Lua / shell - How to "pipe" commands in os.execute() ?

[luc1an0 <luc1an0@...>]

Hi all,

My question can be very stupid, but I want to execute some shell 
commands with a pipe ( | ) like that :
os.execute(ps aux | grep my_program) or a generic way :
os.execute("my_command" "my_args" | another_command | ....)

And another simply question : How to don't return exit code if function 
even return string, for example
io.write(os.execute("pwd")) return this
/home/fred
0

Thanks a lot.