Capture stdout stderr of python subprocess, when it runs from cron or rc.local

问题

I have problem accessing output (stderr stdout) of a command when I do lunch it via cron or rc.local

It works perfectly form regular shell, but fails via rc.local

cat /root/watchdog.py 
import subprocess
    cmd = ( 'echo "TEST" |gnokii --config /root/.config/gnokii/config --sendsms +123456789xx ')
    #p = subprocess.Popen([cmd, '2>&1'], stdout=subprocess.PIPE, stderr=subprocess.STDOUT, shell=True)
    p = subprocess.Popen([cmd, '2>&1'], stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
    output = p.stdout.read()
    output += p.stderr.read()
    logFile = open("/root/logfile", 'a+')
    ##### 
    #Idea to read line by line:
    #output = ''
    #    for line in iter(p.stdout.readline,''):
    #       print "captured line: %s" % line.rstrip()
    #       #logFile.write(line.rstrip())
    #       output += line
    logFile.write(output)
    logFile.close()

The output when run from console looks like:

/root/watchdog.py 
GNOKII Version 0.6.30
Cannot open logfile /root/.cache/gnokii/gnokii-errors
WARNING: cannot open logfile, logs will be directed to stderr
Send succeeded with reference 186!

in my rc.local

/root/watchdog.py  > /root/mywatchPY.out 2>&1 &

This looks interesting: Redirect subprocess stderr to stdout but it does not solve the problem.

Any idea howto capture sdterr/stdout of subprocess run without full shell?

回答1:

There are multiple issues in your code:

pass the command and its args as a string when shell=True otherwise the args are passed to the shell itself instead of the command
you should use stderr=subprocess.STDOUT instead of 2>&1 if you meant to apply the latter to the whole pipeline and not just the last command in it
use p.communicate() instead of p.stdout.read(), p.stderr.read() otherwise the subprocess may stall if any of OS pipe buffers fill up
if you want to redirect the output to a file then you do not need to save it as a string first

import shlex
from subprocess import Popen, PIPE, STDOUT

with open("/root/logfile", 'ab', 0) as logfile:
    p = Popen(shlex.split('gnokii ... +123456789xx'), 
              stdin=PIPE, stdout=logfile, stderr=STDOUT)
    p.communicate(b'TEST')

Redirect subprocess stderr to stdout doesn't apply because you redirect stdout explicitly.

来源：https://stackoverflow.com/questions/25415143/capture-stdout-stderr-of-python-subprocess-when-it-runs-from-cron-or-rc-local

标签

python

cron

subprocess

stdout