Steven D'Aprano wrote:
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
You can set environment variables from within Python using os.putenv:
>>> import os
>>> os.putenv('POWER', '14')
>>> return_value = os.system('echo $POWER')
14
>>> return_value
0
Keep in mind that putenv() only affects processes started by Python
after you call putenv. It does not, for example, affect the shell
process you used to invoke Python:
$ POWER=14
$ python -c 'import os
os.putenv("POWER", "42")
os.system("echo $POWER")'
42
$ echo $POWER
14
$
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?
Probably some form of Unix. The value returned by os.system() is the
exit status shifted left one byte, for example:
>>> os.system("exit 1")
256
Hope this helps,
-- HansM
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