Develop Reference

Print subprocess stdout line by line in real time in Django StreamHttpResponse

I wrote an API in Django. It runs a shell script in the server and prints some logs. When the client calls the API in the browser, I want the browser to print the logs line by line. So far, the snippet is like:
from django.http import StreamingHttpResponse
import subprocess
import shlex
def test(request):
cmd = 'bash test.sh'
args = shlex.split(cmd)
proc = subprocess.Popen(
args,
cwd='/foo/bar',
shell=False,
stdout=subprocess.PIPE,
stderr=subprocess.STDOUT,
)
return StreamingHttpResponse(
(line for line in iter(proc.stdout.readline, '')),
content_type="text/plain",
)
And test.sh is like
#!/bin/bash
echo hello
sleep 1
echo world
sleep 1
echo love
sleep 1
echo and
sleep 1
echo peace
sleep 1
The logs are not printed until bash test.sh is finished. How can I make it print the logs as if test.sh is run in the client?
I'm using Python 2.7 and Django 1.11.29. I know they are old, but I still have to use them now. Thanks for help.

You provide the content to StreamingHttpResponse as:
(line for line in iter(proc.stdout.readline, ''))
Here you make an iterable and at iterate over it at the same time and return a generator. Although this won't really matter and you should be getting real time output. The problem may simply be that the output is too fast to actually see the real time difference or perhaps your configuration is incorrect (See Streaming HTTP response, flushing to the browser), Also another problem can be that you are not using the response as a stream with javascript and instead are downloading it fully before using it. You can try using time.sleep() to see if it is just too fast or instead a configuration problem:
import time
def iter_subproc_with_sleep(proc):
for line in iter(proc.stdout.readline, ''):
time.sleep(1)
yield line
def test(request):
cmd = 'bash test.sh'
args = shlex.split(cmd)
proc = subprocess.Popen(
args,
cwd='/foo/bar',
shell=False,
stdout=subprocess.PIPE,
stderr=subprocess.STDOUT,
)
return StreamingHttpResponse(
iter_subproc_with_sleep(proc),
content_type="text/plain",
)

How to create a subprocess in Python, send multiple commands based on previous output

I am writing a program which initiates a connection to a remote machine, then dynamically sending multiple commands to it by monitoring the response. Instead of using pexpect, what else can I use? I am trying to use subprocess.Popen, but communicate() method will kill the process.
Pexpect version: 2.4, http://www.bx.psu.edu/~nate/pexpect/pexpect.html
Referring to the API for subprocess in:
https://docs.python.org/2/library/subprocess.html#subprocess.Popen.communicate
Popen.communicate(input=None)
Interact with process: Send data to stdin. Read data from stdout and stderr, until end-of-file is reached. Wait for process to terminate. The optional input argument should be a string to be sent to the child process, or None, if no data should be sent to the child.
Thanks

Refer the subprocess documentation to understand the basics here
You could do something like this ...
Again, this is just a pointer... this approach may/may not be a best fit for your use case.
Explore -> and Test to find what works for you!
import shlex
import subprocess
import sys
class Command(object):
""" Generic Command Interface ."""
def execute(self, cmd):
proc = subprocess.Popen(shlex.split(cmd), stdout=subprocess.PIPE)
stdout_value = proc.communicate()[0]
exit_value = proc.poll()
if exit_value:
logger.error('Command execution failed. Command : %s' % cmd)
return exit_value, stdout_value
if __name__ == '__main__':
cmd = Command()
host = '' # HOSTNAME GOES HERE
cmd_str = '' # YOUR COMMAND GOES HERE
cmdline = 'ksh -c "ssh root#{0} "{1}""'.format(host, cmd_str)
exit_value, stdout_value = cmd.execute(cmdline)
if exit_value == 0:
# execute other command/s
# you basically use the same logic as above
else:
# return Or execute other command/s

Flask and subprocess Popen - Send same data to multiple users

The code below is almost what I need. But when second user connect, new subprocess.Popen created, i need run only one subprocess.Popen and send same data to multiple users.
Example: The first user connect, start subprocess.Popen, he begins to receive the result starting from the number 0, when second user connect 30 seconds after, he begins to receive result starting from de number 30.
#!/usr/bin/env python
import os
from functools import partial
from subprocess import Popen, PIPE
from flask import Flask, Response # $ pip install flask
file = 'test.file'
app = Flask(__name__)
#app.route('/' + file)
def stream():
process = Popen([ "bash", "-c", "for ((i=0;i<100;i=i+1)); do echo $i; sleep 1; done" ], stdout=PIPE, bufsize=-1)
read_chunk = partial(os.read, process.stdout.fileno(), 1024)
return Response(iter(read_chunk, b''), mimetype='audio/mp3')
if __name__ == "__main__":
app.run(host='0.0.0.0',threaded=True)

