We Keep Coding

Prevent subprocess PIPE from blocking

I want to utilize subprocess Popen to call strace on Linux.
I also want to catch every line of output strace gives, in realtime if possible.
I came up with the following code for that, but for some reason I can't get it working. I'll only get the output AFTER I terminate the program.
from threading import Thread
from queue import Queue, Empty
pid = 1
def enqueue_output(out, queue):
for line in iter(out.readline, b''):
queue.put(line)
out.close()
p = Popen(["strace", "-p", pid], stdout=subprocess.PIPE, bufsize=1)
q = Queue()
t = Thread(target=enqueue_output, args=(p.stdout, q))
t.daemon = True # thread dies with the program
t.start()
try:
line = q.get_nowait()
print("Got it! "+line)
except Empty:
pass

Here is a short working example:
Please note that:
strace writes to stderr (unless -o filename is given)
all arguments must be strings (or bytes), i.e. pid must be given as "1"
line buffering works only with universal newlines
you must be root to trace process 1
import subprocess
PID = 1
p = subprocess.Popen(
["strace", "-p", str(PID)],
stdin=subprocess.DEVNULL, stderr=subprocess.PIPE,
universal_newlines=True, bufsize=1)
for line in p.stderr:
line = line.rstrip()
print(line)

Python interact with subprocess

I want to interact with a process.
I can start the process and print out the first two lines (something like 'process successfully started').
Now I want to send a new command to the process which should return again something like 'command done' but nothing happens.
Please help me.
import subprocess
def PrintAndPraseOutput(output, p):
print(output)
if 'sucessfully' in output:
p.stdin.write('command')
cmd = ["./programm"]
p = subprocess.Popen(cmd, universal_newlines=True, shell=False, stdout=subprocess.PIPE, stdin=subprocess.PIPE)
while p.poll() is None:
output = p.stdout.readline()
PrintAndPraseOutput(output, p)
Update:
same problem, no output after 'process successfully started'
import subprocess
def print_and_parse_output(output, p):
print(output)
if 'successfully' in output:
p.stdin.write('command\n')
with subprocess.Popen(["./programm"], universal_newlines=True, shell=False, stdout=subprocess.PIPE, stdin=subprocess.PIPE) as proc:
while proc.poll() is None:
output = proc.stdout.readline()
print_and_parse_output(output, proc)

Your I/O should be line buffered, so PrintAndPraseOutput should send a '\n' at the end of the string.
BTW, you have a couple of spelling errors. That function should be named print_and_parse_output to conform to PEP-0008, and "successfully" has 2 c's.
def print_and_parse_output(output, p):
print(output)
if 'successfully' in output:
p.stdin.write('command\n')
When using subprocess like this it's a good idea to put it in a with statement. From the subprocess.Popen` docs:
Popen objects are supported as context managers via the with
statement: on exit, standard file descriptors are closed, and the
process is waited for.
with Popen(["ifconfig"], stdout=PIPE) as proc:
log.write(proc.stdout.read())

returncode of Popen object is None after the process is terminated

I'm running a process with the use of Popen. I need to wait for the process to terminate. I'm checking that the process have terminated through the returncode. When returncode is different from None the process must have terminated. The problem is that when print_output is False the returncode is always None, even when the process have finished running (terminated). This is however not the case when print_output is True. I'm using the following code to run the process:
def run(command, print_output=True):
# code mostly from: http://sharats.me/the-ever-useful-and-neat-subprocess-module.html
from subprocess import Popen, PIPE
from threading import Thread
from queue import Queue, Empty
from time import sleep
io_q = Queue()
def stream_watcher(identifier, stream):
for line in stream:
io_q.put((identifier, line))
if not stream.closed:
stream.close()
with Popen(command, stdout=PIPE, stderr=PIPE, universal_newlines=True) as proc:
if print_output:
Thread(target=stream_watcher, name='stdout-watcher', args=('STDOUT', proc.stdout)).start()
Thread(target=stream_watcher, name='stderr-watcher', args=('STDERR', proc.stderr)).start()
def printer():
while True:
try:
# Block for 1 second.
item = io_q.get(True, 1)
except Empty:
# No output in either streams for a second. Are we done?
if proc.poll() is not None:
break
else:
identifier, line = item
print(identifier + ':', line, end='')
Thread(target=printer, name='printer').start()
while proc.returncode is None:
sleep(2)
proc.poll()
if not proc.returncode == 0:
raise RuntimeError(
'The process call "{}" returned with code {}. The return code is not 0, thus an error '
'occurred.'.format(list(command), proc.returncode))
return proc.stdout, proc.stderr
Any clues to what might cause this problem?
EDIT: Discovered something pretty weird. I'm running the following code:
run(my_command, True)
print('--------done--------')
run(my_command, False)
print('--------done--------')
'--------done--------' is never printed even though run(my_command, False) gets executed.

