# 11.1. Environment variables set for MPI applications

Open MPI provides the following environment variables that will be defined on every MPI process:

- `OMPI_COMM_WORLD_SIZE`: the number of processes in this process’s MPI_COMM_WORLD

- `OMPI_COMM_WORLD_RANK`: the MPI rank of this process in MPI_COMM_WORLD

- `OMPI_COMM_WORLD_LOCAL_SIZE`: the number of ranks from this job that are running on this node.

- `OMPI_COMM_WORLD_LOCAL_RANK`: the relative rank of this process on this node within its job. For example, if four processes in a job share a node, they will each be given a local rank ranging from 0 to 3.

- `OMPI_UNIVERSE_SIZE`: the number of process slots allocated to this job. Note that this may be different than the number of processes in the job.

- `OMPI_COMM_WORLD_NODE_RANK`: the relative rank of this process on this node looking across all jobs.