Popen Question

moogyd moogyd at yahoo.co.uk
Thu Nov 4 15:06:24 CET 2010

I usually use csh for my simulation control scripts, but these scripts
are becoming more complex, so I plan to use python for the next
To this end, I am looking at subprocess.Popen() to actually call the
simulations, and have a very basic question which is demonstrated

[sde:staff at lbux03 ~]$ python
Python 2.6 (r26:66714, Feb 21 2009, 02:16:04)
[GCC 4.3.2 [gcc-4_3-branch revision 141291]] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os, subprocess
>>> os.environ['MYVAR'] = "myval"
>>> p = subprocess.Popen(['echo', '$MYVAR'],shell=True)
>>> p = subprocess.Popen(['echo', '$MYVAR'])
>>> $MYVAR

>>> p = subprocess.Popen('echo $MYVAR',shell=True)
>>> myval

>>> p = subprocess.Popen('echo $MYVAR')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.6/subprocess.py", line 595, in __init__
    errread, errwrite)
  File "/usr/lib64/python2.6/subprocess.py", line 1106, in
    raise child_exception
OSError: [Errno 2] No such file or directory

I am not really sure I understand these results.
1) No idea what is going on
2) As (1). What isn't myval printed out (rather than $MYVAR)
3) Works as I wanted it to
4) Why do I need shell=True ?
The documentation isn't very clear to me (it seems you need to
understand the underlying system calls).

Can anyone explain (or provide link) for this behaviour in simple

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