Getting realtime output using subprocess

Question

I am trying to write a wrapper script for a command line program (svnadmin verify) that will display a nice progress indicator for the operation. This requires me to be able to see each line of output from the wrapped program as soon as it is output.

I figured that I'd just execute the program using subprocess.Popen, use stdout=PIPE, then read each line as it came in and act on it accordingly. However, when I ran the following code, the output appeared to be buffered somewhere, causing it to appear in two chunks, lines 1 through 332, then 333 through 439 (the last line of output)

from subprocess import Popen, PIPE, STDOUT

p = Popen('svnadmin verify /var/svn/repos/config', stdout = PIPE, 
        stderr = STDOUT, shell = True)
for line in p.stdout:
    print line.replace('\n', '')

After looking at the documentation on subprocess a little, I discovered the bufsize parameter to Popen, so I tried setting bufsize to 1 (buffer each line) and 0 (no buffer), but neither value seemed to change the way the lines were being delivered.

At this point I was starting to grasp for straws, so I wrote the following output loop:

while True:
    try:
        print p.stdout.next().replace('\n', '')
    except StopIteration:
        break

but got the same result.

Is it possible to get 'realtime' program output of a program executed using subprocess? Is there some other option in Python that is forward-compatible (not exec*)?

Answer 1

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)

Answer 2

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()

Answer 3

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

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

Answer 4

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.

Answer 5

In Python 3.x the process might hang because the output is a byte array instead of a string. Make sure you decode it into a string.

Starting from Python 3.6 you can do it using the parameter encoding in Popen Constructor. The complete example:

process = subprocess.Popen(
    'my_command',
    stdout=subprocess.PIPE,
    stderr=subprocess.STDOUT,
    shell=True,
    encoding='utf-8',
    errors='replace'
)

while True:
    realtime_output = process.stdout.readline()

    if realtime_output == '' and process.poll() is not None:
        break

    if realtime_output:
        print(realtime_output.strip(), flush=True)

Note that this code redirects stderr to stdout and handles output errors.

Answer 6

Real Time Output Issue resolved: I encountered a similar issue in Python, while capturing the real time output from C program. I added fflush(stdout); in my C code. It worked for me. Here is the code.

C program:

#include <stdio.h>
void main()
{
    int count = 1;
    while (1)
    {
        printf(" Count  %d\n", count++);
        fflush(stdout);
        sleep(1);
    }
}

Python program:

#!/usr/bin/python

import os, sys
import subprocess


procExe = subprocess.Popen(".//count", shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE, universal_newlines=True)

while procExe.poll() is None:
    line = procExe.stdout.readline()
    print("Print:" + line)

Output:

Print: Count  1
Print: Count  2
Print: Count  3

Answer 7

The Streaming subprocess stdin and stdout with asyncio in Python blog post by Kevin McCarthy shows how to do it with asyncio:

import asyncio
from asyncio.subprocess import PIPE
from asyncio import create_subprocess_exec


async def _read_stream(stream, callback):
    while True:
        line = await stream.readline()
        if line:
            callback(line)
        else:
            break


async def run(command):
    process = await create_subprocess_exec(
        *command, stdout=PIPE, stderr=PIPE
    )

    await asyncio.wait(
        [
            _read_stream(
                process.stdout,
                lambda x: print(
                    "STDOUT: {}".format(x.decode("UTF8"))
                ),
            ),
            _read_stream(
                process.stderr,
                lambda x: print(
                    "STDERR: {}".format(x.decode("UTF8"))
                ),
            ),
        ]
    )

    await process.wait()


async def main():
    await run("docker build -t my-docker-image:latest .")


if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

Answer 8

Found this "plug-and-play" function here. Worked like a charm!

import subprocess

def myrun(cmd):
    """from
    http://blog.kagesenshi.org/2008/02/teeing-python-subprocesspopen-output.html
    """
    p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE,
                         stderr=subprocess.STDOUT)
    stdout = []
    while True:
        line = p.stdout.readline()
        stdout.append(line)
        print line,
        if line == '' and p.poll() != None:
            break
    return ''.join(stdout)

Answer 9

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.

(See also here for the same answer.)

