% File src/library/base/man/sink.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2016 R Core Team
% Distributed under GPL 2 or later

\name{sink}
\alias{sink}
\alias{sink.number}
\concept{tee}
\title{Send R Output to a File}
\usage{
sink(file = NULL, append = FALSE, type = c("output", "message"),
     split = FALSE)

sink.number(type = c("output", "message"))
}
\arguments{
  \item{file}{a writable \link{connection} or a character string naming the
    file to write to, or \code{NULL} to stop sink-ing.}
  \item{append}{logical.  If \code{TRUE}, output will be appended to
    \code{file}; otherwise, it will overwrite the contents of
    \code{file}.}
  \item{type}{character string.  Either the output stream or the messages
    stream.  The name will be partially matched so can be abbreviated.}
  \item{split}{logical: if \code{TRUE}, output will be sent to the new
    sink and to the current output stream, like the Unix program \code{tee}.}
}
\description{
  \code{sink} diverts \R output to a connection (and stops such diversions).

  \code{sink.number()} reports how many diversions are in use.

  \code{sink.number(type = "message")} reports the number of the
  connection currently being used for error messages.
}
\details{
  \code{sink} diverts \R output to a connection (and must be used again
  to finish such a diversion, see below!).  If \code{file} is a
  character string, a file connection with that name will be established
  for the duration of the diversion.

  Normal \R output (to connection \code{\link{stdout}}) is diverted by
  the default \code{type = "output"}.  Only prompts and (most)
  messages continue to appear on the console.  Messages sent to
  \code{\link{stderr}()} (including those from \code{\link{message}},
  \code{\link{warning}} and \code{\link{stop}}) can be diverted by
  \code{sink(type = "message")} (see below).

  \code{sink()} or \code{sink(file = NULL)} ends the last diversion (of
  the specified type).  There is a stack of diversions for normal
  output, so output reverts to the previous diversion (if there was
  one).  The stack is of up to 21 connections (20 diversions).

  If \code{file} is a connection it will be opened if necessary (in
  \code{"wt"} mode) and closed once it is removed from the stack of
  diversions.

  \code{split = TRUE} only splits \R output (via \code{Rvprintf}) and
  the default output from \code{\link{writeLines}}: it does not split
  all output that might be sent to \code{\link{stdout}()}.

  Sink-ing the messages stream should be done only with great care.
  For that stream \code{file} must be an already open connection, and
  there is no stack of connections.

  If \code{file} is a character string, the file will be opened using
  the current encoding.  If you want a different encoding (e.g., to
  represent strings which have been stored in UTF-8), use a
  \code{\link{file}} connection --- but some ways to produce \R output
  will already have converted such strings to the current encoding.
}
\value{
  \code{sink} returns \code{NULL}.

  For \code{sink.number()} the number (0, 1, 2, \dots) of diversions of
  output in place.

  For \code{sink.number("message")} the connection number used for
  messages, 2 if no diversion has been used.
}
\section{Warning}{
  Do not use a connection that is open for \code{sink} for any other
  purpose.  The software will stop you closing one such inadvertently.

  Do not sink the messages stream unless you understand the source code
  implementing it and hence the pitfalls.
}
\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth & Brooks/Cole.

  Chambers, J. M. (1998)
  \emph{Programming with Data.  A Guide to the S Language}.
  Springer.
}
\seealso{\code{\link{capture.output}}}
\examples{
sink("sink-examp.txt")
i <- 1:10
outer(i, i, "*")
sink()
\dontshow{unlink("sink-examp.txt")}% don't show what confuses newbies
\donttest{
## capture all the output to a file.
zz <- file("all.Rout", open = "wt")
sink(zz)
sink(zz, type = "message")
try(log("a"))
## revert output back to the console -- only then access the file!
sink(type = "message")
sink()
file.show("all.Rout")
}}
\keyword{file}
\keyword{connection}