Intro. to Environment Modules
As a Unity user, you have access to a wide variety of software. Making all of this software available simultaneously and without conflicts is a complex problem, and the solution is Environment modules.
An environment is a set of shell variables of the form KEY="VALUE".
env command.The PATH environment variable
$PATH is a list of directories (folders) delimited by colons.
$ echo $PATH
/usr/local/bin:/usr/bin:/bin
Most of the commands you use in the shell are actually executable files somewhere on the filesystem. When you enter a command, 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 is found earlier in the $PATH (further to the left) is used.
This means that the commands available in your shell can be changed by changing your environment variables.
Modules
Environment modules are scripts that modify your environment. We use modules to add new directories to your $PATH, making the executables within available for use. We ‘prepend’ the $PATH, making this new directory furthest to the left. This makes sure that the executables within are chosen first by the shell when you call their name.
$ which python3
/usr/bin/python3
$ module load python/3.9.1
$ which python3
/modules/apps/python/3.9.1/bin/python3
$ 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