Python sending command over a socket

Question

I'm having a bit of trouble. I want to create a simple program that connects to the server and executes a command using subprocess then returns the result to the client. It's simple but I can't get it to work. Right now this is what I have: client:

import sys, socket, subprocess
conn = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
host = sys.argv[1]
port = int(sys.argv[2])
socksize = 1024
conn.connect((host, port))
while True:
    shell = raw_input("$ ")
    conn.send(shell)
    data = conn.recv(socksize)
    #msglen = len(data)
    output = data
    iotype = subprocess.PIPE
    cmd = ['/bin/sh', '-c', shell]
    proc = subprocess.Popen(cmd, stdout=iotype).wait()
    stdout,stderr = proc.communicate()
    conn.send(stdout)
    print(output)
    if proc.returncode != 0:
        print("Error")

server:

import sys, socket, subprocess
host = ''               
port = 50106
socksize = 1024

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((host, port))
print("Server started on port: %s" %port)
s.listen(1)
print("Now listening...\n")
conn, addr = s.accept()
while True:
    print 'New connection from %s:%d' % (addr[0], addr[1])
    data = conn.recv(socksize)
    cmd = ['/bin/sh', '-c', data]
    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE).wait()
    stdout,stderr = cmd.communicate()
    if not data:
        break
    elif data == 'killsrv':
        sys.exit()

Answer 1

Danger, Will Robinson!!!

Do you really want to send commands in clear text without authentication over the network? It is very, very dangerous.

Do it over SSH with paramiko .

Alright I've heard this answer too many times. I don't want to use SSH I'm just building it to learn more about sockets. I'm not going to actually use this if I want to send commands to a system. – AustinM

There is no way I could infer this noble quest from your question. :-)

The sockets module is a thin layer over the posix library; plain sockets is tedious and hard to get right. As of today (2014), asynchronous I/O and concurrency are not among Python's strongest traits - 3.4 is starting to change that but libraries will lag behind for a while. My advice is to spent your time learning some higher level API like Twisted ( twistedmatrix.com/trac ). If you are really interested in the low level stuff, dive in the project source.

Alright. Any idea on how I could use twisted for this type of thing? – AustinM

Look at twistedmatrix.com/documents/current/core/examples/#auto2

Answer 2

Well I can understand your frustration Austin; I was in the same boat. However trial and error at last worked out. Hopefully you were looking for this:

print "Command is:",command
op = subprocess.Popen(command, shell=True, stderr=subprocess.PIPE, stdout=subprocess.PIPE)
if op:
    output=str(op.stdout.read())
    print "Output:",output
    conn.sendall(output)

else:
    error=str(op.stderr.read())
    print "Error:",error
    conn.sendall(error)

Answer 3

It's unclear why you are using subprocess.Popen() for the same command in both the client and the server. Here's an outline of what I would try to do (pseudocode):

client

while True:
    read command from user
    send command to server
    wait for and then read response from server
    print response to user

server

while True:
    wait for and then read command from client
    if command is "killsrv", exit
    execute command and capture output
    send output to client

Answer 4

The problem with your code is this line (in both client and server):

proc = subprocess.Popen(cmd, stdout=iotype).wait()
stdout,stderr = proc.communicate()

You are calling wait on the Popen object, which means that the variable proc is getting an int (returned by wait ) instead of a Popen object. You can just get rid of the wait -- since communicate waits for the process to end before returning, and you aren't checking the exit code anyway, you don't need to call it.

Then, in your client, I don't think you even need the subprocess calls, unless you're running some command that the server is sending back.