To be honest I am not sure if this will work. I do not use the subprocess module very much, so I am not sure if this is an appropriate use case. The question in general reminds me of flask extensions.
I was trying to suggest you use a similar pattern to flask extensions...
check if resource exists
if it does not, create it
return it
Store it on a Flask global, which is the API recommendation for extension development.
edit: realized your route was by file name so changed keys to reflect that
#!/usr/bin/env python
import os
from functools import partial
from subprocess import Popen, PIPE
from flask import Flask, Response, _app_ctx_stack as stack
file = 'test.file'
app = Flask(__name__)
def get_chunk(file):
ctx = stack.top
key = "read_chunk_%s" % file
if ctx is not None:
if not hasattr(ctx, key):
process = Popen(["bash", "-c", "for ((i=0;i<100;i=i+1)); do echo $i; sleep 1; done"],
stdout=PIPE, bufsize=-1)
setattr(ctx, key, partial(os.read, process.stdout.fileno(), 1024))
return getattr(ctx, key)
#app.route('/' + file)
def stream():
read_chunk = get_chunk(file)
return Response(iter(read_chunk, b''), mimetype='audio/mp3')
if __name__ == "__main__":
app.run(host='0.0.0.0',threaded=True)

How to control background process in linux

I need to write a script in Linux which can start a background process using one command and stop the process using another.
The specific application is to take userspace and kernel logs for android.
following command should start taking logs
$ mylogscript start
following command should stop the logging
$ mylogscript stop
Also, the commands should not block the terminal. For example, once I send the start command, the script run in background and I should be able to do other work on terminal.
Any pointers on how to implement this in perl or python would be helpful.
EDIT:
Solved: https://stackoverflow.com/a/14596380/443889

I got the solution to my problem. Solution essentially includes starting a subprocess in python and sending a signal to kill the process when done.
Here is the code for reference:
#!/usr/bin/python
import subprocess
import sys
import os
import signal
U_LOG_FILE_PATH = "u.log"
K_LOG_FILE_PATH = "k.log"
U_COMMAND = "adb logcat > " + U_LOG_FILE_PATH
K_COMMAND = "adb shell cat /proc/kmsg > " + K_LOG_FILE_PATH
LOG_PID_PATH="log-pid"
def start_log():
if(os.path.isfile(LOG_PID_PATH) == True):
print "log process already started, found file: ", LOG_PID_PATH
return
file = open(LOG_PID_PATH, "w")
print "starting log process: ", U_COMMAND
proc = subprocess.Popen(U_COMMAND,
stdout=subprocess.PIPE, stderr=subprocess.PIPE,
shell=True, preexec_fn=os.setsid)
print "log process1 id = ", proc.pid
file.write(str(proc.pid) + "\n")
print "starting log process: ", K_COMMAND
proc = subprocess.Popen(K_COMMAND,
stdout=subprocess.PIPE, stderr=subprocess.PIPE,
shell=True, preexec_fn=os.setsid)
print "log process2 id = ", proc.pid
file.write(str(proc.pid) + "\n")
file.close()
def stop_log():
if(os.path.isfile(LOG_PID_PATH) != True):
print "log process not started, can not find file: ", LOG_PID_PATH
return
print "terminating log processes"
file = open(LOG_PID_PATH, "r")
log_pid1 = int(file.readline())
log_pid2 = int(file.readline())
file.close()
print "log-pid1 = ", log_pid1
print "log-pid2 = ", log_pid2
os.killpg(log_pid1, signal.SIGTERM)
print "logprocess1 killed"
os.killpg(log_pid2, signal.SIGTERM)
print "logprocess2 killed"
subprocess.call("rm " + LOG_PID_PATH, shell=True)
def print_usage(str):
print "usage: ", str, "[start|stop]"
# Main script
if(len(sys.argv) != 2):
print_usage(sys.argv[0])
sys.exit(1)
if(sys.argv[1] == "start"):
start_log()
elif(sys.argv[1] == "stop"):
stop_log()
else:
print_usage(sys.argv[0])
sys.exit(1)
sys.exit(0)

There are a couple of different approaches you can take on this:
1. Signal - you use a signal handler, and use, typically "SIGHUP" to signal the process to restart ("start"), SIGTERM to stop it ("stop").
2. Use a named pipe or other IPC mechanism. The background process has a separate thread that simply reads from the pipe, and when something comes in, acts on it. This method relies on having a separate executable file that opens the pipe, and sends messages ("start", "stop", "set loglevel 1" or whatever you fancy).
I'm sorry, I haven't implemented either of these in Python [and perl I haven't really written anything in], but I doubt it's very hard - there's bound to be a ready-made set of python code to deal with named pipes.
Edit: Another method that just struck me is that you simply daemonise the program at start, and then let the "stop" version find your deamonized process [e.g. by reading the "pidfile" that you stashed somewhere suitable], and then sends a SIGTERM for it to terminate.

