Getting realtime output using subprocess

Question

I tried this, and for some reason while the code

for line in p.stdout:
  ...

buffers aggressively, the variant

while True:
  line = p.stdout.readline()
  if not line: break
  ...

does not. Apparently this is a known bug: http://bugs.python.org/issue3907 (The issue is now "Closed" as of Aug 29, 2018)

By setting the buffer size to 1, you essentially force the process to not buffer the output.

p = subprocess.Popen(cmd, stdout=subprocess.PIPE, bufsize=1)
for line in iter(p.stdout.readline, b''):
    print line,
p.stdout.close()
p.wait()

You can direct the subprocess output to the streams directly. Simplified example:

subprocess.run(['ls'], stderr=sys.stderr, stdout=sys.stdout)

You can try this:

import subprocess
import sys

process = subprocess.Popen(
    cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE
)

while True:
    out = process.stdout.read(1)
    if out == '' and process.poll() != None:
        break
    if out != '':
        sys.stdout.write(out)
        sys.stdout.flush()

If you use readline instead of read, there will be some cases where the input message is not printed. Try it with a command the requires an inline input and see for yourself.