先從幫助命令入手，看能做哪些事情。

    db.help()                    help on db methods
    db.mycoll.help()             help on collection methods
    sh.help()                    sharding helpers
    rs.help()                    replica set helpers
    help admin                   administrative help
    help connect                 connecting to a db help
    help keys                    key shortcuts
    help misc                    misc things to know
    help mr                      mapreduce

    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
    show logs                    show the accessible logger names
    show log [name]              prints out the last segment of log in memory, 'global' is default
    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

db.help() 和 db.mycoll.help() 從字面意思很容易看出是數(shù)據(jù)庫 (database慧瘤，類似關系型數(shù)據(jù)庫的庫) 和里面集合 (collection，類似關系型數(shù)據(jù)庫的表)，這兩個命令內(nèi)容非常多艇拍，是一系列核心操作。

sh.help()：google 一番：

Sharding is a type of database partitioning that separates very large databases the into smaller, faster, more easily managed parts called data shards. The word shard means a small part of a whole.

大概是數(shù)據(jù)庫體積變得非常大的時候的優(yōu)化手段宠纯。

rs.help()：mongodb 官網(wǎng)是這樣介紹的：

A replica set in MongoDB is a group of mongod processes that maintain the same data set. Replica sets provide redundancy and high availability, and are the basis for all production deployments.

類似 mysql 的主從數(shù)據(jù)庫概念卸夕。

help admin：一些操作文件，診斷信息的命令婆瓜。

help connect：如何連接到 mongodb快集。

help keys：shell 可用的快捷鍵。

help misc：

To get a list of the wrapper classes available in the mongo shell, such as BinData(), type help misc in the mongo shell

可以通過這個命令查詢數(shù)據(jù)被 BinData() 包裹之后可以執(zhí)行的方法廉白。

help mr：

Map-reduce is a data processing paradigm for condensing large volumes of data into useful aggregated results. For map-reduce operations, MongoDB provides the mapReduce database command.

將巨大的數(shù)據(jù)匯總成直觀的結(jié)果个初。

剩下的都是一些比較簡單的查詢命令和其它命令。