MongoDB - The mongo Shell, Write Scripts for the mongo Shell

Posted 2020-08-21 huey

You can write scripts for the mongo shell in javascript that manipulate data in MongoDB or perform administrative operation. For more information about the mongo shell, see the Running .js files via a mongo shell Instance on the Server section for more information about using these mongo script.

This tutorial provides an introduction to writing JavaScript that uses the mongo shell to access MongoDB.

 

Opening New Connections

From the mongo shell or from a JavaScript file, you can instantiate database connections using the Mongo()constructor:

> new Mongo()
> new Mongo(<host>)
> new Mongo(<host:port>)

Consider the following example that instantiates a new connection to the MongoDB instance running on localhost on the default port and sets the global db variable to myDatabase using the getDB() method:

> conn = new Mongo();
2016-12-01T00:09:03.664+0800 I NETWORK  [initandlisten] connection accepted from 127.0.0.1:29118 #8 (8 connections now open)
connection to 127.0.0.1
> db = conn.getDB("myDatabase");
myDatabase

If connecting to a MongoDB instance that enforces access control, you can use the db.auth() method to authenticate.

Additionally, you can use the connect() method to connect to the MongoDB instance. The following example connects to the MongoDB instance that is running on localhost with the non-default port 27020and set the global db variable:

> db = connect("localhost:27020/myDatabase");

SEE ALSO: mongo Shell Methods

 

Differences Between Interactive and Scripted mongo

When writing scripts for the mongo shell, consider the following:

• To set the db global variable, use the getDB() method or the connect() method. You can assign the database reference to a variable other than db.

• Write operations in the mongo shell use a write concern of { w: 1 } by default. If performing bulk operations, use the Bulk() methods. See Write Method Acknowledgements for more information.

  Changed in version 2.6: Before MongoDB 2.6, call db.getLastError() explicitly to wait for the result of write operations.

• You cannot use any shell helper (e.g. use <dbname>, show dbs, etc.) inside the JavaScript file because they are not valid JavaScript.

  The following table maps the most common mongo shell helpers to their JavaScript equivalents.

  Shell Helpers	JavaScript Equivalents
  show dbs, show databases	db.adminCommand(‘listDatabases‘)
  use <db>	db = db.getSiblingDB(‘<db>‘)
  show collections	db.getCollectionNames()
  show users	db.getUsers()
  show roles	db.getRoles({showBuiltinRoles: true})
  show log <logname>	db.adminCommand({ ‘getLog‘ : ‘<logname>‘ })
  show logs	db.adminCommand({ ‘getLog‘ : ‘*‘ })
  it	cursor = db.collection.find()  if ( cursor.hasNext() ){      cursor.next();  }

• In interactive mode, mongo prints the results of operations including the content of all cursors. In scripts, either use the JavaScript print() function or the mongo specific printjson() function which returns formatted JSON.

  To print all items in a result cursor in mongo shell scripts, use the following idiom:

  cursor = db.collection.find();
  while ( cursor.hasNext() ) {
      printjson( cursor.next() );
  }

 

Scripting

From the system prompt, use mongo to evaluate JavaScript.

--eval option

Use the --eval option to mongo to pass the shell a JavaScript fragment, as in the following:

mongo test --eval "printjson(db.getCollectionNames())"

This returns the output of db.getCollectionNames() using the mongo shell connected to the mongod or mongos instance running on port 27017 on the localhost interface.

Execute a JavaScript file

You can specify a .js file to the mongo shell, and mongo will execute the JavaScript directly. Consider the following example:

mongo localhost:27017/test myjsfile.js

This operation executes the myjsfile.js script in a mongo shell that connects to the test database on the mongod instance accessible via the localhost interface on port 27017.

Alternately, you can specify the mongodb connection parameters inside of the javascript file using the Mongo() constructor. See Opening New Connections for more information.

You can execute a .js file from within the mongo shell, using the load() function, as in the following:

load("myjstest.js")

This function loads and executes the myjstest.js file.

The load() method accepts relative and absolute paths. If the current working directory of the mongo shell is /data/db, and the myjstest.js resides in the /data/db/scripts directory, then the following calls within the mongo shell would be equivalent:

load("scripts/myjstest.js")
load("/data/db/scripts/myjstest.js")

NOTE: There is no search path for the load() function. If the desired script is not in the current working directory or the full specified path, mongo will not be able to access the file.