Suppressing messages in Knitr / Rmarkdown

You should never need to use dev.off() when using knitr. It takes care of all the devices to create plots.

From the package author Yihui

God kills a kitten whenever you dev.off()

null device 
          1

Is the output of dev.off().

It may be that getSymbols returns something given that you haven't defined env

If you want to hide the results (output) (in general) you can use results = 'hide' as an option. No need to wrap anything in invisible()

Ran into this problem as well, I would like to add that it should be warning = FALSE, not warnings = FALSE

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