Python sys.stdout.write（）无法正常工作

Question

I am trying to "scp" a child python script called "whichserver.py" using parent python script called "server.py". I am using "subprocess" in the parent script. The parent script will first "SCP" the child script into the remote server. Both parent and child script are in the same directory. Then run the child script on remote server and show the output back in local terminal. However I am not seeing any output. Here are my script:

Parent script "server.py":

import pexpect
import subprocess
import sys

def server_type(host):
  filepath = "whichserver"
  remotepath = "/tmp/"
  hostname = 'adam@' + host
  HOST = host
  COMMAND="cd /tmp && chmod 755 ./whichserver && ./whichserver"
  subprocess.call(['scp', filepath, ':'.join([hostname,remotepath])])
  ssh = subprocess.Popen(["ssh", "%s" % HOST, COMMAND],shell=False,stdout=subprocess.PIPE,stderr=subprocess.PIPE)  
  result = ssh.stdout.readlines()
  if result == []:
      error = ssh.stderr.readlines()
      print >>sys.stderr, "ERROR: %s" % error
  else:
      print result
      for line in iter(result):
         sys.stdout.write(line)

print('Enter the server name: ')
hostname1 = raw_input()
response = os.system("ping -c 1 " + hostname1)

if response == 0:
  print(hostname1 + ' is up')
  server_type(hostname1)
else:
  print(hostname1 + ' is down')

My Child script called "whichserver.py" is:

#!/bin/bash
server="$(sudo dmidecode | grep -m1 'Manufacturer:' | sed 's/.*Manufacturer://')"
echo
printf $server

Output:

['\n']

Expected Output:

ZT Systems

Can you please suggest why I am only getting newline character ? Is there a way to store the value "ZT Systems" in a variable in localhost after getting output from remote server ?

Answer 1

There are a few things going on here.

1. You should use ssh.communicate() instead of ssh.stdout or ssh.stderr unless you have no choice. This avoids blocking (potentially blocking forever because you're waiting on the wrong pipe).

2. You should check the output status of your subprocess. Instead, you just check if it produces output. But it should produce output, even if it fails, because of the echo statement. So the test for success and failure doesn't work.

3. That shell script is a bit of a mess. It won't handle errors, it puts newlines in funny places. There's a $server variable that serves no purpose (well, other than stripping whitespace).

Here is a fixed shell script for you:

#!/bin/sh
# Note: sh, because we don't need any bash-isms

# Exit on error
set -e

# No need to save to a variable, just print it out
# Also: printf would get rid of the newline at end, but why bother?
sudo dmidecode | grep -m1 'Manufacturer:' | sed 's/[^:]*: *//'

However, this isn't really necessary. Instead of uploading a script with scp and executing it with ssh , we can just execute the script directly with ssh . This saves us some steps.

from __future__ import print_function

def server_type(host):
    cmd = "sudo dmidecode | grep -m1 'Manufacturer:' | sed 's/[^:]*: *//'"
    proc = subprocess.Popen(
        ['ssh', str(host), cmd],
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE)
    stdout, stderr = proc.communicate()
    if proc.returncode != 0:
        print('Error:', stderr.rstrip(), file=sys.stderr)
    else:
        print('Server type:', stdout.rstrip())

Also note that sudo may require a tty. You can configure it to not require a tty, or you can use ssh -t , which makes ssh provide a tty. Both choices have drawbacks.