Answer 10

I used this solution to get realtime output on a subprocess. This loop will stop as soon as the process completes leaving out a need for a break statement or possible infinite loop.

sub_process = subprocess.Popen(my_command, close_fds=True, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

while sub_process.poll() is None:
    out = sub_process.stdout.read(1)
    sys.stdout.write(out)
    sys.stdout.flush()

Answer 11

This is the basic skeleton that I always use for this. It makes it easy to implement timeouts and is able to deal with inevitable hanging processes.

import subprocess
import threading
import Queue

def t_read_stdout(process, queue):
    """Read from stdout"""

    for output in iter(process.stdout.readline, b''):
        queue.put(output)

    return

process = subprocess.Popen(['dir'],
                           stdout=subprocess.PIPE,
                           stderr=subprocess.STDOUT,
                           bufsize=1,
                           cwd='C:\\',
                           shell=True)

queue = Queue.Queue()
t_stdout = threading.Thread(target=t_read_stdout, args=(process, queue))
t_stdout.daemon = True
t_stdout.start()

while process.poll() is None or not queue.empty():
    try:
        output = queue.get(timeout=.5)

    except Queue.Empty:
        continue

    if not output:
        continue

    print(output),

t_stdout.join()

Answer 12

I ran into the same problem awhile back. My solution was to ditch iterating for the read method, which will return immediately even if your subprocess isn't finished executing, etc.

Answer 13

You may use an iterator over each byte in the output of the subprocess. This allows inline update (lines ending with '\r' overwrite previous output line) from the subprocess:

from subprocess import PIPE, Popen

command = ["my_command", "-my_arg"]

# Open pipe to subprocess
subprocess = Popen(command, stdout=PIPE, stderr=PIPE)


# read each byte of subprocess
while subprocess.poll() is None:
    for c in iter(lambda: subprocess.stdout.read(1) if subprocess.poll() is None else {}, b''):
        c = c.decode('ascii')
        sys.stdout.write(c)
sys.stdout.flush()

if subprocess.returncode != 0:
    raise Exception("The subprocess did not terminate correctly.")

Answer 14

if you just want to forward the log to console in realtime

Below code will work for both

 p = subprocess.Popen(cmd,
                         shell=True,
                         cwd=work_dir,
                         bufsize=1,
                         stdin=subprocess.PIPE,
                         stderr=sys.stderr,
                         stdout=sys.stdout)

Answer 15

Complete solution:

import contextlib
import subprocess

# Unix, Windows and old Macintosh end-of-line
newlines = ['\n', '\r\n', '\r']
def unbuffered(proc, stream='stdout'):
    stream = getattr(proc, stream)
    with contextlib.closing(stream):
        while True:
            out = []
            last = stream.read(1)
            # Don't loop forever
            if last == '' and proc.poll() is not None:
                break
            while last not in newlines:
                # Don't loop forever
                if last == '' and proc.poll() is not None:
                    break
                out.append(last)
                last = stream.read(1)
            out = ''.join(out)
            yield out

def example():
    cmd = ['ls', '-l', '/']
    proc = subprocess.Popen(
        cmd,
        stdout=subprocess.PIPE,
        stderr=subprocess.STDOUT,
        # Make all end-of-lines '\n'
        universal_newlines=True,
    )
    for line in unbuffered(proc):
        print line

example()

Answer 16

Using pexpect with non-blocking readlines will resolve this problem. It stems from the fact that pipes are buffered, and so your app's output is getting buffered by the pipe, therefore you can't get to that output until the buffer fills or the process dies.

Answer 17

Here is what worked for me:

import subprocess
import sys

def run_cmd_print_output_to_console_and_log_to_file(cmd, log_file_path):
    make_file_if_not_exist(log_file_path)
    logfile = open(log_file_path, 'w')

    proc=subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, shell = True)
    for line in proc.stdout:
        sys.stdout.write(line.decode("utf-8") )
        print(line.decode("utf-8").strip(), file=logfile, flush=True)
    proc.wait()

    logfile.close()

Answer 18

These are all great examples, but I've found they either (a) handle partial lines (eg "Are you sure (Y/n):") but are really slow or b) are quick but hang on partial lines.

