With the default `mpiexec`, which is `mpiexec.hydra`, it sets environment variable `PMI_RANK`, and `PMI_SIZE`. You can check that with mpiexec -n 1 env |grep PMI_ -- Hui Zhou From: Sajid Ali via discuss <[email protected]> Date: Monday, January 4, 2021 at 8:13 PM To: [email protected] <[email protected]> Cc: Sajid Ali <[email protected]> Subject: [mpich-discuss] MPICH equivalent to OMPI_COMM_WORLD_RANK Hi MPICH-developers, Does MPICH set an environment variable describing the MPI rank equivalent to OpenMPI's `OMPI_COMM_WORLD_RANK` ? If so, what is it called ? Thank You, Sajid Ali (he/him) | PhD Candidate Applied Physics Northwestern University s-sajid-ali.github.io<http://s-sajid-ali.github.io>