TL;DR
add popen.wait() after subprocess.Popen()
Explanation Part (sort of)
Python goes too fast and the child process is ended but returncode can't be read
(I don't really know why it does that. Explanations welcome)
Why did I use this:
Shell command execution and get both return code and output (stdout)
def exec_cmd(cmd):
pop = subprocess.Popen(shlex.split(cmd), stderr=subprocess.STDOUT, stdout=subprocess.PIPE)
pop.wait()
return [pop.returncode, pop.communicate()[0]]
Also: please read the .wait warning on the popen page

I'm not sure why it did not work, but I think it has something to do with not closing the streams. The following code works:
def run(command, print_output=True):
from subprocess import Popen, PIPE, STDOUT
from io import StringIO
popen = Popen(command, stdout=PIPE, stderr=STDOUT, universal_newlines=True)
out = StringIO()
for line in popen.stdout:
if print_output:
print(line, end='')
else:
out.write(line)
popen.stdout.close()
return_code = popen.wait()
if not return_code == 0:
raise RuntimeError(
'The process call "{}" returned with code {}. The return code is not 0, thus an error '
'occurred.'.format(list(command), return_code))
stdout_string = out.getvalue()
out.close()
return stdout_string

Python subprocess output on windows?

I'm running into some difficulties getting output from a subprocess stdout pipe. I'm launching some third party code via it, in order to extract log output. Up until a recent update of the third party code, everything worked fine. After the update, python has started blocking indefinitely, and not actually showing any output. I can manually launch the third party app fine and see output.
A basic version of the code I'm using:
import subprocess, time
from threading import Thread
def enqueue_output(out):
print "Hello from enqueue_output"
for line in iter(out.readline,''):
line = line.rstrip("\r\n")
print "Got %s" % line
out.close()
proc = subprocess.Popen("third_party.exe", stdout=subprocess.PIPE, bufsize=1)
thread = Thread(target=enqueue_output, args=(proc.stdout,))
thread.daemon = True
thread.start()
time.sleep(30)
This works perfectly if I substitute third_party.exe for this script:
import time, sys
while True:
print "Test"
sys.stdout.flush()
time.sleep(1)
So I'm unclear as to magic needs to be done to get this working with the original command.
These are all variants of the subprocess.Popen line I've tried with no success:
proc = subprocess.Popen("third_party.exe", stdout=subprocess.PIPE, bufsize=0)
proc = subprocess.Popen("third_party.exe", stdout=subprocess.PIPE, shell=True)
proc = subprocess.Popen("third_party.exe", stdout=subprocess.PIPE, creationflags=subprocess.CREATE_NEW_CONSOLE)
si = subprocess.STARTUPINFO()
si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW
proc = subprocess.Popen("third_party.exe", stdout=subprocess.PIPE, startupinfo=si)
Edit 1:
I can't actually use .communicate() in this case. The app I'm launching remains running for long periods of time (days to weeks). The only way I could actually test .communicate() would be to kill the app shortly after it launches, which I don't feel would give me valid results.
Even the non-threaded version of this fails:
import subprocess, time
from threading import Thread
proc = subprocess.Popen("third_party.exe", stdout=subprocess.PIPE, stderr=subprocess.PIPE)
print "App started, reading output..."
for line in iter(proc.stdout.readline,''):
line = line.rstrip("\r\n")
print "Got: %s" % line
Edit 2:
Thanks to jdi, the following works okay:
import tempfile, time, subprocess
w = "test.txt"
f = open("test.txt","a")
p = subprocess.Popen("third_party.exe", shell=True, stdout=f,
stderr=subprocess.STDOUT, bufsize=0)
time.sleep(30)
with open("test.txt", 'r') as r:
for line in r:
print line
f.close()

