Mongo shell: iterate over all dbs

iterate_dbs.js

/* switch to admin database and get list of databases */
db = db.getSiblingDB("admin");
dbs = db.runCommand({ "listDatabases": 1 }).databases;

/* iterate through each database and get its collections */
dbs.forEach(function(database) {
  print(database.name);
  db = db.getSiblingDB(database.name);
  cols = db.getCollectionNames();

  cols.forEach(function(col) {
    if (col === 'emails') { print(col); }
  });
});