 |
 |
4.3.2 Environment Variables
When Linux programs are run they have the open file descriptors described above. They also carry with them a list of “name=value” pairs called their environment. These environment variables allow for context to be shared among
several successively executed programs. Some examples of environment variables are:
• USER is the name you used to log in.
• HOME is the directory where you start when you log in.
• PATH is the list of directories searched for executable files.
To see the environment variables defined in your current shell, type env
at the command prompt:
$ env HOME=/home/user01 USER=user01
PATH=/bin:/usr/bin:/usr/local/bin:/home/user01/bin
...
$
The names of environment variables, sometimes referred to as shell vari- ables, are traditionally uppercase, though that is only a convention. The variable names are treated in a case sensitive fashion (e.g., Home != HOME).
You can set environment variables for use in the current shell with a simple assignment statement:
$ VAR=value
That will set the value for the duration of this shell, but not for any of its sub- processes. Since running another program is a subprocess, such an assignment won’t be visible in your running program. Instead, you can export the variable so that it is carried forward to all subprocesses:1
$ export VAR=value
|