在R中的函数中捕获系统命令的输出

Question

I'm working with a function in a package that uses system2() to run a command within the function. This particular command prints some stuff to standard out. I want to capture the output of that system2() call, however capture.output() does not work. I understand how to run system2() directly and capture the standard output, but not how to get it in this specific situation.

Simple example:

my_ls <- function() {
  system2("ls")
  return("Hello")
}
my_ls()
output <- capture.output(my_ls())
output

Answer 1

Here's a solution... But be warned, it works for me on Linux, might work on a Mac, and I doubt it works on Windows...

Create two Rcpp functions:

library(Rcpp)
cppFunction('void redir(){FILE* F=freopen("/tmp/capture.txt","w+",stdout);}')
cppFunction('void resetredir(){FILE* F=freopen("/dev/tty","w+",stdout);}')

The first will send everything to that file. The second will reset it. The problem is that interactively after the first you'll not be able to see anything. So beware...

So initially system2 sends to the console:

> system2("echo", "hello")
hello

But wrapping a call in redir / resetredir sends it to the file:

> redir(); system2("echo","hello world this time") ; resetredir()
> # prompt returns!

Now we have:

$ cat /tmp/capture.txt 
hello world this time

and if this is the output from some other package you'll have to read it in with R's file I/O routines.

The dodgy bit is the use of /dev/tty in the reset code - I'm not sure it works on a Mac or Windows. If you don't care about resetting the stdout then skip it, and just make sure you know how to quite R without seeing what you type. I'm also unsure if this will work in RStudio which probably has a different concept of the console....