Specifications

Chapter 2 Using the Command-Line Shell Interactively 21
Using Environment Variables
The shell uses environment variables to store information, such as the name of the
current user, the name of the host computer, and the default paths to any commands.
Environment variables are inherited by all commands executed in the shell’s context,
and some commands depend on environment variables.
You can create environment variables and use them to control the behavior of
a command without modifying the command itself. For example, you can use an
environment variable to have a 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 modies the environment variable PATH with the value assigned.
To view all environment variables, enter:
$ 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 congure the application dynamically. For example, your
application can check for the presence (or value) of an environment variable and
change its behavior accordingly.
Dierent shells support dierent semantics for exporting environment variables.
For information, see the man page for your preferred shell.
Although child processes of a shell inherit the environment of that shell, shells are
separate execution contexts that don’t share environment information with each other.
Variables you set in one Terminal window aren’t 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 across sessions and in all Terminal windows, you
must set it in a shell startup script. For information about modifying your bash shell
startup script (~bashrc) to keep variables and other settings across multiple sessions,
see the “Invocation” section of the bash man page.
Another way to set environment variables in Mac OS X is with a property list le in
your home folder. When you log in, the computer looks for a ~/.MacOSX/environment.
plist le. If the le is present, the computer registers the environment variables in the
property list le.