I don't know if this is the optimum way to do it in perl, but for example:
system("sleep 60 &")
This starts a background process that will sleep for 60 seconds without blocking the terminal. The ampersand in shell means to do something in the background.
A simple mechanism for telling the process when to stop is to have it periodically check for the existence of a certain file. If the file exists, it exits.

How can I run an external command asynchronously from Python?

I need to run a shell command asynchronously from a Python script. By this I mean that I want my Python script to continue running while the external command goes off and does whatever it needs to do.
I read this post:
Calling an external command in Python
I then went off and did some testing, and it looks like os.system() will do the job provided that I use & at the end of the command so that I don't have to wait for it to return. What I am wondering is if this is the proper way to accomplish such a thing? I tried commands.call() but it will not work for me because it blocks on the external command.
Please let me know if using os.system() for this is advisable or if I should try some other route.

subprocess.Popen does exactly what you want.
from subprocess import Popen
p = Popen(['watch', 'ls']) # something long running
# ... do other stuff while subprocess is running
p.terminate()
(Edit to complete the answer from comments)
The Popen instance can do various other things like you can poll() it to see if it is still running, and you can communicate() with it to send it data on stdin, and wait for it to terminate.

If you want to run many processes in parallel and then handle them when they yield results, you can use polling like in the following:
from subprocess import Popen, PIPE
import time
running_procs = [
Popen(['/usr/bin/my_cmd', '-i %s' % path], stdout=PIPE, stderr=PIPE)
for path in '/tmp/file0 /tmp/file1 /tmp/file2'.split()]
while running_procs:
for proc in running_procs:
retcode = proc.poll()
if retcode is not None: # Process finished.
running_procs.remove(proc)
break
else: # No process is done, wait a bit and check again.
time.sleep(.1)
continue
# Here, `proc` has finished with return code `retcode`
if retcode != 0:
"""Error handling."""
handle_results(proc.stdout)
The control flow there is a little bit convoluted because I'm trying to make it small -- you can refactor to your taste. :-)
This has the advantage of servicing the early-finishing requests first. If you call communicate on the first running process and that turns out to run the longest, the other running processes will have been sitting there idle when you could have been handling their results.

This is covered by Python 3 Subprocess Examples under "Wait for command to terminate asynchronously". Run this code using IPython or python -m asyncio:
import asyncio
proc = await asyncio.create_subprocess_exec(
'ls','-lha',
stdout=asyncio.subprocess.PIPE,
stderr=asyncio.subprocess.PIPE)
# do something else while ls is working
# if proc takes very long to complete, the CPUs are free to use cycles for
# other processes
stdout, stderr = await proc.communicate()
The process will start running as soon as the await asyncio.create_subprocess_exec(...) has completed. If it hasn't finished by the time you call await proc.communicate(), it will wait there in order to give you your output status. If it has finished, proc.communicate() will return immediately.
The gist here is similar to Terrels answer but I think Terrels answer appears to overcomplicate things.
See asyncio.create_subprocess_exec for more information.

What I am wondering is if this [os.system()] is the proper way to accomplish such a thing?
No. os.system() is not the proper way. That's why everyone says to use subprocess.
For more information, read http://docs.python.org/library/os.html#os.system
The subprocess module provides more
powerful facilities for spawning new
processes and retrieving their
results; using that module is
preferable to using this function. Use
the subprocess module. Check
especially the Replacing Older
Functions with the subprocess Module
section.

