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\name{shQuote}
\alias{shQuote}
\title{Quote Strings for Use in OS Shells}
\description{
  Quote a string to be passed to an operating system shell.
}
\usage{
shQuote(string, type = c("sh", "csh", "cmd"))
}
\arguments{
  \item{string}{a character vector, usually of length one.}
  \item{type}{character: the type of shell.  Partial matching is
    supported. \code{"cmd"} refers to the Windows NT shell.}
}
\details{
  The default type of quoting supported is that for the Bourne shell
  \code{sh}.  If the string does not contain single quotes, we can just
  surround it with single quotes.  Otherwise, the string is surrounded in
  double quotes, which suppresses all special meanings of metacharacters
  except dollar, backquote and backslash, so these (and of course double
  quote) are preceded by backslash.  This type of quoting is also
  appropriate for \code{ksh} and \code{bash}.

  The other type of quoting is for the C-shell (\code{csh} and
  \code{tcsh}).  Once again, if the string does not contain single
  quotes, we can just surround it with single quotes.  If it does
  contain single quotes, we can use double quotes provided it does not
  contain dollar or backquote (and we need to escape backslash,
  exclamation mark and double quote).  As a last resort, we need to
  split the string into pieces not containing single quotes and surround
  each with single quotes, and the single quotes with double quotes.

  The Windows shell supports only double quoting.  All this
  implementation does is to surround by double quotes and escape
  internal double quotes by a backslash.
}
\references{
  Loukides, M. et al (2002)
  \emph{Unix Power Tools} Third Edition.  O'Reilly.  Section 27.12.

  \url{http://www.mhuffman.com/notes/dos/bash_cmd.htm}
}
\seealso{
  \code{\link{sQuote}} for quoting in text.
}
\examples{
test <- "abc$def`gh`i\\j"
cat(shQuote(test), "\n")
\dontrun{system(paste("echo", shQuote(test)))}
test <- "don't do it!"
cat(shQuote(test), "\n")

tryit <- "use the `-c' switch\nlike this"
cat(shQuote(tryit), "\n")
\dontrun{system(paste("echo", shQuote(tryit)))}
cat(shQuote(tryit, type="csh"), "\n")

## Windows-only example.
perlcmd <- 'print "Hello World\\n";'
\dontrun{shell(paste("perl -e", shQuote(perlcmd, type="cmd")))}
}
\keyword{utilities}