Question

When I execute environment commands, such as env, set, and unset, something happens, but what?

set hello='hello world!' unset find 

What do you do with the commands? Are the changes permanent or temporary? Where can you see the changes? I am an Ubuntu-newbie.

Answer 1

Each process that is created gets its own environment which lives as long as the process. Your shell is just like any other process. Its environment is its own.

If you type ‘set’ with no arguments, you’ll see what exists. Many of these settings are there to control program behavior.. your search path, desired X11 display, home directory (if not /home/yourname), etc.

The use is really whatever you need it to be. Any time you need to store some useful bit of information (such as long list of command line options to some program) into a variable that other applications can read, or that you can access from the shell, use the environment.

For instance:

USUAL_CONFIGURE_OPTS='--prefix=/home/charlie --sysconfdir=/home/charlie/tmp-etc' ./configure $USUAL_CONFIGURE_OPTS --and-additional-arguments 

Edit:

As a programmer, I read the environment to determine the user’s preferences and obey them. For instance, the variable POSIXLY_CORRECT influences the output of my programs if it is set. The environment is where the user tells programs how to behave. It also happens, the environment is a handy place for the user to store useful bits, as I described above.

Again (responding to your comment), every program that is executed is a process. A process gets its own address space (own memory), its environment is stored in that space. This means, the environment is specific to that process and lives only as long as the process itself.

Edit 2:

I think that I now fully understand your question. If someone says ‘virtual environment’, they are just noting that the environment resides in the application’s address space, which is mapped by the kernel as virtual memory (some pages might be in physical memory, some might be in swap, shared dynamic objects, etc).

No process can access another’s environment unless the process explicitly creates a map to that specific region and shares it with another process. Again, a process’ address space is completely private and isolated from other processes. Environmental variables live within that address space, otherwise, the process could not access or manipulate them.