I tried both pexpect and subprocess.Popen from python to call an external long term background process (this process use socket to communicate with external applications), with following details.
subprocess.Popen(launchcmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
This works fine. I do not need to do anything else. However, because I have to get the output immediately, I choose pexpect to avoid the pipe file buffer problem.
obj= pexpect.spawn(launchcmd, timeout=None)
after launching external process, I use a separate thread to do "readline" to read the output of the launched process "obj", and everything is ok.
obj= pexpect.spawn(launchcmd, timeout=None)
after launching external process, I did nothing further, i.e., just leave it there. Although, by using the "ps -e" command I can find the launched process, but the launched process seems blocked and cannot communicate in sockets with other applications.
OK. To be more specific, I put some sample code to formulate my question.
import subprocess
import pexpect
import os
t=1
while(True):
if(t==1):
background_process="./XXX.out"
launchcmd = [background_process]
#---option 3--------
p=pexpect.spawn(launchcmd, timeout=None) # process launced, problem with socket.
#---option 1--------
p=subprocess.Popen(launchcmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE) # process launced, everything fine
t=0
Could anyone tell me what's wrong with the 3rd option? And if it is due to the fact that I did not use a separate thread to manipulate the output, why 1st option works with subprocess.popen? I suspect there is something wrong with pexpect to launch a process using socket, but I am not sure, especially considering option 2 works well.
I think that you are making this too complicated.
Yes, it is a good idea to use a pty instead of a pipe to communicate with the background process because most applications recognize tty/pty devices and switch to using unbuffered output, (or at least line-buffered).
But why pexpect? Just use Python's pty module. First call openpty to get some filehandles and then use Popen to spawn the process. Example code is found in the following question (the answer with the green checkmark) Python Run a daemon sub-process & read stdout
Related
I have a web server that needs to manage a separate multi-process subprocess (i.e. starting it and killing it).
For Unix-based systems, the following works fine:
# save the pid as `pid`
ps = subprocess.Popen(cmd, preexec_fn=os.setsid)
# elsewhere:
os.killpg(os.getpgid(pid), signal.SIGTERM)
I'm doing it this way (with os.setsid) because otherwise killing the progress group will also kill the web server.
On Windows these os functions are not available -- so if I wanted to accomplish something similar on Windows, how would I do this?
I'm using Python 3.5.
THIS ANSWER IS PROVIDED BY eryksun IN COMMENT. I PUT IT HERE TO HIGHLIGHT IT FOR SOMEONE MAY ALSO GET INVOLVED IN THIS PROBLEM。
Here is what he said:
You can create a new process group via ps = subprocess.Popen(cmd, creationflags=subprocess.CREATE_NEW_PROCESS_GROUP). The group ID is the process ID of the lead process. That said, it's only useful for processes in the tree that are attached to the same console (conhost.exe instance) as your process, if your process even has a console. In this case, you can send the group a Ctrl+Break via ps.send_signal(signal.CTRL_BREAK_EVENT). Processes shouldn't ignore Ctrl+Break. They should either exit gracefully or let the default handler execute, which calls ExitProcess(STATUS_CONTROL_C_EXIT)
I tried it with this and succeed:
process = Popen(args=shlex.split(command), shell=shell, cwd=cwd, stdout=PIPE, stderr=PIPE,creationflags=subprocess.CREATE_NEW_PROCESS_GROUP)
/*...*/
process .send_signal(signal.CTRL_BREAK_EVENT)
process .kill()
I am writing a test framework in Python for a command line application. The application will create directories, call other shell scripts in the current directory and will output on the Stdout.
I am trying to treat {Python-SubProcess, CommandLine} combo as equivalent to {Selenium, Browser}. The first component plays something on the second and checks if the output is expected. I am facing the following problems
The Popen construct takes a command and returns back after that command is completed. What I want is a live handle to the process so I can run further commands + verifications and finally close the shell once done
I am okay with writing some infrastructure code for achieveing this since we have a lot of command line applications that need testing like this.
Here is a sample code that I am running
p = subprocess.Popen("/bin/bash", cwd = test_dir)
p.communicate(input = "hostname") --> I expect the hostname to be printed out
p.communicate(input = "time") --> I expect current time to be printed out
but the process hangs or may be I am doing something wrong. Also how do I "grab" the output of that sub process so I can assert that something exists?
subprocess.Popen allows you to continue execution after starting a process. The Popen objects expose wait(), poll() and many other methods to communicate with a child process when it is running. Isn't it what you need?
See Popen constructor and Popen objects description for details.
