Apple Mac OS X Server manual Using Environment Variables, Following command in a Terminal window

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Using Environment Variables

Some commands require the use of environment variables for their execution. Environment variables are variables inherited by all commands executed in the shell’s context. The shell itself uses environment variables to store information, such as the name of the current user, the name of the host computer, and the paths to any commands. You can also create environment variables and use them to control the behavior of your command without modifying the command itself. For example, you might use an environment variable to tell your command to print debug information to the console.

To set the value of an environment variable, you use the appropriate shell command to associate a variable name with a value. For example, to set the variable PATH to the

value /bin:/sbin:/user/bin:/user/sbin:/system/Library/, you would enter the

following command in a Terminal window:

$ PATH=/bin:/sbin:/user/bin:/user/sbin:/system/Library/ export PATH

This will modify the environment variable PATH with the value assigned. To view all of the environment variables, enter the following:

$ env

When you launch an application from a shell, the application inherits much of the shell’s environment, including any exported environment variables. This form of inheritance can be a useful way to configure the application dynamically. For example, your application can check for the presence (or value) of an environment variable and change its behavior accordingly. Different shells support different semantics for exporting environment variables, so see the man page for your preferred shell for further information.

Although child processes of a shell inherit the environment of that shell, shells are separate execution contexts that do not share environment information with one another. Thus, variables you set in one Terminal window are not set in other Terminal windows. Once you close a Terminal window, any variables you set in that window are gone. If you want the value of a variable to persist between sessions and in all Terminal windows, you must set it in a shell startup script.

Another way to set environment variables in Mac OS X is with a special property list in your home folder. At login, the computer looks for the ~/.MacOSX/environment.plist file. If the file is present, the computer registers the environment variables in the property-list file.

Chapter 1 Executing Commands