python subprocess.check_output returns CalledProcessError on some commands

Question

>>> import subprocess
>>> subprocess.check_output("smartctl -d ata -a /dev/sda", shell=True)
"output of above command prints normally"
>>> subprocess.check_output("smartctl -d ata -a /dev/sdb", shell=True)
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/usr/lib/python2.7/subprocess.py", line 544, in check_output
       raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command 'smartctl -d ata -a /dev/sdb' returned non-zero exit status 64

Can anyone explain to me why the above command with sda works fine but returns an error with sdb? sdc works too, for the record. Also, I am aware of the risk of using shell=True. I am learning python and ran into this problem while writing a test script.

Eugene Primako · Accepted Answer · 2016-01-19 20:39:10Z

2

You can easily see what was wrong with sdb:

try:
    subprocess.check_output("smartctl -d ata -a /dev/sdb", shell=True)
except subprocess.CalledProcessError, e:
    print e.output

edited Jan 19, 2016 at 20:39

answered Jan 19, 2016 at 20:29

Eugene Primako

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5 Comments

user2503227 Over a year ago

Interestingly, you code gives me the following error: NameError: name 'CalledProcessError' is not defined. edit fixed it to subprocess.CalledProcessError.

Eugene Primako Over a year ago

Thanks, I've edited my answer. Alternatively you could write from subprocess import CalledProcessError before my piece of code.

user2503227 Over a year ago

When I change to subprocess.CalledProcessError and print e.output, I get the output of the smartctl command with seemingly no errors. Strange, is this somehow expected?

Eugene Primako Over a year ago

Then nothing can be done from python side: the command really returns nonzero exit status. But googling helped to find the answer: sourceforge.net/p/smartmontools/mailman/message/10780193. "64 (at least, a 1 in the 7th bit) is an indication that there are items in the Error Log"

mgilson Over a year ago

nit: The modern way to catch exceptions is except SomeException as e rather than except SomeException, e.

vks · Accepted Answer · 2016-01-19 20:28:50Z

1

To know outputs and errors use

import subprocess
c=subprocess.Popen("smartctl -d ata -a /dev/sdb",stdout=subprocess.PIPE,stderr=subprocess.PIPE,shell=True)
output,error=c.communicate()
#now check your error or output

answered Jan 19, 2016 at 20:28

vks

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Comments

Roland Smith · Accepted Answer · 2016-01-19 21:32:39Z

1

The smartctl command returns the value 64. According to the manual page the return value is a bitfield. Converting 64 to binary yields 01000000, so bit 6 is set (rightmost bit is bit 0). According to the aforementioned manpage:

Bit 6: The device error log contains records of errors.

If you can use Python 3.5, you could use the new high-level API subprocess.run(). This allows you to both capture return value and standard output/error.

prog = ['smartctl',  '-d', 'ata', '-a',  '/dev/sda']
result = subprocess.run(prog, stdout=subprocess.PIPE,
                        stderr=subprocess.PIPE)
if result.returncode == 0:
    print(result.stdout)
else:
   print('smartctl returned error', result.returncode)

edited Jan 19, 2016 at 21:32

answered Jan 19, 2016 at 21:21

Roland Smith

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1 Comment

user2503227 Over a year ago

Ended up going with the subprocess.Popen option since that better suits my needs. Preferably looking for options compatible with 2.7 and 3.5 but it's gotta work with 2.7. Thanks for the information though

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