How to get the last N lines of a subprocess' stderr stream output?

问题

I am a Python newbie writing a Python (2.7) script that needs to exec a number of external applications, one of which writes a lot of output to its stderr stream. What I am trying to figure out is a concise and succinct way (in Python) to get the last N lines from that subprocess' stderr output stream.

Currently, I am running that external application from my Python script like so:

p = subprocess.Popen('/path/to/external-app.sh', stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stdout, stderr = p.communicate()

if p.returncode != 0:
    print "ERROR: External app did not complete successfully (error code is " + str(p.returncode) + ")"
    print "Error/failure details: ", stderr
    status = False
else:
    status = True

I'd like to capture the last N lines of output from its stderr stream so that they can be written to a log file or emailed, etc.

回答1:

N = 3 # for 3 lines of output
p = subprocess.Popen(['/path/to/external-app.sh'], 
    stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stdout, stderr = p.communicate()

if p.returncode != 0:
    print ("ERROR: External app did not complete successfully "
           "(error code is %s)" % p.returncode)
    print "Error/failure details: ", '\n'.join(stderr.splitlines()[-N:])
    status = False
else:
    status = True

回答2:

If the whole output can't be stored in RAM then:

import sys

from collections import deque
from subprocess  import Popen, PIPE
from threading   import Thread

ON_POSIX = 'posix' in sys.builtin_module_names

def start_thread(func, *args):
    t = Thread(target=func, args=args)
    t.daemon = True
    t.start()
    return t

def consume(infile, output):
    for line in iter(infile.readline, ''):
        output(line)
    infile.close()

p = Popen(['cat', sys.argv[1]], stdout=PIPE, stderr=PIPE,
          bufsize=1, close_fds=ON_POSIX)

# preserve last N lines of stdout,  print stderr immediately
N = 100 
queue = deque(maxlen=N)
threads  = [start_thread(consume, *args)
            for args in (p.stdout, queue.append), (p.stderr, sys.stdout.write)]
for t in threads: t.join() # wait for IO completion

print ''.join(queue), # print last N lines
retcode = p.wait()

来源：https://stackoverflow.com/questions/4995095/how-to-get-the-last-n-lines-of-a-subprocess-stderr-stream-output

标签

python

scripting

subprocess