Welcome back. In
this series of lessons, we'll discuss
environment variables. These are mechanism in program and program
libraries for preserving values or information that lasts over several runs
of a program, yet are easily
changed by the user. Lets get started.
Environment Variables provide a mechanism for preserving values or information that lasts over
several runs of the program. But also allowing it
to be easily changed. You don't have to
go edit the file. They're very widely used. In Windows, this is
the equivalent of the registry. But in Linux and similar systems, it's an actual variable stored in the memory of
the command interpreter. These are typically under
the control of the user, that is you can change
them, their values. Programs use them,
libraries use them, and other parts of
the system use them. Home and user identify the user and the home
directory for example. Editor, pager, and so forth all indicate programs that should be run when another program wants for example to
invoke an editor. It'll look at
the editor environment variable to see which
one you prefer. When the command interpreter
goes looking for a program, it will look down the directories listed in the path
environment variable. We'll get to what IFS
means in a minute. But basically it separates words and we'll explain that
in a couple of minutes. Then you've got
others that involve the time zone where to look for manual pages, what
locale basically, what language and
cultural symbols you're using for dollar
and things like that. In general, they have a name followed by a value
separated by an equal sign. When the command interpreter
starts up or you type that, it reads the name and reads
the value and then stores the value as under the name. Now in some command
interpreters notably bash which is a very widely used
command interpreter in Linux. You can do functions as well. The format of the value
here is specialized. It has to begin with
an open and close parentheses, and then the rest of the value has to be
enclosed in curly braces, which contains really
the function body.