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

Write to terminal byte by byte for interactive applications

This method write any bytes it gets to stdout immediately, which more closely simulates the behavior of tee, especially for interactive applications.

main.py

#!/usr/bin/env python3
import os
import subprocess
import sys
with subprocess.Popen(sys.argv[1:], stdout=subprocess.PIPE, stderr=subprocess.STDOUT) as proc, \
        open('logfile.txt', 'bw') as logfile:
    while True:
        byte = proc.stdout.read(1)
        if byte:
            sys.stdout.buffer.write(byte)
            sys.stdout.flush()
            logfile.write(byte)
            # logfile.flush()
        else:
            break
exit_status = proc.returncode

sleep.py

#!/usr/bin/env python3
import sys
import time
for i in range(10):
    print(i)
    sys.stdout.flush()
    time.sleep(1)

First we can do a non-interactive sanity check:

./main.py ./sleep.py

And we see it counting to stdout on real time.

Next, for an interactive test, you can run:

./main.py bash

Then the characters you type appear immediately on the terminal as you type them, which is very important for interactive applications. This is what happens when you run:

bash | tee logfile.txt

Also, if you want the output to show on the ouptut file immediately, then you can also add a:

logfile.flush()

but tee does not do this, and I'm afraid it would kill performance. You can test this out easily with:

tail -f logfile.txt

Related question: live output from subprocess command

Tested on Ubuntu 18.04, Python 3.6.7.

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.

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)