Dynamically updated output with `run`

[hongooi73]

It would be nice if run had better support for programs that continually update the screen. Taking the example of docker again:

• system2("docker", "pull <foo>") prints the status updates to the screen as they occur, with progress bars that update properly. It mimics what you'd see if you ran docker from the commandline.
• processx::run("docker", c("pull", "<foo>"), echo=TRUE) doesn't show the progress bars at all, and only shows the updated status as each layer operation completes.

[gaborcsardi]

This is essentially a duplicate of #72.

Also, it is up to docker to decide what it outputs. For the processx case, seeing that stdout / stderr are pipes, it decides not to output progress bars.

On Unix we might be able to fix this issue via pseudo terminals, which processx already supports

[hongooi73]

Oh, seeing the mention of pseudo-terminals reminded me of https://github.com/Microsoft/Terminal

Maybe also worth looking into supporting this on Win 10?

[gaborcsardi]

Yeah, we could support them on Win 10.

Also, run() needs to support them as well, currently only process has them, I'll open some issues.

[gaborcsardi]

There are issues for pty support in run() and on Windows, so I am going to close this now.