Official command return status specification??
 

I'm working on an embedded linux project, I have to concern every aspect of the small collection of software which I'm using with it.
I want to make a specification of the return status(the integer number returned by the command when it exits) for every command(especially the shell scripts I wrote myself) in the system. I guess there would be some worldwidely accepted standard of which number means what status, but I haven't found it in my familiar sources of information.
Can anyone help? Thanx!

The standard is:
= 0 -> Success
!= 0 -> Failure

There is no other convention, each command is free to choose what a non zero return status means.