I have a script which runs a subprocess as follows:
child_process = subprocess.Popen(["python", testset['dir'] + testname, \
output_spec_file, plugin_directory],\
stderr=subprocess.PIPE, stdout=subprocess.PIPE)
In that process, I am trying to insert print statements but they are not appearing to stdout. I tried using sys.stdout.write() in that subprocess and then sys.stduout.read() right after child_process but it is not capturing the output.
I am new to Python and I haven't gotten to that level of complexity in Python. I am actually working low level in C and there are some Python test scripts and I'm not sure how to print out from the subprocess.
Any suggestions?
sys.stdout.read (and write) are for standard input/output of the current process (not the subprocess). If you want to write to stdin of the child process, you need to use:
child_process.stdin.write("this goes to child") #Popen(..., stdin=subprocess.PIPE)
and similar for reading from the child's stdout stream:
child_process = subprocess.Popen( ... , stdout=subprocess.PIPE)
child_process.stdout.read("This is the data that comes back")
Of course, it is generally more idiomatic to use:
stdoutdata, stderrdata = child_process.communicate(stdindata)
(taking care to pass subprocess.PIPE to the Popen constructor where appropriate) provided that your input data can be passed all at once.
Related
I have already read the documentation about subprocesses in python, but still cannot quite understand this.
When using Popen, and we set the parameter stdout (or stdin) to subprocesses.PIPE, what does that actually mean?
The documentation says
stdin, stdout and stderr specify the executed program’s standard
input, standard output and standard error file handles,
respectively... PIPE indicates that a new pipe to the child should be
created.
what does this mean?
For example, if I have two subprocesses both with stdout to PIPE, are the ouptuts mixed? (I don't think so)
more importantly, if I have a subprocess with stdout set to PIPE and later another subprocess with stdin set to PIPE , is that pipe the same, the output of one goes to the other?
Can someone explain me that part of the documentation that seems criptic to me?
Additional notes:
For example
import os
import signal
import subprocess
import time
# The os.setsid() is passed in the argument preexec_fn so
# it's run after the fork() and before exec() to run the shell.
pro = subprocess.Popen("sar -u 1 > mylog.log", stdout=subprocess.PIPE,
shell=True, preexec_fn=os.setsid)
// Here another subprocess
subprocess.Popen(some_command, stdin=subprocess.PIPE)
time.sleep(10)
os.killpg(os.getpgid(pro.pid), signal.SIGTERM)
Does the output of sar goes as input to "some command"?
Please see the document.
So as you can see, the PIPE is a special value, it "indicates that a new pipe to the child should be created." Which means, stdout=subprocess.PIPE and stderr=subprocess.PIPE results in two different pipes.
And for your example, the answer is no. These are two different pipes.
Actually you can print out the subprocess.PIPE:
print(subprocess.PIPE)
# -1
print(type(subprocess.PIPE))
# int
# So it is just an integer to represent a special case.
I have a c program (I'm not the author) that reads from stderr. I call it using subprocess.Popen as below. Is there any way to write to stderr of the subprocess.
proc = subprocess.Popen(['./std.bin'],stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
Yes, maybe, but you should be aware of the irregularity of writing to the standard output or standard error output of a subprocess. The vast majority of processes only writes to these and almost none is actually trying to read (because in almost all cases there's nothing to read).
What you could try is to open a socket and supply that as the stderr argument.
What you most probably want to do is the opposite, to read from the stderr from the subprocess (the subprocesses writes, you read). That can be done by just setting it to subprocess.PIPE and then access the stderr attribute of the subprocess:
proc subprocess(['./std.bin'], stderr=subprocess.PIPE)
for l in proc.stderr:
print(l)
Note that you could specify more than one of stdin, stdout and stderr as being subprocess.PIPE. This will not mean that they will be connected to the same pipe (subprocess.PIPE is no actuall file, but just a placeholder to indicate that a pipe should be created). If you do this however you should take care to avoid deadlocks, this can for example be done by using the communicate method (you can inspect the source of the subprocess module to see what communicate does if you want to do it yourself).
If the child process reads from stderr (note: normally stderr is opened for output):
#!/usr/bin/env python
"""Read from *stderr*, write to *stdout* reversed bytes."""
import os
os.write(1, os.read(2, 512)[::-1])
then you could provide a pseudo-tty (so that all streams point to the same place), to work with the child as if it were a normal subprocess:
#!/usr/bin/env python
import sys
import pexpect # $ pip install pexpect
child = pexpect.spawnu(sys.executable, ['child.py'])
child.sendline('abc') # write to the child
child.expect(pexpect.EOF)
print(repr(child.before))
child.close()
Output
u'abc\r\n\r\ncba'
You could also use subprocess + pty.openpty() instead pexpect.
