python with echo

[Steven D'Aprano]

On Wed, 11 Nov 2009 17:24:37 -0800, hong zhang wrote:

> List,
> 
> I have a question of python using echo.
> 
> POWER = 14
> return_value = os.system('echo 14 >
> /sys/class/net/wlan1/device/tx_power')
> 
> can assign 14 to tx_power
> 
> But
> return_value = os.system('echo $POWER >
> /sys/class/net/wlan1/device/tx_power')

POWER = 14 doesn't create an environment variable visible to echo. It is 
a Python variable.


>>> POWER = 14
>>> import os
>>> return_value = os.system('echo $POWER')

>>> return_value
0



> return_value is 256 not 0. It cannot assign 14 to tx_power.


I don't understand that. Exit status codes on all systems I'm familiar 
with are limited to 0 through 255. What operating system are you using?

Assuming your system allows two-byte exit statuses, you should check the 
documentation for echo and the shell to see why it is returning 256.

Have you tried this in the shell, without involving Python? I will almost 
guarantee that Python is irrelevant to the problem.




-- 
Steven