MongoDB是一个NoSQL数据库系统：一个数据库可以包含多个集合（Collection），每个集合对应于关系数据库中的表；而每个集合中可以存储一组由列标识的记录，列是可以自由定义的，非常灵活，由一组列标识的实体的集合对应于关系数据库表中的行。下面通过熟悉MongoDB的基本管理命令，来了解MongoDB提供的DBMS的基本功能和行为。
MongoDB命令帮助系统
在安装MongoDB后，启动服务器进程（mongod），可以通过在客户端命令mongo实现对MongoDB的管理和监控。看一下MongoDB的命令帮助系统：
- root@dev2:~# mongo
- MongoDB shell version: 1.8.3
- connecting to: test
- > help
- db.help() help on db methods
- db.mycoll.help() help on collection methods
- rs.help() help on replica set methods
- help connect connecting to a db help
- help admin administrative help
- help misc misc things to know
- help mr mapreduce help
- show dbs show database names
- show collections show collections in current database
- show users show users in current database
- show profile show most recent system.profile entries with time >= 1ms
- use <db_name> set current database
- db.foo.find() list objects in collection foo
- db.foo.find( { a : 1 } ) list objects in foo where a == 1
- it result of the last line evaluated; use to further iterate
- DBQuery.shellBatchSize = x set default number of items to display on shell
- exit quit the mongo shell
这是MongoDB最顶层的命令列表，主要告诉我们管理数据库相关的一些抽象的范畴：数据库操作帮助、集合操作帮助、管理帮助。如果你想了解数据库操作更详细的帮助命令，可以直接使用db.help()，如下所示：
- > db.help()
- DB methods:
- db.addUser(username, password[, readOnly=false])
- db.auth(username, password)
- db.cloneDatabase(fromhost)
- db.commandHelp(name) returns the help for the command
- db.copyDatabase(fromdb, todb, fromhost)
- db.createCollection(name, { size : ..., capped : ..., max : ... } )
- db.currentOp() displays the current operation in the db
- db.dropDatabase()
- db.eval(func, args) run code server-side
- db.getCollection(cname) same as db['cname'] or db.cname
- db.getCollectionNames()
- db.getLastError() - just returns the err msg string
- db.getLastErrorObj() - return full status object
- db.getMongo() get the server connection object
- db.getMongo().setSlaveOk() allow this connection to read from the nonmaster member of a replica pair
- db.getName()
- db.getPrevError()
- db.getProfilingLevel() - deprecated
- db.getProfilingStatus() - returns if profiling is on and slow threshold
- db.getReplicationInfo()
- db.getSiblingDB(name) get the db at the same server as this one
- db.isMaster() check replica primary status
- db.killOp(opid) kills the current operation in the db
- db.listCommands() lists all the db commands
- db.printCollectionStats()
- db.printReplicationInfo()
- db.printSlaveReplicationInfo()
- db.printShardingStatus()
- db.removeUser(username)
- db.repairDatabase()
- db.resetError()
- db.runCommand(cmdObj) run a database command. if cmdObj is a string, turns it into { cmdObj : 1 }
- db.serverStatus()
- db.setProfilingLevel(level,<slowms>) 0=off 1=slow 2=all