先从帮助命令入手，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() 从字面意思很容易看出是数据库 (database，类似关系型数据库的库) 和里面集合 (collection，类似关系型数据库的表)，这两个命令内容非常多，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.
大概是数据库体积变得非常大的时候的优化手段。
rs.help()：mongodb 官网是这样介绍的：
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.
类似 mysql 的主从数据库概念。
help admin：一些操作文件，诊断信息的命令。
help connect：如何连接到 mongodb。
help keys：shell 可用的快捷键。
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
可以通过这个命令查询数据被 BinData() 包裹之后可以执行的方法。
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.
将巨大的数据汇总成直观的结果。
剩下的都是一些比较简单的查询命令和其它命令。
