progress: Output to stderr - not stdout

[HenrikBengtsson]

Issue

Download progress such as:

Downloading: 1.2 kB

is outputted to standard output (stdout). This means for instance that the progress is captured by report generators. Outputting to standard error (stderr) avoids this.

Troubleshooting

This is because cat() and utils::txtProgressBar() are called with defaults, which results in output to stdout. Passing file=stderr() to these functions would probably be enough.

Related code issues: Issue #344

[hadley]

I made it an option - I couldn't change the default because it breaks the default progress behaviour (each update gets a new line when using stderr)

[HenrikBengtsson]

Thanks.

Could you clarify what you mean by "each update gets a new line when using stderr"? Why would stdout() and stderr() differ in what (character sequence) is outputted (other than to where)? You're not referring to behavior of message() here, which shouldn't be involved, correct?

[hadley]

Just try it out yourself - stderr() doesn't seem to respond to "\r" in the same way as stdout() (at least in RStudio)

[HenrikBengtsson]

I see. It seems to come down to how RStudio handles \r (CR) in the two cases. Here's what I found using RStudio 0.99.902 (on Windows):

## Ask expected
> cat("abc\rABC\n", file=stdout())
ABC
> cat("abc\rABC\n", file=stderr())
ABC

## Different behavior between stdout and stderr
> cat("abc", file=stdout()); cat("\rABC\n", file=stdout())
ABC
> cat("abc", file=stderr()); cat("\rABC\n", file=stderr())
abcABC

## Also, it seems as '\r' is "forgotten" between cat() regardless of output
> cat("abc\r", file=stdout()); cat("ABC\n", file=stdout())
abcABC
> cat("abc\r", file=stderr()); cat("ABC\n", file=stderr())
abcABC

I've tried this on Windows (Rterm and Rgui) as well as Linux and I cannot reproduce this there.