Here is a small example that runs Bash on Unix systems and executes a command:
from subprocess import Popen, PIPE
p = Popen (['/bin/sh'], stdout=PIPE, stderr=PIPE, stdin=PIPE)
sout, serr = p.communicate('ls\n')
print 'OUT:'
print sout
print 'ERR:'
print serr
UPD: communicate() waits for process termination. If you do not need that, you may use the appropriate pipes directly, though that usually gives you rather ugly code.
UPD2: You updated the question. Yes, you cannot call communicate twice for a single process. You may either give all commands you need to execute in a single call to communicate and check the whole output, or work with pipes (Popen.stdin, Popen.stdout, Popen.stderr). If possible, I strongly recommend the first solution (using communicate).
Otherwise you will have to put a command to input and wait for some time for desired output. What you need is non-blocking read to avoid hanging when there is nothing to read. Here is a recipe how to emulate a non-blocking mode on pipes using threads. The code is ugly and strangely complicated for such a trivial purpose, but that's how it's done.
Another option could be using p.stdout.fileno() for select.select() call, but that won't work on Windows (on Windows select operates only on objects originating from WinSock). You may consider it if you are not on Windows.
Instead of using plain subprocess you might find Python sh library very useful:
http://amoffat.github.com/sh/
Here is an example how to build in an asynchronous interaction loop with sh:
http://amoffat.github.com/sh/tutorials/2-interacting_with_processes.html
Another (old) library for solving this problem is pexpect:
http://www.noah.org/wiki/pexpect
I'm writing a Python script that will use subprocesses. The main idea is to have one parent script that runs specialised child scripts, which e.g. run other programs or do some stuff on their own. There are pipes between parent script and subprocesses. I use them to control whether subprocess is still responding by sending some characters on regular basis and checking the response. The problem is that when the subprocess prints anything on screen (i.e. writes to stdout or stderr), the pipes are broken and everything crashes. So my main question is whether it is possible to block writing to std* in the subprocess, so only legitimate response written to pipe would be possible? I have already tried Stop a function from writing to stdout but without any success.
Also other ideas for communcation between parent and subprocess are welcome (except file based pipes). However, the subprocesses must be used.
I strongly believe that you do not just have to accept "that when the subprocess prints anything on screen (i.e. writes to stdout or stderr), the pipes are broken and everything crashes". You can solve this problem. Then you do not need to "block" the subprocesses from writing to standard streams.
Make proper use of all the power of the subprocess module. First of all, connect a subprocess.PIPE to each of the standard streams of a subprocess:
p = subprocess.Popen(
[executable, arg1, arg2],
stdin=subprocess.PIPE,
stdout=subprocess.PIPE,
stderr=subprocess.PIPE)
Run the subprocess and interact with it through those pipes:
stdout, stderr = p.communicate(stdin="command")
If communicate() is not flexible enough (if you need to monitor several subprocesses at the same time and/or if the stdin data to a certain subprocess depends on its output in response to a previous command) you can directly interact with the p.stdout, p.stderr, p.stdin attributes. In this case, you will likely have to build your own monitoring loop and make use of p.poll() and/or p.returncode. Controlling the subprocesses can also be realized via p.send_signal().
You can pass a function to subprocess.Popen that is executed prior to executing the requested program:
def close_std():
os.close(0)
os.close(1)
os.close(2)
p = subprocess.Popen(cmd, preexec_fn=close_std)
Note the use of low-level os.close; closing sys.std* will only have effect in the forked Python process. Also, be aware that if your underlying programs are Python scripts, they may die due to an exception when they try to write to closed file descriptors.
I'd like to execute multiple commands in a standalone application launched from a python script, using pipes. The only way I could reliably pass the commands to the stdin of the program was using Popen.communicate but it closes the program after the command gets executed. If I use Popen.stdin.write than the command executes only 1 time out of 5 or so, it does not work reliable. What am I doing wrong?
To elaborate a bit :
I have an application that listens to stdin for commands and executes them line by line.
I'd like to be able to run the application and pass various commands to it, based on the users interaction with a GUI.
This is a simple test example:
import os, string
from subprocess import Popen, PIPE
command = "anApplication"
process = Popen(command, shell=False, stderr=None, stdin=PIPE)
process.stdin.write("doSomething1\n")
process.stdin.flush()
process.stdin.write("doSomething2\n")
process.stdin.flush()
I'd expect to see the result of both commands but I don't get any response. (If I execute one of the Popen.write lines multiple times it occasionally works.)
And if I execute:
process.communicate("doSomething1")
it works perfectly but the application terminates.
If I understand your problem correctly, you want to interact (i.e. send commands and read the responses) with a console application.
