RStudio knit button is not showing options - continued

问题

The question has been already asked (and answered) in 2017 RStudio knit button is not showing options and in 2014 Rstudio knit to PDF.

Three cases are considered:

a) You are not running R 3.0 (which is required for RMarkdown v2);
b) You have a custom markdown renderer

getOption(“rstudio.markdownToHTML”)

c) output: pdf_document in YAML header

• Option c) has worked time ago but it has no effect now.
• Option a) I am using "R version 3.5.1 (2018-07-02)" >> version 3.0
• Option b)

    > getOption("rstudio.markdownToHTML")
    function (inputFile, outputFile) 
    {
        system(paste("pandoc", shQuote(inputFile), "-o", shQuote(outputFile)))
    }
    
    > getOption("rstudio.markdownToPDF")
    NULL

"toHTML" is set (not by me, anyway it looks ok)
"toPDF" does not exist

I tried erasing all I could concerning R and Rstudio and re-installing everything. No change: the options sub-button beside 'knit' button does not re-appear.

来源：https://stackoverflow.com/questions/51694198/rstudio-knit-button-is-not-showing-options-continued