Python3 subprocess check_output returns empty for some commands

Question

I am a python3 beginner. I am trying to get the java version with a python3 script. After I checked the docs, I saw that subprocess.check_output might be what I need.

output = subprocess.check_output(["java", "-version"])
print("Output is {}".format(output))

The problem is the output I am getting is

Output is b''

Why am I not getting the correct string that I get with bash?

Thanks

greschd · Accepted Answer · 2014-10-20 19:50:06Z

7

For some reason your output lands in the stderr. You can pipe that to the return value like this:

output = subprocess.check_output(["java", "-version"], stderr=subprocess.STDOUT)

If somebody knows why it goes to stderr, I'd be happy to hear it. ["python", "--version"], for example, works as expected.

answered Oct 20, 2014 at 19:50

greschd

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1 Comment

jfs Over a year ago

I can confirm: java -version command prints to stderr on my system. man java doesn't specify where the output should go. Your code covers both stdout/stderr therefore unless java -version prints directly to terminal; it should work.

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