Question

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.

Answer 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