Learning Perl, Fourth Edition

The Environment Variables

When you’re starting another process (with any of the methods discussed here), you may need to set up its environment in one way or another. You could start the process with a certain working directory, which it inherits from your process. Another common configuration detail is the environment variables.

One of the best-known environment variables is PATH . (If you’ve never heard of it, you probably haven’t used a system that has environment variables.) On Unix and similar systems, PATH is a colon-separated list of directories that may hold programs. When you type a command such as rm fred, the system will look for the rm command in that list of directories in order. Perl (or your system) will use PATH whenever it needs to find the program to run. If the program runs other programs, those may also be found along the PATH. (If you give a complete name for a command, such as /bin/echo, there’ll be no need to search PATH. But that’s less convenient.)

In Perl, the environment variables are available via the special %ENV hash, and each key in this hash represents one environment variable. At the start of your program’s execution, %ENV holds values it has inherited from its parent process (generally the shell). Modifying this hash changes the environment variables, which will be inherited by new processes and possibly used by Perl as well. For example, suppose you wished to run the system’s make utility (which typically runs other programs), and you want to use ...