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24 Chapter 1 Executing Commands
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.