The R Project SVN R

% File src/library/base/man/system.Rd

% Part of the R package, https://www.R-project.org

% Copyright 1995-2018 R Core Team

% Distributed under GPL 2 or later

\name{system2}

\alias{system2}

\title{Invoke a System Command}

\description{

  \code{system2} invokes the OS command specified by \code{command}.

}

\usage{

system2(command, args = character(),

        stdout = "", stderr = "", stdin = "", input = NULL,

        env = character(), wait = TRUE,

        minimized = FALSE, invisible = TRUE, timeout = 0)

}

\arguments{

  \item{command}{the system command to be invoked, as a character string.}

  \item{args}{a character vector of arguments to \command{command}.}

  \item{stdout, stderr}{where output to \file{stdout} or

    \file{stderr} should be sent.  Possible values are \code{""}, to the \R

    console (the default), \code{NULL} or \code{FALSE} (discard output),

    \code{TRUE} (capture the output in a character vector) or a

    character string naming a file.}

  \item{stdin}{should input be diverted?  \code{""} means the default,

    alternatively a character string naming a file.  Ignored

    if \code{input} is supplied.}

  \item{input}{if a character vector is supplied, this is copied one

    string per line to a temporary file, and the standard input of

    \code{command} is redirected to the file.}

  \item{env}{character vector of name=value strings to set environment

    variables.}

  \item{wait}{a logical (not \code{NA}) indicating whether the \R

    interpreter should wait for the command to finish, or run it

    asynchronously.  This will be ignored (and the interpreter will

    always wait) if \code{stdout = TRUE} or \code{stderr = TRUE}. When

    running the command asynchronously, no output will be displayed on

    the \code{Rgui} console in Windows (it will be dropped, instead).}

  \item{timeout}{timeout in seconds, ignored if 0.  This is a limit for the

    elapsed time running \code{command} in a separate process.   Fractions

    of seconds are ignored.}

#ifdef unix

  \item{minimized, invisible}{arguments that are accepted on Windows but

    ignored on this platform, with a warning.}

#endif

#ifdef windows

  \item{minimized}{logical (not \code{NA}), indicates whether the

    command window should be displayed initially as a minimized window.}

  \item{invisible}{logical (not \code{NA}), indicates whether the

    command window should be visible on the screen.}

#endif

}

\details{

  Unlike \code{\link{system}}, \code{command} is always quoted by

  \code{\link{shQuote}}, so it must be a single command without arguments.

  For details of how \code{command} is found see \code{\link{system}}.

  On Windows, \code{env} is only supported for commands such as

  \command{R} and \command{make} which accept environment variables on

  their command line.

  Some Unix commands (such as some implementations of \code{ls}) change

  their output if they consider it to be piped or redirected:

  \code{stdout = TRUE} uses a pipe whereas \code{stdout =

  "some_file_name"} uses redirection.

  Because of the way it is implemented, on a Unix-alike \code{stderr =

    TRUE} implies \code{stdout = TRUE}: a warning is given if this is

  not what was specified.

  When \code{timeout} is non-zero, the command is terminated after the given

  number of seconds.  The termination works for typical commands, but is not

  guaranteed: it is possible to write a program that would keep running

  after the time is out.  Timeouts can only be set with \code{wait = TRUE}.

#ifdef unix

  Timeouts cannot be used with interactive commands: the command is run with

  standard input redirected from \code{/dev/null} and it must not modify

  terminal settings.  As long as tty \code{tostop} option is disabled, which

  it usually is by default, the executed command may write to standard

  output and standard error.

#endif

}

%% We use popen, and that pipes stdout only

\value{

  If \code{stdout = TRUE} or \code{stderr = TRUE}, a character vector

  giving the output of the command, one line per character string.

  (Output lines of more than 8095 bytes will be split.)  If the command

  could not be run an \R error is generated.  If \code{command} runs but

  gives a non-zero exit status this will be reported with a warning and

  in the attribute \code{"status"} of the result: an attribute

  \code{"errmsg"} may also be available.

  In other cases, the return value is an error code (\code{0} for

  success), given the \link{invisible} attribute (so needs to be printed

  explicitly).  If the command could not be run for any reason, the

  value is \code{127} and a warning is issued (as from \R 3.5.0).

  Otherwise if \code{wait = TRUE} the value is the exit status returned

  by the command, and if \code{wait = FALSE} it is \code{0} (the

  conventional success value).

  If the command times out, a warning is issued and the exit status is

  \code{124}.

#ifdef windows

  Some Windows commands return out-of-range status values

  (e.g., \code{-1}) and so only the bottom 16 bits of the value are used.

#endif

}

\note{

  \code{system2} is a more portable and flexible interface than

  \code{\link{system}}.  It allows redirection of output without needing

  to invoke a shell on Windows, a portable way to set environment

  variables for the execution of \code{command}, and finer control over

  the redirection of \code{stdout} and \code{stderr}.  Conversely,

  \code{system} (and \code{shell} on Windows) allows the invocation of

  arbitrary command lines.

  There is no guarantee that if \code{stdout} and \code{stderr} are both

  \code{TRUE} or the same file that the two streams will be interleaved

  in order.  This depends on both the buffering used by the command and

  the OS.

}

\seealso{

  \code{\link{system}}.

#ifdef windows

  \code{\link{shell}} and \code{\link{shell.exec}}.

#endif

}

\keyword{interface}

\keyword{file}

\keyword{utilities}