catching stdout in realtime from subprocess

Question

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   gt  gt  gt      str line rstrip         p stdout flush

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if you run something like this in a thread and save the ffmpeg time property in a property of a method so you can access it  it would work very nice I get outputs like this  output be like if you use threading in tkinter input    path input file mp4  output    path input file mp4  command    quot ffmpeg -y -v quiet -stats -i   quot  quot    str input     quot   quot  -metadata title   quot  alaa sanatisharif  quot  -preset ultrafast -vcodec copy -r 50 -vsync 1 -async 1   quot  quot    output    quot   quot  quot  process   subprocess Popen command  stdout subprocess PIPE  stderr subprocess STDOUT  universal newlines True  shell True  for line in self process stdout      reg   re search   d d  d d  d d   line      ffmpeg time   reg group 0  if reg else        print ffmpeg time

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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

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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

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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

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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   gt  gt  gt      format line rstrip

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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

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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

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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

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Your problem is   for line in p stdout      print   gt  gt  gt      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

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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

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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

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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   gt  gt  gt      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

User · Answer

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     lt cmd gt  1 gt  amp 2  should do it   Open the process as follows  myproc   subprocess Popen   lt cmd gt  1 gt  amp 2   stderr subprocess PIPE  You cannot distinguish from stdout or stderr  but you get all output immediately   Hope this helps anyone tackling this problem

[python] catching stdout in realtime from subprocess

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