writeLines('
<<echo=FALSE, results="asis">>=
writeLines("Hello World!")
@
', "foo.Rnw")

knitr::knit("foo.Rnw", quiet = TRUE)
writeLines(readLines("foo.tex"))
##
## Hello World!
##

knitr::render_sweave()
knitr::knit("foo.Rnw", quiet = TRUE)
writeLines(readLines("foo.tex"))
##
## begin{Soutput}
## Hello World!
## end{Soutput}
##

writeLines('
<<echo=FALSE, results=tex>>=
writeLines("Hello World!")
@
', "foo.Rnw")

utils::Sweave("foo.Rnw", quiet = TRUE)
writeLines(readLines("foo.tex"))
##
## Hello World!