I'sorry to inform you that I`m watching #43 bug now in p7zip 9.20 (archlinux, x86-64).
ps ax |grep 7z
shows me all my passwords
As a workaround :
enter the password manually (don't use -pmy_password")
you can try "expect" to automatically enter the password manually ...
example of script :
spawn 7z x protected_archive.7z
expect "Enter pasword (will not be echoed) :"
I attached a patch that allows to read the password from a file. This should solve the problem.
The usage is as follows:
7za x -fpsecret.txt protected_archive.7z
the file secret.txt contains the password.