Fix Python – Subprocess changing directory

Question 1

I want to execute a script inside a subdirectory/superdirectory (I need to be inside this sub/super-directory first). I can’t get subprocess to enter my subdirectory:

tducin@localhost:~/Projekty/tests/ve$ python
Python 2.7.4 (default, Sep 26 2013, 03:20:26) 
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> import os
>>> os.getcwd()
'/home/tducin/Projekty/tests/ve'
>>> subprocess.call(['cd ..'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/subprocess.py", line 524, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib/python2.7/subprocess.py", line 711, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1308, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

Python throws OSError and I don’t know why. It doesn’t matter whether I try to go into an existing subdir or go one directory up (as above) – I always end up with the same error.

Question 2

What your code tries to do is call a program named cd ... What you want is call a command named cd.

But cd is a shell internal. So you can only call it as

subprocess.call('cd ..', shell=True) # pointless code! See text below.

But it is pointless to do so. As no process can change another process’s working directory (again, at least on a UNIX-like OS, but as well on Windows), this call will have the subshell change its dir and exit immediately.

What you want can be achieved with os.chdir() or with the subprocess named parameter cwd which changes the working directory immediately before executing a subprocess.

For example, to execute ls in the root directory, you either can do

wd = os.getcwd()
os.chdir("/")
subprocess.Popen("ls")
os.chdir(wd)

or simply

subprocess.Popen("ls", cwd="/")

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Answer