If so, you may want to check an Expect-like library, like pexpect for Python: http://pexpect.sourceforge.net
It will make your life easier, because it will take care of synchronization, the problem that ddaa also describes. See also:
http://www.noah.org/wiki/Pexpect#Q:_Why_not_just_use_a_pipe_.28popen.28.29.29.3F
The real issue here is whether the application is buffering its output, and if it is whether there's anything you can do to stop it. Presumably when the user generates a command and clicks a button on your GUI you want to see the output from that command before you require the user to enter the next.
Unfortunately there's nothing you can do on the client side of subprocess.Popen to ensure that when you have passed the application a command the application is making sure that all output is flushed to the final destination. You can call flush() all you like, but if it doesn't do the same, and you can't make it, then you are doomed to looking for workarounds.
Your code in the question should work as is. If it doesn't then either your actual code is different (e.g., you might use stdout=PIPE that may change the child buffering behavior) or it might indicate a bug in the child application itself such as the read-ahead bug in Python 2 i.e., your input is sent correctly by the parent process but it is stuck in the child's internal input buffer.
The following works on my Ubuntu machine:
#!/usr/bin/env python
import time
from subprocess import Popen, PIPE
LINE_BUFFERED = 1
#NOTE: the first argument is a list
p = Popen(['cat'], bufsize=LINE_BUFFERED, stdin=PIPE,
universal_newlines=True)
with p.stdin:
for cmd in ["doSomething1\n", "doSomethingElse\n"]:
time.sleep(1) # a delay to see that the commands appear one by one
p.stdin.write(cmd)
p.stdin.flush() # use explicit flush() to workaround
# buffering bugs on some Python versions
rc = p.wait()
It sounds like your application is treating input from a pipe in a strange way. This means it won't get all of the commands you send until you close the pipe.
So the approach I would suggest is just to do this:
process.stdin.write("command1\n")
process.stdin.write("command2\n")
process.stdin.write("command3\n")
process.stdin.close()
It doesn't sound like your Python program is reading output from the application, so it shouldn't matter if you send the commands all at once like that.
I'm writing a GUI for using the oracle exp/imp commands and starting sql-scripts through sqlplus. The subprocess class makes it easy to launch the commands, but I need some additional functionality. I want to get rid of the command prompt when using my wxPython GUI, but I still need a way to show the output of the exp/imp commands.
I already tried these two methods:
command = "exp userid=user/pwd#nsn file=dump.dmp"
process = subprocess.Popen(command, stdout=subprocess.PIPE)
output = process.communicate()[0]
process = subprocess.Popen(command, stdout=subprocess.PIPE)
process.wait()
output = process.stdout.read()
Through one of these methods (forgot which one) I really got the output of exp/imp, but only after the command finishes, which is quite worthless to me, as I need a frequent update during these potentially long running operations. And sqlplus made even more problems, as sqlplus mostly wants some input when an error occurs. When this happens python waits for the process to finish but the user can't see the prompt, so you don't know how long to wait or what to do...
What I'd like to have is a wrapper that outputs everything I can see on the standard commandline. I want to log this to a file and show it inside a wxPython control.
I also tried the code from this page: http://code.activestate.com/recipes/440554/
but this can't read the output either.
The OutputWrapper from this answer doesn't work either: How can I capture all exceptions from a wxPython application?
Any help would be appreciated!
EDIT:
The subprocesses don't seem to flush their output. I already tried it with .readline().
My Tool has to run on windows and unix, so pexpect is no solution if there's no windows version. And using cx_oracle would be extreme overkill as I would have to rebuild the whole functionality of exp, imp and sqlplus.
The solution is to use a list for your command
command = ["exp", "userid=user/pwd#nsn", "file=dump.dmp"]
process = subprocess.Popen(command, stdout=subprocess.PIPE)
then you read process.stdout in a line-by-line basis:
line = process.stdout.readline()
that way you can update the GUI without waiting. IF the subprocess you are running (exp) flushes output. It is possible that the output is buffered, then you won't see anything until the output buffer is full. If that is the case then you are probably out of luck.
If you're on Linux, check out pexpect. It does exactly what you want.
If you need to work on Windows, maybe you should bite the bullet and use Python bindings to Oracle, such as cx_Oracle, instead of running CL stuff via subprocess.
Are these solutions able to capture stderr as well? I see you have stdout= option above. How do you make sure to get stderr as well? Another question is is there a way to use import logging/import logging.handlers to capture command stdout/stderr. It would be interesting to be able to use the logger with its buildt in formatters/rotaters,etc.
Try this:
import subprocess
command = "ping google.com"
process = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE)
output = process.stdout
while 1:
print output.readline(),