如果想查看当前连接在哪个数据库下面，可以直接输入db

> db
Admin

想切换到test数据库下面

> use test
switched to db test
> db

Test想查看test下有哪些表或者叫collection，可以输入

> show collections

system.indexes

user想知道mongodb支持哪些命令，可以直接输入help

> help
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   set curent database to
db.help()  help on DB methods
db.foo.help()  help on collection methods
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

如果想知道当前数据库支持哪些方法：

> db.help();
DB methods:
db.addUser(username, password) 添加数据库授权用户
db.auth(username, password)  访问认证
db.cloneDatabase(fromhost) 克隆数据库
db.commandHelp(name) returns the help for the command
db.copyDatabase(fromdb, todb, fromhost)  复制数据库
db.createCollection(name, { size : ..., capped : ..., max : ... } ) 创建表
db.currentOp() displays the current operation in the db
db.dropDatabase()  删除当前数据库
db.eval_r(func, args) run code server-side
db.getCollection(cname) same as db['cname'] or db.cname
db.getCollectionNames()  获取当前数据库的表名
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()
db.getReplicationInfo()
db.getSisterDB(name) get the db at the same server as this onew
db.killOp() kills the current operation in the db
db.printCollectionStats()  打印各表的状态信息
db.printReplicationInfo()  打印主数据库的复制状态信息
db.printSlaveReplicationInfo()  打印从数据库的复制状态信息
db.printShardingStatus()  打印分片状态信息
db.removeUser(username) 删除数据库用户
db.repairDatabase() 修复数据库
db.resetError()
db.runCommand(cmdObj) run a database command.  if cmdObj is a string, turns it into { cmdObj : 1 }
db.setProfilingLevel(level) 0=off 1=slow 2=all
db.shutdownServer()
db.version() current version of the server