Or you could write a code specific to the weird stderr behavior:
#!/usr/bin/env python
import os
import sys
from subprocess import Popen, PIPE
r, w = os.pipe()
p = Popen([sys.executable, 'child.py'], stderr=r, stdout=PIPE,
universal_newlines=True)
os.close(r)
os.write(w, b'abc') # write to subprocess' stderr
os.close(w)
print(repr(p.communicate()[0]))
Output
'cba'
for line in proc.stderr:
sys.stdout.write(line)
This is write the stderr of the subprocess. Hope it answers your question.
We are having some problems with the dreaded "too many open files" on our Ubuntu Linux machine rrunning a python Twisted application. In many places in our program, we are using subprocess Popen, something like this:
Popen('ifconfig ' + iface, shell=True, stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
output = process.stdout.read()
while in other places we use subprocess communicate:
process = subprocess.Popen(['/usr/bin/env', 'python', self._get_script_path(script_name)],
stdin=subprocess.PIPE,
stdout=subprocess.PIPE,
close_fds=True)
out, err = process.communicate(data)
What exactly do I need to do in both cases in order to close any open file descriptors? Python documentation is not clear on this. From what I gather (which could be wrong) both communicate() and wait() will indeed clean up any open fds on their own. But what about Popen? Do I need to close stdin, stdout, and stderr explicitly after calling Popen if I don't call communicate or wait?
According to this source for the subprocess module (link) if you call communicate you should not need to close the stdout and stderr pipes.
Otherwise I would try:
process.stdout.close()
process.stderr.close()
after you are done using the process object.
For instance, when you call .read() directly:
output = process.stdout.read()
process.stdout.close()
Look in the above module source for how communicate() is defined and you'll see that it closes each pipe after it reads from it, so that is what you should also do.
If you're using Twisted, don't use subprocess. If you were using spawnProcess instead, you wouldn't need to deal with annoying resource-management problems like this.
We have:
A Python based server (A)
A running command-line application (on the same Linux machine) which is able to read stdin, computes something and provides the output into stdout (B)
What is the best (most elegant) way how to send an input from (A) to stdin of (B), and wait for an answer from (B), i.e read its stdout?
If you spawn (B) using Python's subprocess module from the standard library, you can set up (B)'s stdin and stdout as byte buffers readable and writable by (A).
b = Popen(["b.exe"], stdin=PIPE, stdout=PIPE)
b.stdin.write("OHAI\n")
print(b.stdout.readline())
For your given example, it's easiest to use communicate, as that takes care to avoid deadlocks for you:
b = Popen(["b.exe"], stdin=PIPE, stdout=PIPE)
b_out = b.communicate("OHAI\n")[0]
print(b_out)
http://docs.python.org/release/3.1.3/library/subprocess.html
http://docs.python.org/release/3.1.3/library/subprocess.html#subprocess.Popen.communicate
If there's a lot of 2-way communication, you should take care to avoid deadlocks because of full buffers. If your communication pattern gives this type of problem, you should consider using socket communication instead.
As #Deestan pointed subprocess,module, is an elegant and proven one. We use subprocess a lot when we have to run commands from python.
Ours mostly involves running a command, mostly in-house built, and capturing its output. Our wrapper to run such commands looks thus.
import subprocess
def _run_command( _args, input=[],withShell=False):
"""
Pass args as array, like ['echo', 'hello']
Waits for completion and returns
tuple (returncode, stdout, stderr)
"""
p = subprocess.Popen(_args, shell = withShell,
stdin=subprocess.PIPE, stdout=subprocess.PIPE)
[p.stdin.write(v) for v in input]
stdout, stderr = p.communicate()
return p.returncode, stdout, stderr
_,op,er = _run_command(['cat'],["this","is","for","testing"])
value="".join(op)
print value
_,op,er = _run_command(['ls',"/tmp"])
value="".join(op)
print value
If your input to B is minimal then subprocess is a yes.
I have a shell script that gets whois info for domains, and outputs taken or available to the shell depending on the domain.
I'd like to execute the script, and be able to read this value inside my Python script.
I've been playing around with subprocess.call but can't figure out how to get the output.
e.g.,
subprocess.call('myscript www.google.com', shell=True)
will output taken to the shell.
subprocess.call() does not give you the output, only the return code. For the output you should use subprocess.check_output() instead. These are friendly wrappers around the popen family of functions, which you could also use directly.
For more details, see: http://docs.python.org/library/subprocess.html
Manually using stdin and stdout with Popen was such a common pattern that it has been abstracted into a very useful method in the subprocess module: communicate
Example:
p = subprocess.Popen(['myscript', 'www.google.com'], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
(stdoutdata, stderrdata) = p.communicate(input="myinputstring")
# all done!
import subprocess as sp
p = sp.Popen(["/usr/bin/svn", "update"], stdin=sp.PIPE, stdout=sp.PIPE, close_fds=True)
(stdout, stdin) = (p.stdout, p.stdin)
data = stdout.readline()
while data:
# Do stuff with data, linewise.
data = stdout.readline()
stdout.close()
stdin.close()
Is the idiom I use, obviously in this case I was updating an svn repository.
try subprocess.check_output.