mplayer -vo null -ao alsa test.mp3 & disown
sh: 1 : disown: not find
Why it doesn't work?
Likely, your shell is not
dash which doesn't have
fgrep $(id -n) /etc/password and see whether your shell is
bash. If it's not, then update it, re-login, and you will be able to disown processes.
If you want to continue using the current shell (which lacks the
disown builtin) you can use
nohup your_command_here & to achieve almost the same behaviour.
The difference between
nohup is that
disown prevents shell from sending HUP to disowned process, while
nohup installs a handler that ignores the HUP signal sent by shell at the exit.