First I would recommend that you simplify this example to make sure you can actually read anything. Remove the complication of the thread from the mix:
proc = subprocess.Popen("third_party.exe", stdout=subprocess.PIPE, bufsize=1)
print proc.communicate()
If that works, great. Then you are having problems possibly with how you are reading the stdout directly or possibly in your thread.
If this does not work, have you tried piping stderr to stdout as well?
proc = subprocess.Popen("third_party.exe",
stdout=subprocess.PIPE,
stderr=subprocess.STDOUT, bufsize=1)
Update
Since you say communicate() is deadlocking, here is another approach you can try to see if its a problem with the internal buffer of subprocess...
import tempfile
import subprocess
w = tempfile.NamedTemporaryFile()
p = subprocess.Popen('third_party.exe', shell=True, stdout=w,
stderr=subprocess.STDOUT, bufsize=0)
with open(w.name, 'r') as r:
for line in r:
print line
w.close()

args = ['svn','log','-v']
def foo(info=''):
import logging
import subprocess
import tempfile
try:
pipe = subprocess.Popen(args,bufsize = 0,\
stdout = subprocess.PIPE,\
stderr=subprocess.STDOUT)
except Exception as e:
logging.error(str(e))
return False
while 1:
s = pipe.stdout.read()
if s:
print s,
if pipe.returncode is None:
pipe.poll()
else:
break
if not 0 == pipe.returncode:
return False
return True
print foo()
This one should works,not thread,temp file magic.

How to replicate tee behavior in Python when using subprocess?

I'm looking for a Python solution that will allow me to save the output of a command in a file without hiding it from the console.
FYI: I'm asking about tee (as the Unix command line utility) and not the function with the same name from Python intertools module.
Details
Python solution (not calling tee, it is not available under Windows)
I do not need to provide any input to stdin for called process
I have no control over the called program. All I know is that it will output something to stdout and stderr and return with an exit code.
To work when calling external programs (subprocess)
To work for both stderr and stdout
Being able to differentiate between stdout and stderr because I may want to display only one of the to the console or I could try to output stderr using a different color - this means that stderr = subprocess.STDOUT will not work.
Live output (progressive) - the process can run for a long time, and I'm not able to wait for it to finish.
Python 3 compatible code (important)
References
Here are some incomplete solutions I found so far:
http://devlishgenius.blogspot.com/2008/10/logging-in-real-time-in-python.html (mkfifo works only on Unix)
http://blog.kagesenshi.org/2008/02/teeing-python-subprocesspopen-output.html (doesn't work at all)
Diagram http://blog.i18n.ro/wp-content/uploads/2010/06/Drawing_tee_py.png
Current code (second try)
#!/usr/bin/python
from __future__ import print_function
import sys, os, time, subprocess, io, threading
cmd = "python -E test_output.py"
from threading import Thread
class StreamThread ( Thread ):
def __init__(self, buffer):
Thread.__init__(self)
self.buffer = buffer
def run ( self ):
while 1:
line = self.buffer.readline()
print(line,end="")
sys.stdout.flush()
if line == '':
break
proc = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stdoutThread = StreamThread(io.TextIOWrapper(proc.stdout))
stderrThread = StreamThread(io.TextIOWrapper(proc.stderr))
stdoutThread.start()
stderrThread.start()
proc.communicate()
stdoutThread.join()
stderrThread.join()
print("--done--")
#### test_output.py ####
#!/usr/bin/python
from __future__ import print_function
import sys, os, time
for i in range(0, 10):
if i%2:
print("stderr %s" % i, file=sys.stderr)
else:
print("stdout %s" % i, file=sys.stdout)
time.sleep(0.1)
Real output
stderr 1
stdout 0
stderr 3
stdout 2
stderr 5
stdout 4
stderr 7
stdout 6
stderr 9
stdout 8
--done--
Expected output was to have the lines ordered. Remark, modifying the Popen to use only one PIPE is not allowed because in the real life I will want to do different things with stderr and stdout.
Also even in the second case I was not able to obtain real-time like out, in fact all the results were received when the process finished. By default, Popen should use no buffers (bufsize=0).

I see that this is a rather old post but just in case someone is still searching for a way to do this:
proc = subprocess.Popen(["ping", "localhost"],
stdout=subprocess.PIPE,
stderr=subprocess.PIPE)
with open("logfile.txt", "w") as log_file:
while proc.poll() is None:
line = proc.stderr.readline()
if line:
print "err: " + line.strip()
log_file.write(line)
line = proc.stdout.readline()
if line:
print "out: " + line.strip()
log_file.write(line)

