How to know the exact output from a Python subprocess.check_output() call?

Question

I'm running some terminal commands from within Python using the subprocess.check_output() call. This works all fine when it returns correct results. When interacting with the bitcoin daemon I can get several responses however. For example, the raw output (as seen on a normal bash command line) can be:

: Cannot obtain a lock on data directory /home/kramer65/.bitcoin. Bitcoin is probably already running.

or

error: couldn't connect to server

Both these responses give an error code 1 however. I tried printing out the following attributes of the subprocess.CalledProcessError as e, which in both cases results in the same outputs (except for the cmd attribute of course:

print e.args, e.cmd, e.message, e.output, e.returncode
#       ()   'BTC'                            1
#       ()   'BTC getinfo'                    1

I guess the only thing that distincts the two errors, is the raw string that is outputted on the command line which I listed above. So my question is: how can I get the raw string that is shown on the command line from within Python?

wim · Accepted Answer · 2014-08-27 09:25:24Z

3

It is likely coming on the stderr.

You could setup your subprocess with

stderr=subprocess.STDOUT

as a kwarg to merge the stdout and stderr together.

Alternatively, if you need them separately, do something like

proc = subprocess.Popen(..., stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stdout, stderr = proc.communicate()

Note: unlike check_output, this method will not raise exception if the return code was nonzero, so you will have to do that manually if that's the behaviour you wanted.

answered Aug 27, 2014 at 9:25

wim

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