\HeaderA{system}{Invoke a System Command}{system}
\keyword{interface}{system}
\keyword{file}{system}
\keyword{utilities}{system}
\begin{Description}\relax
\code{system} invokes the OS command specified by \code{command}.
\end{Description}
\begin{Usage}
\begin{verbatim}
system(command, intern = FALSE, ignore.stderr = FALSE,
       wait = TRUE, input = NULL, show.output.on.console = TRUE,
       minimized = FALSE, invisible = TRUE)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{command}] the system command to be invoked, as a string.
\item[\code{intern}] a logical (not \code{NA}) which indicates whether to
make the output of the command an \R{} object.
Not available unless \code{popen} is supported on the platform.

\item[\code{ignore.stderr}] a logical indicating whether error messages written
to \file{stderr} should be ignored.
\item[\code{wait}] a logical 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{intern = TRUE}.
\item[\code{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[\code{show.output.on.console, minimized, invisible}] arguments
that are accepted on other platforms but ignored on this one, with a
warning.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{command} is parsed as a command plus arguments separated by spaces.
So if the path to the command (or a filepath argument) contains
spaces, it must be quoted e.g. by \code{\LinkA{shQuote}{shQuote}}.

How the command is run differs by platform: Unix-alikes use a shell
(\file{/bin/sh} by default), and Windows executes the command directly
(extensions \code{.exe}, \code{.com}) or as a batch file (extensions
\code{.cmd} and \code{.bat}).

If \code{intern} is \code{TRUE} then \code{popen} is used to invoke the
command and the output collected, line by line, into an \R{}
\code{\LinkA{character}{character}} vector.  If \code{intern} is \code{FALSE} then
the C function \code{system} is used to invoke the command.

The ordering of arguments after the first two has changed from time to
time: it is recommended to name all arguments after the first.
\end{Details}
\begin{Value}
If \code{intern = TRUE}, a character vector giving the output of the
command, one line per character string.  (Output lines of more than
8095 characters will be split.)  If the command could not be run or
gives an error
this will be reported on the shell's \file{stderr} (unless
\code{popen} is not supported, when there is an \R{} error).

If \code{intern = FALSE}, the return value is an error code (\code{0}
for success), given the invisible attribute (so needs to be printed
explicitly).  If the command could not be run for any reason, the
value is
\code{256*127 = 52512}.
Otherwise if \code{wait = TRUE} the value is
256 times
the error code returned by the command, and if \code{wait = FALSE} it
is \code{0} (the conventional success value).
\end{Value}
\begin{Section}{Stdout and stderr}
Error messages written to \file{stderr} will be sent by the shell to
the terminal unless \code{ignore.stderr = TRUE}.  They can be captured
(in the most likely shells) by
\begin{alltt}
    system("some command 2>\&1", intern=TRUE)
\end{alltt}
What happens to output sent to \file{stdout} and \file{stderr} if
\code{intern = FALSE} is interface-specific, and it is unsafe to
assume that such messages will appear on the console (they do on the
MacOS X console but not on the \pkg{gnomeGUI} console, for example).
\end{Section}
\begin{Note}\relax
\code{wait} is implemented by appending \code{\&} to the command: this
is shell-dependent, but required by POSIX and so widely supported.
\end{Note}
\begin{SeeAlso}\relax
\code{\LinkA{.Platform}{.Platform}} for platform-specific variables.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
# list all files in the current directory using the -F flag
## Not run: system("ls -F")

# t1 is a character vector, each one
# representing a separate line of output from who
# (if the platform has popen and who)
t1 <- try(system("who", intern = TRUE))

try(system("ls fizzlipuzzli", intern = TRUE, ignore.stderr = TRUE))
# empty since file doesn't exist
\end{ExampleCode}
\end{Examples}