The accepted answer is very old.
I found a better modern answer here:
https://kevinmccarthy.org/2016/07/25/streaming-subprocess-stdin-and-stdout-with-asyncio-in-python/
and made some changes:
make it work on windows
make it work with multiple commands
import sys
import asyncio
if sys.platform == "win32":
asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())
async def _read_stream(stream, cb):
while True:
line = await stream.readline()
if line:
cb(line)
else:
break
async def _stream_subprocess(cmd, stdout_cb, stderr_cb):
try:
process = await asyncio.create_subprocess_exec(
*cmd, stdout=asyncio.subprocess.PIPE, stderr=asyncio.subprocess.PIPE
)
await asyncio.wait(
[
_read_stream(process.stdout, stdout_cb),
_read_stream(process.stderr, stderr_cb),
]
)
rc = await process.wait()
return process.pid, rc
except OSError as e:
# the program will hang if we let any exception propagate
return e
def execute(*aws):
""" run the given coroutines in an asyncio loop
returns a list containing the values returned from each coroutine.
"""
loop = asyncio.get_event_loop()
rc = loop.run_until_complete(asyncio.gather(*aws))
loop.close()
return rc
def printer(label):
def pr(*args, **kw):
print(label, *args, **kw)
return pr
def name_it(start=0, template="s{}"):
"""a simple generator for task names
"""
while True:
yield template.format(start)
start += 1
def runners(cmds):
"""
cmds is a list of commands to excecute as subprocesses
each item is a list appropriate for use by subprocess.call
"""
next_name = name_it().__next__
for cmd in cmds:
name = next_name()
out = printer(f"{name}.stdout")
err = printer(f"{name}.stderr")
yield _stream_subprocess(cmd, out, err)
if __name__ == "__main__":
cmds = (
[
"sh",
"-c",
"""echo "$SHELL"-stdout && sleep 1 && echo stderr 1>&2 && sleep 1 && echo done""",
],
[
"bash",
"-c",
"echo 'hello, Dave.' && sleep 1 && echo dave_err 1>&2 && sleep 1 && echo done",
],
[sys.executable, "-c", 'print("hello from python");import sys;sys.exit(2)'],
)
print(execute(*runners(cmds)))
It is unlikely that the example commands will work perfectly on your system, and it doesn't handle weird errors, but this code does demonstrate one way to run multiple subprocesses using asyncio and stream the output.

I've had good success with the asyncproc module, which deals nicely with the output from the processes. For example:
import os
from asynproc import Process
myProc = Process("myprogram.app")
while True:
# check to see if process has ended
poll = myProc.wait(os.WNOHANG)
if poll is not None:
break
# print any new output
out = myProc.read()
if out != "":
print out

Using pexpect with non-blocking readlines is another way to do this. Pexpect solves the deadlock problems, allows you to easily run the processes in the background, and gives easy ways to have callbacks when your process spits out predefined strings, and generally makes interacting with the process much easier.

Considering "I don't have to wait for it to return", one of the easiest solutions will be this:
subprocess.Popen( \
[path_to_executable, arg1, arg2, ... argN],
creationflags = subprocess.CREATE_NEW_CONSOLE,
).pid
But... From what I read this is not "the proper way to accomplish such a thing" because of security risks created by subprocess.CREATE_NEW_CONSOLE flag.
The key things that happen here is use of subprocess.CREATE_NEW_CONSOLE to create new console and .pid (returns process ID so that you could check program later on if you want to) so that not to wait for program to finish its job.

I have the same problem trying to connect to an 3270 terminal using the s3270 scripting software in Python. Now I'm solving the problem with an subclass of Process that I found here:
http://code.activestate.com/recipes/440554/
And here is the sample taken from file:
def recv_some(p, t=.1, e=1, tr=5, stderr=0):
if tr < 1:
tr = 1
x = time.time()+t
y = []
r = ''
pr = p.recv
if stderr:
pr = p.recv_err
while time.time() < x or r:
r = pr()
if r is None:
if e:
raise Exception(message)
else:
break
elif r:
y.append(r)
else:
time.sleep(max((x-time.time())/tr, 0))
return ''.join(y)
def send_all(p, data):
while len(data):
sent = p.send(data)
if sent is None:
raise Exception(message)
data = buffer(data, sent)
if __name__ == '__main__':
if sys.platform == 'win32':
shell, commands, tail = ('cmd', ('dir /w', 'echo HELLO WORLD'), '\r\n')
else:
shell, commands, tail = ('sh', ('ls', 'echo HELLO WORLD'), '\n')
a = Popen(shell, stdin=PIPE, stdout=PIPE)
print recv_some(a),
for cmd in commands:
send_all(a, cmd + tail)
print recv_some(a),
send_all(a, 'exit' + tail)
print recv_some(a, e=0)
a.wait()

There are several answers here but none of them satisfied my below requirements:
I don't want to wait for command to finish or pollute my terminal with subprocess outputs.
I want to run bash script with redirects.
I want to support piping within my bash script (for example find ... | tar ...).
The only combination that satiesfies above requirements is:
subprocess.Popen(['./my_script.sh "arg1" > "redirect/path/to"'],
stdout=subprocess.PIPE,
stderr=subprocess.PIPE,
shell=True)