for \code{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 \env{CURL_CA_BUNDLE} to the path to a certificate
  bundle file, usually named \file{ca-bundle.crt} or
  \file{curl-ca-bundle.crt}.  (This is normally done for a binary
  installation of \R, which installs \file{etc/curl-ca-bundle.crt}.)
}
\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.
}
\seealso{
  \code{\link{options}} to set the \code{HTTPUserAgent}, \code{timeout}
  and \code{internet.info} options.

  \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 package \CRANpkg{RCurl} provides more comprehensive
  facilities to download from URLs.
}
\keyword{utilities}