This document discusses the MongoDB shell, which is a JavaScript interpreter with built-in support for MongoDB. It can be used for interactive development, testing, administration, and learning MongoDB. The shell allows running queries, saving documents, and accessing MongoDB helpers and methods. It also supports command line editing, tab completion, and examining JavaScript code to understand how the shell works. While JavaScript has limitations, the shell provides a useful interface for working with MongoDB.

1. Mastering the Shell — MongoBerlin
Richard M Kreuter
10gen Inc.
richard@10gen.com
March 25, 2011
Mastering the Shell — MongoBerlin

2. The Mongo Shell
What is the shell?
The mongo shell is a JavaScript interpreter with built-in support for
MongoDB connectivity.
Mastering the Shell — MongoBerlin

3. What’s the shell good for?
Interactive development/prototyping.
Test scripting (cf. MongoDB’s own regression framework).
Administrative operations, lightweight scripting.
Learning MongoDB (and teaching it, too).
Mastering the Shell — MongoBerlin

4. Running the Shell
$ mongo
MongoDB shell version: 1.6.4
connecting to: test
> db.people.save({name:"Washington", no: 1});
> db.people.save({name:"Adams", no: 2});
> db.people.save({name:"Jefferson", no: 3});
> for (i=0; i<1024; i++) db.numbers.save({ num: i });
Mastering the Shell — MongoBerlin

5. Running the shell (continued)
You can execute a file of code either by specifying command-line
argument or with the built-in load extension function.
$ mongo foo.js # executes foo.js and exits
$ mongo
MongoDB shell version: 1.6.4
connecting to: test
> load("foo.js") // executes foo.js and returns to prompt
Mastering the Shell — MongoBerlin

6. Running the shell (continued continued)
As of 1.8, you can also use mongo as a “shebang” interpreter.
$ cat ~/foo.js
#!/Users/kreuter/10gen/mongo/mongo
...
$ ./foo.js
Mastering the Shell — MongoBerlin

7. Shell helpers
The shell has some built-in helpers that resemble the MySQL
shell’s.
// Change database to "foo"
use foo
// List the collections in "foo"
show collections
Note that these helpers aren’t strictly JavaScript; they’re sort of
preprocessors available only when the shell is run interactively.
There are proper JavaScript methods for doing these things
programmatically (e.g., db=db.getSisterDB("foo")).
Mastering the Shell — MongoBerlin

8. Completion in interactive mode
The shell supports completion (improved notably in 1.8).
Completion can introspect on JavaScript objects to find
attributes/methods:
db.<TAB> // print methods on the db object
db.people.<TAB> // print methods on the people collection
// etc...
Mastering the Shell — MongoBerlin

9. Command-line editing
The shell uses readline for command completion, history, editing,
etc. (for now, anyway). So it has similar keybindings by default to
those in bash, et. al.
Mastering the Shell — MongoBerlin

10. Getting help
> help // help is a helper...
...
> db.help() // db.help() is a method on the db object
...
> db.collection.help() // a method on the collection
Mastering the Shell — MongoBerlin

11. Working with the shell
The shell runs all queries in “SafeMode”, i.e., it executes
getLastError and prints any error message after data
manipulations. So, for example,
> db.foo.insert({_id:1});
> db.foo.insert({_id:1});
E11000 duplicate key error index: test.foo.$_id_
dup key: { : 1.0 }
Mastering the Shell — MongoBerlin

12. Working with the shell (continued)
Because the shell uses JavaScript, a little care is called for when
handling types MongoDB supports that JavaScript doesn’t:
JavaScript’s only number type is double-floats. (Use
NumberLong to construct 64-bit integers.)
Documents having multiple values for the same key aren’t
supported in JavaScript (but you shouldn’t really use these
anyway).
Binary data is represented by the BinData type.
Also, note that in JavaScript, Date(string) returns a string;
you almost always want new Date(string), which returns an
object. (In 1.8, see the ISODate() function.)
Mastering the Shell — MongoBerlin

13. Cursors in the shell
By default, cursors in the shell are printed by iterating the cursor
some number of times, and assigning the variable it to an iterator:
> db.numbers.find()
{ "_id" : ObjectId("4cf91b32e3f85d1561593dfc"), "num" : 0 }
...
has more
> it
{ "_id" : ObjectId("4cf91b32e3f85d1561593e10"), "num" : 20
...
has more
Mastering the Shell — MongoBerlin

14. Cursors in the shell (continued)
The shell supports cursors as first-class objects:
> var cur=db.people.find()
> cur.hasNext()
true
> while (cur.hasNext()) { printjson(cur.next().name); }
"Washington"
"Adams"
"Jefferson"
> var cur2=db.people.find({}, {name:1})
> cur2.forEach(printjson)
{ "_id" : ..., "name" : "Washington" }
{ "_id" : ..., "name" : "Adams" }
{ "_id" : ..., "name" : "Jefferson" }
Mastering the Shell — MongoBerlin

15. Examining JavaScript code
Most of the shell’s functionality is implemented in JavaScript itself,
with the consequence that you can examine (and so cargo-cult) it
yourself:
> db.people.findOne
function (query, fields) {
var cursor = this._mongo.find(this._fullName,
this._massageObject(query) || {}, fields, -1, 0, 0);
if (!cursor.hasNext())
return null;
var ret = cursor.next();
if (cursor.hasNext())
throw "findOne has more than 1 result!";
if (ret.$err)
throw "error " + tojson(ret);
return ret;
}
Mastering the Shell — MongoBerlin

16. A nifty function
Here’s a nifty administrative function (stolen from Scott
Hernandez’s talk on the shell):
var cursor = db.coll.find();
var biggest=0;
var doc = {};
cursor.forEach(function (x) {
var size = Object.bsonsize(x);
if (size > biggest) { biggest=size; doc = x; }
});
Mastering the Shell — MongoBerlin

17. Gotchas
JavaScript isn’t the fastest language around.
JavaScript lacks features for “programming in the large”
(modules/packages/namespaces/etc.)
Iterating arrays (in index order) is slow.
Some data types require special care.
Mastering the Shell — MongoBerlin

18. So give the shell another spin!
www.mongodb.org — downloads, docs, community
mongodb-user@googlegroups.com — mailing list
#mongodb on irc.freenode.net
try.mongodb.org — web-based shell
10gen is hiring. Email jobs@10gen.com.
10gen offers support, training, and advising services for
mongodb
Mastering the Shell — MongoBerlin