UNIX for Beginners Questions & Answers

OK, so, to understand the difference between background and foreground processes, and where system services get their environment, I must explain how almost everything works from the ground up... Wish me luck.

For starters, the "environment". Every process has a block of memory devoted to storing text variables. These are not shell variables: They are used by programs but not actually a part of them. Any process can get to these variables, shells, editors, anything. A large part of the shell's job is setting up these variables for everything else.

UNIX creates processes by cloning your own process then replacing it with something else. The environment is part of the memory which gets cloned. So to run nano, your shell calls fork to clone itself, then the clone calls exec to replace itself with a text editor. The environment isn't touched by this replacement -- nano still has a perfect copy of your shell's environment after it loads.

To clone a process, one must first have a process, so when a UNIX kernel starts, it runs the program init as the very first process. It has no controlling terminal (or gets rid of its controlling terminal shortly after starting - either way.) init will stay running for as long as your system does.

There's only a few things run by init: It will run a copy of getty on each physical terminal to provide "Username:" login prompts on them, and it will run "rc" to start system services. Unlike init, rc does not stay running, it just starts processes then quits.

rc runs shell scripts to start system services, traditionally found in /etc/rc.d/ or /etc/init.d/, called "init scripts". It loads /etc/profile first before loading an init script to start a system service.

So the point to my really longwinded answer is that the rc program (or whatever equivalent your system has) is responsible for running everything that's not a terminal login, and therefore controls their environments. Its own environment is probably next to nothing. If you really need a custom environment for a service, its the job of its init script to set it up. (though many UNIX and Linux distributions now have set up a better place for it in /etc/conf.d/ which is more or less a per-service /etc/profile kind of thing. rc is responsible for loading it.)

Now, foreground and background processes. These are meaningless terms without a terminal, because all "foreground" means is "in control of the terminal". When you run nano, your shell hands control of the terminal to it, and when nano quits, it gets that control back.