Especially in batch mode it is often useful to run an external process, letting it write to stdout and stderr as if it was the main executed process. As far as I see (and I have spent quite a lot of time investigating this), with Emacs it is not possible.
I found two options for stdout+stderr, but with neither allowing me to achieve quite what I want.
1. You can run a process with stdout and stderr combined, then use e.g. process filter function to e.g. `princ' received output. The disadvantage is that stdout and stderr get bunched together. For example, if someone runs your script with `> /dev/null', he will also miss stderr output of the child process. Perhaps even more importantly, robust automated processing of output becomes impossible, as it can contain "garbage", i.e. output meant for stderr.
2. You can split stdout and stderr e.g. by using `make-pipe-process' for stderr and then e.g. use `princ' in the filter for the "real" process (only stdout output) and `message' for stderr process. The problem is that here output comes completely unsynchronized and (at least usually) in larger chunks. If the child process intermixes stdout and stderr output, it comes out not in the original order. You also lose interactivity if the child process e.g. runs for minutes and prints progress messages regularly during that time.
In comparison, with Python (or pretty much any other "real" language) this is trivial, see attached `output-forwarder.py'. I don't see a way to replicate these three lines in Elisp.
It would be nice if e.g. `make-process' grew an argument `:forward' (possible values `t', `stderr' and `stdout'; the latter to forward only one stream). It would be fine if this was incompatible with `:buffer' and `:stderr', i.e. if this couldn't be specified together with those options. I wouldn't mind any other interface either, as long as it worked.
Attachments:
* `mixed-output.py': a simple Python scripts that writes to both stdout and stderr with small delays;
* `output-forwarder.py': a simple Python scripts that runs whatever command is specified on the command line and forwards it output cleanly; usage, e.g.: `$ ./output-forwarder.py ./mixed-output.py' or `$ ./output-forwarder.py ./mixed-output.py >/dev/null'.
Paul
mixed-output.py