Popen Question

[Ravi]

On Nov 4, 7:06 pm, moogyd <moo... at yahoo.co.uk> wrote:
> Hi,
> 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
> project.
> To this end, I am looking at subprocess.Popen() to actually call the
> simulations, and have a very basic question which is demonstrated
> below.
>
> [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
> _execute_child
>     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
> English?
> Thanks,
> Steven

try giving /bin/echo