Python subprocess module giving an OSError while running UNIX commands

Question

Here's the context: I am using python 2.7.5. And I would like to run UNIX commands as well as maven commands in a python script. I was successful to do so, using os.system("cmd"), but I need to work on the result of the given command. After reading the doc and some threads in here, I decided to use the subprocess module to redirect the output to the stdout using PIPE. Unexpectedly, I am getting an OSError as shown in the attached image. Your help will be much appreciated. In addition to the given sample in the attached image, I have tried:

p = os.popen("java -version")
result = subprocess.check_output(p, shell=True)
subprocess.call("ls /usr", shell=True)
p.s. Using shell=True is strongly discouraged (doc), since it can be dangerous when coupled with unsanitized input.

Also, I took a look at the given script in the error message /usr/lib64/python2.7/subprocess.py, line 711 adn 1327 but didn't learn more than what is mentionned in the error message: raise child_exception Subprocess Terminal Output

Gil Hamilton · Accepted Answer · 2017-01-03 17:38:29Z

1

You aren't using subprocess.check_output correctly. You're trying to pass a pipe file object (the return value of os.popen) to check_output but it's expecting a command argument or argument vector.

Also, the subprocess.call function won't capture the executed command's output, so you would only use that if you want the output of ls /usr (or whatever) to be seen by the user running the script interactively. (Which is pretty much the same as os.system.)

Try this instead (showing with and without the shell):

import subprocess

out1a = subprocess.check_output(['java', '-version'], stderr=subprocess.STDOUT)
print(out1a)

out1b = subprocess.check_output('java -version', stderr=subprocess.STDOUT, shell=True)
print(out1b)

out2a = subprocess.check_output(['ls', '/usr'])
print(out2a)

out2b = subprocess.check_output('ls /usr', shell=True)
print(out2b)

# Cannot capture output this way, but it will be visible to user
subprocess.call('ls /usr', shell=True)

Note that in the case of the java -version command, the version info gets printed to the command's standard error output so you must redirect that in order to capture it as the returned value of check_output (hence the stderr=subprocess.STDOUT).

answered Jan 3, 2017 at 17:38

Gil Hamilton

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3 Comments

Mams Over a year ago

Thanks, Gil. the command out1a and its printing is working now. But how could I fextract first line, since I want to check if the version match a certain number? I have tried to redirect the output to a file.txt but not working. Is there a command that helps to extract and work directly with the terminal output?

Gil Hamilton Over a year ago

The result you get back is a string containing multiple lines of text. Use out1a.splitlines() to decompose that into individual lines. That returns a tuple from which you can select components you care about. Try, for example, print(out1a.splitlines()[0]). Then use other string methods to isolate the version number you need (see docs.python.org/2/library/…)

Mams Over a year ago

Alright, that helped me solve my problem. Again, thanks Gil. Here's how I completed:

import subprocess import re  res = subprocess.check_output(['java', '-version'], stderr=subprocess.STDOUT) res2 = res.splitlines()[0].split()[2]  versionNumber = re.findall('[0-9._]+', res2) print(versionNumber)  if versionNumber[0] >= 1.7: 	print "Cool" else: 	print "Install a newer version"

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