Using the knitr package:
```{r, engine='bash', code_block_name} ...
E.g.:
```{r, engine='bash', count_lines}
wc -l en_US.twitter.txt
```
You can also use:
engine='sh'
for shellengine='python'
for Pythonengine='perl'
, engine='haskell'
and a bunch of other C-like languages and even gawk
, AWK, etc.