Include code from an external R script, run in, display both code and output

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Is it possible to include code from an external R script in an .Rmd and simultaneously run the code, display the code, and display its results in the output .HTML file? For example, if I have

x <- 1 y <- 3 z <- x + y z 

in external.R. In the output document I want to see the code above along with the result of z, i.e. 4. Essentially, I want the equivalent of what would happen if I copy/pasted what's above in an R chunk. So I want

```{r} some.library::some.function("external.R") ``` 

to be the equivalent of

```{r} x <- 1 y <- 3 z <- x + y z ``` 

In the output HTML file. I've tried things like knitr::read_chunk('external.R) and source('external.R)`, but these don't display the code. Am I missing something simple?


I found that source('external.R', echo = TRUE) will produce what I ask, but each line of the output's displayed code/results is prepended by ##. Any way to make it look like it would if the code was simply copy/pasted in a chunk in the .Rmd?


Although the accepted answer provides a simple and working solution, I think the most idiomatic way of doing this (without having to modify the external script at all) is to use the chunk option code to set the contents of external.R as chunk code:

```{r, code = readLines("external.R")} ``` 


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