catching stdout in realtime from subprocess

问题

I want to subprocess.Popen() rsync.exe in Windows, and print the stdout in Python.

My code works, but it doesn't catch the progress until a file transfer is done! I want to print the progress for each file in real time.

Using Python 3.1 now since I heard it should be better at handling IO.

import subprocess, time, os, sys

cmd = "rsync.exe -vaz -P source/ dest/"
p, line = True, 'start'


p = subprocess.Popen(cmd,
                     shell=True,
                     bufsize=64,
                     stdin=subprocess.PIPE,
                     stderr=subprocess.PIPE,
                     stdout=subprocess.PIPE)

for line in p.stdout:
    print(">>> " + str(line.rstrip()))
    p.stdout.flush()

回答1:

Some rules of thumb for subprocess.

Never use shell=True. It needlessly invokes an extra shell process to call your program.
When calling processes, arguments are passed around as lists. sys.argv in python is a list, and so is argv in C. So you pass a list to Popen to call subprocesses, not a string.
Don't redirect stderr to a PIPE when you're not reading it.
Don't redirect stdin when you're not writing to it.

Example:

import subprocess, time, os, sys
cmd = ["rsync.exe", "-vaz", "-P", "source/" ,"dest/"]

p = subprocess.Popen(cmd,
                     stdout=subprocess.PIPE,
                     stderr=subprocess.STDOUT)

for line in iter(p.stdout.readline, b''):
    print(">>> " + line.rstrip())

That said, it is probable that rsync buffers its output when it detects that it is connected to a pipe instead of a terminal. This is the default behavior - when connected to a pipe, programs must explicitly flush stdout for realtime results, otherwise standard C library will buffer.

To test for that, try running this instead:

cmd = [sys.executable, 'test_out.py']

and create a test_out.py file with the contents:

import sys
import time
print ("Hello")
sys.stdout.flush()
time.sleep(10)
print ("World")

Executing that subprocess should give you "Hello" and wait 10 seconds before giving "World". If that happens with the python code above and not with rsync, that means rsync itself is buffering output, so you are out of luck.

A solution would be to connect direct to a pty, using something like pexpect.

回答2:

I know this is an old topic, but there is a solution now. Call the rsync with option --outbuf=L. Example:

cmd=['rsync', '-arzv','--backup','--outbuf=L','source/','dest']
p = subprocess.Popen(cmd,
                     stdout=subprocess.PIPE)
for line in iter(p.stdout.readline, b''):
    print '>>> {}'.format(line.rstrip())

回答3:

On Linux, I had the same problem of getting rid of the buffering. I finally used "stdbuf -o0" (or, unbuffer from expect) to get rid of the PIPE buffering.

proc = Popen(['stdbuf', '-o0'] + cmd, stdout=PIPE, stderr=PIPE)
stdout = proc.stdout

I could then use select.select on stdout.

See also https://unix.stackexchange.com/questions/25372/

回答4:

for line in p.stdout:
  ...

always blocks until the next line-feed.

For "real-time" behaviour you have to do something like this:

while True:
  inchar = p.stdout.read(1)
  if inchar: #neither empty string nor None
    print(str(inchar), end='') #or end=None to flush immediately
  else:
    print('') #flush for implicit line-buffering
    break

The while-loop is left when the child process closes its stdout or exits. read()/read(-1) would block until the child process closed its stdout or exited.

回答5:

Your problem is:

for line in p.stdout:
    print(">>> " + str(line.rstrip()))
    p.stdout.flush()

the iterator itself has extra buffering.

Try doing like this:

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

回答6:

You cannot get stdout to print unbuffered to a pipe (unless you can rewrite the program that prints to stdout), so here is my solution:

Redirect stdout to sterr, which is not buffered. '<cmd> 1>&2' should do it. Open the process as follows: myproc = subprocess.Popen('<cmd> 1>&2', stderr=subprocess.PIPE)
You cannot distinguish from stdout or stderr, but you get all output immediately.

Hope this helps anyone tackling this problem.

回答7:

Depending on the use case, you might also want to disable the buffering in the subprocess itself.

If the subprocess will be a Python process, you could do this before the call:

os.environ["PYTHONUNBUFFERED"] = "1"

Or alternatively pass this in the env argument to Popen.

Otherwise, if you are on Linux/Unix, you can use the stdbuf tool. E.g. like:

cmd = ["stdbuf", "-oL"] + cmd

See also here about stdbuf or other options.

回答8:

Change the stdout from the rsync process to be unbuffered.

p = subprocess.Popen(cmd,
                     shell=True,
                     bufsize=0,  # 0=unbuffered, 1=line-buffered, else buffer-size
                     stdin=subprocess.PIPE,
                     stderr=subprocess.PIPE,
                     stdout=subprocess.PIPE)

回答9:

To avoid caching of output you might wanna try pexpect,

child = pexpect.spawn(launchcmd,args,timeout=None)
while True:
    try:
        child.expect('\n')
        print(child.before)
    except pexpect.EOF:
        break

PS : I know this question is pretty old, still providing the solution which worked for me.

PPS: got this answer from another question

回答10:

    p = subprocess.Popen(command,
                                bufsize=0,
                                universal_newlines=True)

I am writing a GUI for rsync in python, and have the same probelms. This problem has troubled me for several days until i find this in pyDoc.

If universal_newlines is True, the file objects stdout and stderr are opened as text files in universal newlines mode. Lines may be terminated by any of '\n', the Unix end-of-line convention, '\r', the old Macintosh convention or '\r\n', the Windows convention. All of these external representations are seen as '\n' by the Python program.

It seems that rsync will output '\r' when translate is going on.

回答11:

I've noticed that there is no mention of using a temporary file as intermediate. The following gets around the buffering issues by outputting to a temporary file and allows you to parse the data coming from rsync without connecting to a pty. I tested the following on a linux box, and the output of rsync tends to differ across platforms, so the regular expressions to parse the output may vary:

import subprocess, time, tempfile, re

pipe_output, file_name = tempfile.TemporaryFile()
cmd = ["rsync", "-vaz", "-P", "/src/" ,"/dest"]

p = subprocess.Popen(cmd, stdout=pipe_output, 
                     stderr=subprocess.STDOUT)
while p.poll() is None:
    # p.poll() returns None while the program is still running
    # sleep for 1 second
    time.sleep(1)
    last_line =  open(file_name).readlines()
    # it's possible that it hasn't output yet, so continue
    if len(last_line) == 0: continue
    last_line = last_line[-1]
    # Matching to "[bytes downloaded]  number%  [speed] number:number:number"
    match_it = re.match(".* ([0-9]*)%.* ([0-9]*:[0-9]*:[0-9]*).*", last_line)
    if not match_it: continue
    # in this case, the percentage is stored in match_it.group(1), 
    # time in match_it.group(2).  We could do something with it here...

回答12:

In Python 3, here's a solution, which takes a command off the command line and delivers real-time nicely decoded strings as they are received.

Receiver (receiver.py):

import subprocess
import sys

cmd = sys.argv[1:]
p = subprocess.Popen(cmd, stdout=subprocess.PIPE)
for line in p.stdout:
    print("received: {}".format(line.rstrip().decode("utf-8")))

Example simple program that could generate real-time output (dummy_out.py):

import time
import sys

for i in range(5):
    print("hello {}".format(i))
    sys.stdout.flush()  
    time.sleep(1)

Output:

$python receiver.py python dummy_out.py
received: hello 0
received: hello 1
received: hello 2
received: hello 3
received: hello 4

来源：https://stackoverflow.com/questions/1606795/catching-stdout-in-realtime-from-subprocess

标签

python

subprocess

stdout