Environment Variables

There are many parameters in the shell that define parts of your working environment and which can be set interactively at the command prompt, in shell scripts, or in user-specific profile files (see subsection "The Profile Files" below). Each shell has one or more startup files. Work is currently in progress developing a set of standard startup files (for further information contact Alan Silverman or Tony Cass).

• Shell parameters that are local to your current shell and not passed to any subshell or subprocess are called shell variables. 
• Shell parameters that are global are called environment variables. They are valid in the current shell, where they are set, and in all subshells. They are not valid however in 'higher' shells, from where the current shell is invoked as subshell. The way in which a parameter is declared to be global depends on which family of shell you are using (see below).

There is one restriction if you define environment variables in a shell script. If the environment variables are to be valid also in the current shell you must invoke the script in a special way without spawning a new process:

 
               		   . myscript 		 for the Bourne Shell

               		  source  myscript 		 for the C shell

A set of environment variables is already defined by the operating system and can be changed by each user for his personal working environment. In addition, he can of course also define new environment variables. The profile files which serve as an interface for initialisation of these variables at login are described later. By convention, environment variable names are upper case.

A list of the currently-valid environment variables may be obtained with the command

 
               		  env

               		 also  printenv in the C shell

• Setting Environment Variables
• Getting Values of Environment Variables
• A Summary of some useful Environment variables
• Changing Your Command Prompt