I've worked on the following which:

• provides real-time output for both stdout and stderr to their respective streams
• is extremely fast as it works with stream buffering
• allows for using timeouts as it never blocks on read()
• efficiently saves stdout and stderr independently
• handles text encoding (though easily adaptable to binary streams)
• works on Python 3.6+

import os
import subprocess
import sys
import selectors
import io

def run_command(command: str) -> (int, str):

    proc = subprocess.Popen(
        command, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE
    )

    sel = selectors.DefaultSelector()
    for fobj in [ proc.stdout, proc.stderr ]:
        os.set_blocking(fobj.fileno(), False)
        sel.register(fobj, selectors.EVENT_READ)

    out=io.StringIO()
    err=io.StringIO()

    # loop until all descriptors removed
    while len(sel.get_map()) > 0:
        events = sel.select()
        if len(events) == 0:
            # timeout or signal, kill to prevent wait hanging
            proc.terminate()
            break
        for key, _ in events:
            # read all available data
            buf = key.fileobj.read().decode(errors='ignore')
            if buf == '':
                sel.unregister(key.fileobj)
            elif key.fileobj == proc.stdout:
                sys.stdout.write(buf)
                sys.stdout.flush()
                out.write(buf)
            elif key.fileobj == proc.stderr:
                sys.stderr.write(buf)
                sys.stderr.flush()
                err.write(buf)

    sel.close()
    proc.wait()
    if proc.returncode != 0:
        return (proc.returncode, err.getvalue())
    return (0, out.getvalue())

I didn't include the timeout logic (as the subject is real-time output), but it's simple to add them to select()/wait() and no longer worry about infinite hangs.

I've timed cat '25MB-file' and compared to the .read(1) solutions, it's roughly 300 times faster.

Answer 19

(This solution has been tested with Python 2.7.15)
You just need to sys.stdout.flush() after each line read/write:

while proc.poll() is None:
    line = proc.stdout.readline()
    sys.stdout.write(line)
    # or print(line.strip()), you still need to force the flush.
    sys.stdout.flush()

Answer 20

Few answers suggesting python 3.x or pthon 2.x , Below code will work for both.

 p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT,)
    stdout = []
    while True:
        line = p.stdout.readline()
        if not isinstance(line, (str)):
            line = line.decode('utf-8')
        stdout.append(line)
        print (line)
        if (line == '' and p.poll() != None):
            break

Answer 21

def run_command(command):
process = subprocess.Popen(shlex.split(command), stdout=subprocess.PIPE)
while True:
    output = process.stdout.readline()
    if output == '' and process.poll() is not None:
        break
    if output:
        print(output.strip())
rc = process.poll()
return rc

Answer 22

Yet another answer! I had the following requirements:

• Run some command and print the output to stdout as though the user ran it
• Display to the user any prompts from the command. E.g. pip uninstall numpy will prompt with ... Proceed (Y/n)? (which does not end in a newline)
• Capture the output (that the user saw) as a string

This worked for me (only tested in Python 3.10 on Windows):

def run(*args: list[str]) -> str:
    proc = subprocess.Popen(
        *args,
        text=True,
        stdout=subprocess.PIPE,
        stderr=subprocess.STDOUT,
    )

    result = ""

    while proc.poll() is None:
        output = proc.stdout.read(1)

        if output:
            sys.stdout.write(output)
            sys.stdout.flush()
            result += output

    return result

Answer 23

Here is my solution:

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

error_output = ""

while True:

    # The empty string is important to fulfill the exit condition (see below)
    stdout_line = ""
    if process.stdout:
        stdout = process.stdout.readline()
        if stdout:
            stdout_line = stdout.decode("utf-8")
            log.debug(stdout_line)

    # The empty string is important to fulfill the exit condition (see below)
    stderr_line = ""
    if process.stderr:
        stderr = process.stderr.readline()
        if stderr:
            stderr_line = stderr.decode("utf-8")
            error_output += stderr_line
            log.debug(stderr_line)

    # It might be the case that the process is finished but reading the
    # output is not finished. This is why we check both conditions:
    # Condition for readline:
    #   https://docs.python.org/3.6/tutorial/inputoutput.html#methods-of-file-objects
    # Condition for poll:
    #   https://docs.python.org/3/library/subprocess.html#subprocess.Popen.poll
    if stdout_line == "" and stderr_line == "" and process.poll() != None:
        break

if process.returncode != 0:
    raise Exception(error_output)

Answer 24

Late answer, but the following works for Python3:

import subprocess
import sys

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

while True:
    out = process.stdout.read(1)
    if process.poll() is not None:
        break
    if out != '':
        sys.stdout.buffer.write(out)
        sys.stdout.flush()