This discusses the use of environment variables
in programs. Next slide shows how to get values and create
values explicitly. Getenv will simply returns a value and if it's not defined, it simply returns
the null pointer. Putenv puts it into
the environment. Now, it's important
understand there, it just puts it into
the environment for the program that's running. So if you write a program
that calls this function, and you run it from
your command interpreter, the program will have that
variable in its environment, but the command
interpreter will not. Setenv is a mechanism
similar to putenv. Now, the next slide shows you how to access these from programs because when
your program runs, it's going to inherit the set
of environment variables. It's the third argument
to the main routine. If you look on the slide, you'll see envp. That's the environment. That's a pointer to
the environment variables, and they're stored as
a list of strings. You can also access
them globally. There's a variable
called environ, that again is a pointer just like envp to an array of strings which are
the environment variables. You can also explicitly
put them into a function called execve or execvp
or things like that. So you can define which
variables you want to parse. If you don't define any, all of them get parsed. If you want to use
them in your program, use getenv for
example, to grab them. The little segment of
code on this next slide shows a process being confined to the user's home
directory area. Now, the getenv home tells you what the value of the
environment variable home is, which is typically
the user's home directory. In the next couple of lines, if you see chroot
that changes the root processes notion of root
to be that home directory. If the home is not defined, then it simply uses /tmp
directory /tmp as the root. We'll talk about chroot
later on in this module. Now, environment variables
are also used indirectly that is
not named explicitly. There are two library
functions system and p open. Which again we'll talk
about in more detail later. But those essentially
run a sub-shell, a command interpreter, which then interprets the argument
given to that function. So what will happen here is, system will run a
shell usually bin/sh, it may be bash it may be
csh, or some other shell. But it's usually bin/sh or bash, and it will have
that shell execute the "echo -n today is; date". Now, in order to find
date which is simply a command since there's
no slash in the word date, it looks for a date in the first directory named in the path environment variable. If it doesn't find an
executable there named date, it will go to the second
directory, and so forth. It will continue going
all the way down. Then, the addition system sets the environment variables shell to whichever shell it's running. In this example, it's bin/sh, and it sets home to
the user's home directory.