Apple Xserve Up Mac OS X Server Specifications download pdf (Page 24)

24 Chapter 1 Executing Commands

Using Environment Variables

Some commands require the use of environment variables for their execution.

Environment variables are inherited by all commands executed in the shell’s context.

The shell 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 create environment variables and use them to control the behavior of your

command without modifying the command itself. For example, you can use an

environment variable to have your command print debug information to the console.

To set the value of an environment variable, 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 modifies the environment variable PATH with the value assigned.

To view all environment variables, enter the following:

$ env

When you launch an application from a shell, the application inherits much of the

shell’s environment, including exported environment variables. This form of inheritance

can be a useful way to configure the application dynamically. For example, your

application can verify 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.

After you close a Terminal window, 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.

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Apple Xserve Up Mac OS X Server Specifications Page 24