Introduction to environment modules
Unity offers a wide variety of software tools that are available for you to use. Although you have access to all the software available on Unity, conflicts may occur if you enable all of the software simultaneously. Instead, you should use environment modules to enable only the software that you need.
In shell programming, an environment is a set of shell variables and their corresponding values. These variables are stored and managed by the shell and can be accessed or modified by the user. The shell variables are of the form KEY="VALUE" which is also known as key-value pairs. The KEY acts as a label or a name for the specific information stored inside. The "VALUE" is the actual data associated with the key. You can think of the key as a short keyword for the information you want to find or use.
env command.The PATH environment variable
Most of the commands you use in the shell are actually executable files somewhere in the filesystem. When you want to run an executable you have multiple options. One way is to enter the full path to the executable. While this method works, it can be tedious to memorize and type in the path of each executable you want to run. Alternatively, you can use $PATH, which is an environment variable that contains a list of directories (folders) separated by colons. To view the directories in your $PATH, use the command echo $PATH.
$ echo $PATH
/usr/local/bin:/usr/bin:/bin
In this example there are three directories, /usr/local/bin, /usr/bin, and /bin.
If you want to run an executable file directly from the command line, without specifying its full path or location, you have to use executables within the $PATH directories. When you enter just the name of the executable into the command line, the shell searches the directories in $PATH (from left to right) for an executable by that name. If there are multiple executables of the same name, whichever executable is found earlier in the $PATH (further to the left) will be executed.
Using environment modules, you can modify your environment variables and change which commands are available in your shell. By changing the files within $PATH, you can change which programs you can run.
Environment modules
Environment modules are scripts that modify your environment. These modules can be used to add new directories to your $PATH, which makes the executables within the module available for use. Loading a module will add a directory to the beginning of the $PATH, or furthest to the left, ensuring that the executables within the module are chosen first by the shell when you call their name.
The which command can be used to identify what directory the following executable will run from, as shown in the following code sample:
$ which python3
/usr/bin/python3
$ module load python/3.9.1
$ which python3
/modules/apps/python/3.9.1/bin/python3
In the example above, the executable python3 would run from the directory /usr/bin/python3. After loading the module python/3.9.1, it would run from /modules/apps/python/3.9.1/bin/python3 instead.
Another way to see how $PATH changes is to use echo $PATH, as shown in the following code sample:
$ echo $PATH
/usr/local/bin:/usr/bin:/bin
$ module load python/3.9.1
$ echo $PATH
/modules/apps/python/3.9.1/bin:/usr/local/bin:/usr/bin:/bin
In the example above, the command module load python/3.9.1 prepends the $PATH. To learn more about how to use environment modules, go to Unity’s Module Usage page.