Subprocess module errors with 'export' in python on linux?

Question

I'm setting up a program to connect my computer to our schools proxy and currently have something like this:

import subprocess
import sys

username = 'fergus.barker'
password = '*************'
proxy = 'proxy.det.nsw.edu.au:8080'
options = '%s:%s@%s' % (username, password, proxy)

subprocess.Popen('export http_proxy=' + options)

But upon running I get:

Traceback (most recent call last):
File "school_proxy_settings.py", line 19, in <module>
 subprocess.Popen('export http_proxy=' + options)
File "/usr/lib/python2.6/subprocess.py", line 621, in __init__
 errread, errwrite)
File "/usr/lib/python2.6/subprocess.py", line 1126, in _execute_child
 raise child_exception
OSError: [Errno 2] No such file or directory

Why is this happening please guys?

Answer 1

The problem is that export is not an actual command or file. It is a built-in command to shells like bash and sh, so when you attempt a subprocess.Popen you will get an exception because it can not find the export command. By default Popen does an os.execvp() to spawn a new process, which would not allow you to use shell intrinsics.

You can do something like this, though you have to change your call to Popen.

http://docs.python.org/library/subprocess.html

You can specify shell=True to make it use shell commands.

class subprocess.Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=False, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0)

On Unix, with shell=True: If args is a string, it specifies the command string to execute through the shell. This means that the string must be formatted exactly as it would be when typed at the shell prompt. This includes, for example, quoting or backslash escaping filenames with spaces in them. If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional arguments to the shell itself. That is to say, Popen does the equivalent of:

Popen(['/bin/sh', '-c', args[0], args[1], ...])

Answer 2

export is not a separate binary on your system, it is actually just a command within the shell itself. For example, try using which rm on your system. You'll probably see something like:

[21:16:28] ~ $ which rm
/bin/rm

Now try using which export. You'll get something like:

[21:16:37] ~ $ which export
/usr/bin/which: no export in (/usr/lib/qt-3.3/bin:/usr/kerberos/sbin:
/usr/kerberos/bin:/usr/lib/ccache:/usr/local/bin:/usr/bin:/bin:
/usr/local/sbin:/usr/sbin:/sbin:/home/carter/bin)

So you can't actually invoke an export process/subprocess by default. You may want to look at os.putenv() and os.environ() instead.