How to write to stdout AND to log file simultaneously with Popen?

Question

I am using Popen to call a shell script that is continuously writing its stdout and stderr to a log file. Is there any way to simultaneously output the log file continuously (to the screen), or alternatively, make the shell script write to both the log file and stdout at the same time?

I basically want to do something like this in Python:

cat file 2>&1 | tee -a logfile #"cat file" will be replaced with some script

Again, this pipes stderr/stdout together to tee, which writes it both to stdout and my logfile.

I know how to write stdout and stderr to a logfile in Python. Where I'm stuck is how to duplicate these back to the screen:

subprocess.Popen("cat file", shell=True, stdout=logfile, stderr=logfile)

Of course, I could just do something like this, but is there any way to do this without tee and shell file descriptor redirection?:

subprocess.Popen("cat file 2>&1 | tee -a logfile", shell=True)

Answer 1

You can use a pipe to read the data from the program's stdout and write it to all the places you want:

import sys
import subprocess

logfile = open('logfile', 'w')
proc=subprocess.Popen(['cat', 'file'], stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
for line in proc.stdout:
    sys.stdout.write(line)
    logfile.write(line)
proc.wait()

UPDATE

In python 3, the universal_newlines parameter controls how pipes are used. If False, pipe reads return bytes objects and may need to be decoded (e.g., line.decode('utf-8')) to get a string. If True, python does the decode for you

Changed in version 3.3: When universal_newlines is True, the class uses the encoding locale.getpreferredencoding(False) instead of locale.getpreferredencoding(). See the io.TextIOWrapper class for more information on this change.

Answer 2

To emulate: subprocess.call("command 2>&1 | tee -a logfile", shell=True) without invoking the tee command:

#!/usr/bin/env python2
from subprocess import Popen, PIPE, STDOUT

p = Popen("command", stdout=PIPE, stderr=STDOUT, bufsize=1)
with p.stdout, open('logfile', 'ab') as file:
    for line in iter(p.stdout.readline, b''):
        print line,  #NOTE: the comma prevents duplicate newlines (softspace hack)
        file.write(line)
p.wait()

To fix possible buffering issues (if the output is delayed), see links in Python: read streaming input from subprocess.communicate().

Here's Python 3 version:

#!/usr/bin/env python3
import sys
from subprocess import Popen, PIPE, STDOUT

with Popen("command", stdout=PIPE, stderr=STDOUT, bufsize=1) as p, \
     open('logfile', 'ab') as file:
    for line in p.stdout: # b'\n'-separated lines
        sys.stdout.buffer.write(line) # pass bytes as is
        file.write(line)