python中的re.search（）函数未找到子字符串

Question

The substring 'Failed to connect to host' is not being found in the 'text' variable. I am fairly new to python and I would appreciate any help.

import subprocess
import re

host = 'XXXXXXX'
output = subprocess.Popen(['/usr/bin/ssh', '-q', host, 'ifconfig|', 'grep', 'Mask'], stdout=subprocess.PIPE)
text = output.communicate()[0].decode('utf-8')
pattern = 'Failed to connect to host'

if re.search(pattern, text, re.IGNORECASE):
    print('found')
else:
    print('not found')

The expected result should be "found" since the text str contains the pattern but for some reason, the result is "not found".

Actual Output:

Failed to connect to host XXXXXXX port 22: No route to host
not found

Answer 1

It's because the error message is printed to stderr, but you're only matching against the text printed on stdout. If you try print(repr(text)) you'll see it prints '' .

Try adding stderr=subprocess.STDOUT to your Popen call, like this:

output = subprocess.Popen([...], stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
#                                                        ^-- this

What it does is it redirects stderr to stdout. See the docs here: https://docs.python.org/3.7/library/subprocess.html#subprocess.STDOUT