禁止子流程的输出

Question

I want to use the subprocess module to control some processes spawned via ssh.

By searching and testing I found that this works:

import subprocess
import os
import time

node = 'guest@localhost'
my_cmd = ['sleep','1000']
devnull = open(os.devnull, 'wb')
cmd = ['ssh', '-t', '-t', node] + my_cmd
p = subprocess.Popen(cmd, stderr=devnull, stdout=devnull)


while True:
    time.sleep(1)
    print 'Normal output'

The -t -t option I provide allows me to terminate the remote process instead of just the ssh command. But this, also scrambles my program output as newlines are no longer effective making it a long and hard to read string.

How can I make ssh not affecting the formatting of the python program?

Sample output:

guest:~$ python2 test.py 
Normal output
             Normal output
                          Normal output
                                       Normal output
                                                    Normal output
                                                                 Normal output
                                                                              Normal output
(First ctrl-c)
Normal output
Normal output
Normal output
(Second ctrl-c)
^CTraceback (most recent call last):
  File "test.py", line 13, in <module>
    time.sleep(1)
KeyboardInterrupt

Answer 1

Ok, the output is now clear. I do not exactly know why, but the command ssh -t -t puts the local terminal in raw mode. It makes sense anyway, because it is intended to allow you to directly use curses programs (such as vi) on the remote, and in that case, no conversion should be done, not even the simple \\n -> \\r\\n that allows a simple new line to leave the cursor on first column. But I could not find a reference on this in ssh documentation.

It ( -t -t ) allows you to kill the remote process because the raw mode let the Ctrl + C to be sent to the remote instead of being processed by the local tty driver.

IMHO, this is design smell , because you only use a side effect of the pty allocation to pass a Ctrl + C to the remote and you suffer for another side effect which is the raw mode on local system. You should rather process the standard input ( stdinput = subprocess.PIPE ) and explicitely send a chr(3) when you input a special character on local keyboard, or install a signal handler for SIG-INT that does it.

Alternatively, as a workaround, you can simply use something like os.system("stty opost -igncr") (or better its subprocess equivalent) after starting the remote command to reset the local terminal in an acceptable mode.