https://www.mongodb.com/docs/manual/reference/program/mongod/#std-option-mongod .–replSet {{}} Running: "mongod --replSet s0 --logpath "/data/logs/s0-r0.log" --dbpath /data/shard0/rs0 --bind_ip_all --port 37017 --fork -- outputConfig "   produces the following YAML output.  Output below (generated by --outputConfig) to be used as a mongod configuration file. net: bindIp: "*" port: 37017 outputConfig: true processManagement: fork: true replication: replSet: s0 storage: dbPath: /data/shard0/rs0 systemLog: destination: file path: /data/logs/s0-r0.log However, if you use this output as a configuration file, mongod will not start as you get the following error: "Unrecognized option: replication.replSet" The following manual correction in the YAML file  below allows mongod to start replication: replSetName: s0   https://www.mongodb.com/docs/manual/reference/configuration-options/#mongodb-setting-replication.replSetName   --outputConfig should output the correct name (replSetName) for use in the YAML file as it is different from the name that needs to be used as a command line option (replSet)