Environment Modules are a useful resource for allowing users to control their Linux environment by loading and unloading modules to modify environment variables. This allows users flexibility in controlling which version of a specific software package will be used when compiling or running a program. Below is a list of common commands for using modules.

Command	Use
module avail	lists all available modules
module avail <software package>	shows all available versions of a specific software package
module display <module>	shows all the commands executed by specified module
module load <module>	loads a software module
module unload <module>	unloads a software module
module switch <module 1> <module 2>	unloads module 1 and loads module 2
module list shows all	currently loaded modules

Environment Modules Example

Imagine you wish to compile a C program written with mpi using the compiler mpicc. To see what implementations of mpi are available use the command module avail:

[shesprich@vs-bcmshane ~]$ module avail
----------------------------- /cm/local/modulefiles ------------------------------
cluster-tools/9.0         cmsh             luajit       python37
cm-image/9.0              dot              module-git   shared
cm-scale/cm-scale.module  freeipmi/1.6.4   module-info  slurm/slurm/19.05.5
cm-setup/9.0              gcc/9.2.0        null
cmd                       ipmitool/1.8.18  openldap
cmjob                     lua/5.3.5        python3

----------------------------- /cm/shared/modulefiles -----------------------------
blacs/openmpi/gcc/64/1.1patch03  intel-tbb-oss/ia32/2019_20191006oss     pgi/18.4
blas/gcc/64/3.8.0                intel-tbb-oss/intel64/2019_20191006oss  ucx/1.6.1
bonnie++/1.98                    iozone/3_487
cm-pmix3/3.1.4                   lapack/gcc/64/3.8.0
default-environment              mpich/ge/gcc/64/3.3.2
fftw3/openmpi/gcc/64/3.3.8       mvapich2/gcc/64/2.3.2
gaussian/g16                     netcdf/gcc/64/gcc/64/4.7.3
gdb/8.3.1                        netperf/2.7.0
globalarrays/openmpi/gcc/64/5.7  openblas/dynamic(default)
hdf5/1.10.1                      openblas/dynamic/0.2.20
hdf5_18/1.8.21                   openmpi/gcc/64/1.10.7
hwloc/1.11.11                    pgi/13.9

We can see here that we have mpich, openmpi and mvapich2 as our options.

Let's use the mpich implementation. First, let's examine the module using module display then add it to our environment using module load.

[shesprich@vs-bcmshane ~]$ module list
Currently Loaded Modulefiles:
 1) gcc/9.2.0   2) slurm/slurm/19.05.5
[shesprich@vs-bcmshane ~]$ module display mpich/ge/gcc/64/3.3.2
-------------------------------------------------------------------
/cm/shared/modulefiles/mpich/ge/gcc/64/3.3.2:

module-whatis   {adds MPICH-gcc to your environment variables}
prepend-path    PATH /cm/shared/apps/mpich/ge/gcc/64/3.3.2/bin
prepend-path    MANPATH /cm/shared/apps/mpich/ge/gcc/64/3.3.2/share/man
setenv          MPI_HOME /cm/shared/apps/mpich/ge/gcc/64/3.3.2
setenv          MPI_RUN /cm/shared/apps/mpich/ge/gcc/64/3.3.2/bin/mpirun
setenv          MPICH_USE_SHLIB yes
setenv          P4_GLOBMEMSIZE 67108864
prepend-path    LD_RUN_PATH /cm/shared/apps/mpich/ge/gcc/64/3.3.2/lib
prepend-path    LD_LIBRARY_PATH /cm/shared/apps/mpich/ge/gcc/64/3.3.2/lib
-------------------------------------------------------------------
[shesprich@vs-bcmshane ~]$ module load mpich/ge/gcc/64/3.3.2
[shesprich@vs-bcmshane ~]$ module list
Currently Loaded Modulefiles:
 1) gcc/9.2.0   2) slurm/slurm/19.05.5   3) mpich/ge/gcc/64/3.3.2
[shesprich@vs-bcmshane ~]$ which mpicc
/cm/shared/apps/mpich/ge/gcc/64/3.3.2/bin/mpicc

Finally, if during the process of developing your software you realize that openmpi may be a better option for your specific needs. You can easily change implementations using module switch: 

[shesprich@vs-bcmshane ~]$ module switch mpich/ge/gcc/64/3.3.2 \
> openmpi/gcc/64/1.10.7
[shesprich@vs-bcmshane ~]$ which mpicc
/cm/shared/apps/openmpi/gcc/64/1.10.7bin/mpicc

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