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mongo <dbname> --eval "db.dropDatabase()"
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// First show what databases have you gotten
show dbs
// then, use the database name that you want to drop
use YOUR_DATABASE_NAME
// now you can drop it
db.dropDatabase()
// show collections should be empty now
show collections
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use DB_NAME // go to the db you want to delete
db.dropDatabase()
show dbs
// droppped db will not be shown