If requiring python 3.6 isn't an issue there is now a way of doing this using asyncio. This method allows you to capture stdout and stderr separately but still have both stream to the tty without using threads. Here's a rough outline:
class RunOutput:
def __init__(self, returncode, stdout, stderr):
self.returncode = returncode
self.stdout = stdout
self.stderr = stderr
async def _read_stream(stream, callback):
while True:
line = await stream.readline()
if line:
callback(line)
else:
break
async def _stream_subprocess(cmd, stdin=None, quiet=False, echo=False) -> RunOutput:
if isWindows():
platform_settings = {"env": os.environ}
else:
platform_settings = {"executable": "/bin/bash"}
if echo:
print(cmd)
p = await asyncio.create_subprocess_shell(
cmd,
stdin=stdin,
stdout=asyncio.subprocess.PIPE,
stderr=asyncio.subprocess.PIPE,
**platform_settings
)
out = []
err = []
def tee(line, sink, pipe, label=""):
line = line.decode("utf-8").rstrip()
sink.append(line)
if not quiet:
print(label, line, file=pipe)
await asyncio.wait(
[
_read_stream(p.stdout, lambda l: tee(l, out, sys.stdout)),
_read_stream(p.stderr, lambda l: tee(l, err, sys.stderr, label="ERR:")),
]
)
return RunOutput(await p.wait(), out, err)
def run(cmd, stdin=None, quiet=False, echo=False) -> RunOutput:
loop = asyncio.get_event_loop()
result = loop.run_until_complete(
_stream_subprocess(cmd, stdin=stdin, quiet=quiet, echo=echo)
)
return result
The code above was based on this blog post: https://kevinmccarthy.org/2016/07/25/streaming-subprocess-stdin-and-stdout-with-asyncio-in-python/

This is a straightforward port of tee(1) to Python.
import sys
sinks = sys.argv[1:]
sinks = [open(sink, "w") for sink in sinks]
sinks.append(sys.stderr)
while True:
input = sys.stdin.read(1024)
if input:
for sink in sinks:
sink.write(input)
else:
break
I'm running on Linux right now but this ought to work on most platforms.
Now for the subprocess part, I don't know how you want to 'wire' the subprocess's stdin, stdout and stderr to your stdin, stdout, stderr and file sinks, but I know you can do this:
import subprocess
callee = subprocess.Popen(
["python", "-i"],
stdin=subprocess.PIPE,
stdout=subprocess.PIPE,
stderr=subprocess.PIPE,
)
Now you can access callee.stdin, callee.stdout and callee.stderr like normal files, enabling the above "solution" to work. If you want to get the callee.returncode, you'll need to make an extra call to callee.poll().
Be careful with writing to callee.stdin: if the process has exited when you do that, an error may be rised (on Linux, I get IOError: [Errno 32] Broken pipe).

This is how it can be done
import sys
from subprocess import Popen, PIPE
with open('log.log', 'w') as log:
proc = Popen(["ping", "google.com"], stdout=PIPE, encoding='utf-8')
while proc.poll() is None:
text = proc.stdout.readline()
log.write(text)
sys.stdout.write(text)

If you don't want to interact with the process you can use the subprocess module just fine.
Example:
tester.py
import os
import sys
for file in os.listdir('.'):
print file
sys.stderr.write("Oh noes, a shrubbery!")
sys.stderr.flush()
sys.stderr.close()
testing.py
import subprocess
p = subprocess.Popen(['python', 'tester.py'], stdout=subprocess.PIPE,
stdin=subprocess.PIPE, stderr=subprocess.PIPE)
stdout, stderr = p.communicate()
print stdout, stderr
In your situation you can simply write stdout/stderr to a file first. You can send arguments to your process with communicate as well, though I wasn't able to figure out how to continually interact with the subprocess.

On Linux, if you really need something like the tee(2) syscall, you can get it like this:
import os
import ctypes
ld = ctypes.CDLL(None, use_errno=True)
SPLICE_F_NONBLOCK = 0x02
def tee(fd_in, fd_out, length, flags=SPLICE_F_NONBLOCK):
result = ld.tee(
ctypes.c_int(fd_in),
ctypes.c_int(fd_out),
ctypes.c_size_t(length),
ctypes.c_uint(flags),
)
if result == -1:
errno = ctypes.get_errno()
raise OSError(errno, os.strerror(errno))
return result
To use this, you probably want to use Python 3.10 and something with os.splice (or use ctypes in the same way to get splice). See the tee(2) man page for an example.

My solution isn't elegant, but it works.
You can use powershell to gain access to "tee" under WinOS.
import subprocess
import sys
cmd = ['powershell', 'ping', 'google.com', '|', 'tee', '-a', 'log.txt']
if 'darwin' in sys.platform:
cmd.remove('powershell')
p = subprocess.Popen(cmd)
p.wait()