printing stdout in realtime from a subprocess that requires stdin
python, subprocess
Solution
In order to grab stdout from the subprocess in real time you need to decide exactly what behavior you want; specifically, you need to decide whether you want to deal with the output line-by-line or character-by-character, and whether you want to block while waiting for output or be able to do something else while waiting.
It looks like it will probably suffice for your case to read the output in line-buffered fashion, blocking until each complete line comes in, which means the convenience functions provided by `subprocess` are good enough:
p = subprocess.Popen(some_cmd, stdout=subprocess.PIPE)
# Grab stdout line by line as it becomes available. This will loop until
# p terminates.
while p.poll() is None:
l = p.stdout.readline() # This blocks until it receives a newline.
print l
# When the subprocess terminates there might be unconsumed output
# that still needs to be processed.
print p.stdout.read()
If you need to write to the stdin of the process, just use another pipe:
p = subprocess.Popen(some_cmd, stdout=subprocess.PIPE, stdin=subprocess.PIPE)
# Send input to p.
p.stdin.write("some input\n")
p.stdin.flush()
# Now start grabbing output.
while p.poll() is None:
l = p.stdout.readline()
print l
print p.stdout.read()
Pace the other answer, there's no need to indirect through a file in order to pass input to the subprocess.
Problem
This is a follow up to this question, but if I want to pass an argument to `stdin` to `subprocess`, how can I get the output in real time? This is what I currently have; I also tried replacing `Popen` with `call` from the `subprocess` module and this just leads to the script hanging. ``` from subprocess import Popen, PIPE, STDOUT cmd = 'rsync --rsh=ssh -rv --files-from=- thisdir/ servername:folder/' p = Popen(cmd.split(), stdout=PIPE, stdin=PIPE, stderr=STDOUT) subfolders = '\n'.join(['subfolder1','subfolder2']) output = p.communicate(input=subfolders)[0] print output ``` In the former question where I did not have to pass `stdin` I was suggested to use `p.stdout.readline`, there there is no room there to pipe anything to `stdin`. Addendum: This works for the transfer, but I see the output only at the end and I would like to see the details of the transfer while it's happening.