Mongo Shell Commands

• Mongo shell is a fully functioning JavaScript interpreter, which means that you can create things like JavaScript functions and variables in it.

• Once mongo shell is installed, we can use the following commands when we dont have a GUI to interact with mongodb.

Common commands

1. Show databases

show dbs

1. Select database.

use <database_name> //no angular brackets

• The above command can be used to view the available namespaces.

1. To view collections in the database once a database is selected using above command.

show collections

1. Admin database consists of all the information about users.

2. Find Command - To query the collections.

   • Returns a cursor, basically a pointer to the query result.

     • To find all documents in a collection

       db.<collection_name>.find({}) OR db.<collection_name>.find()

       • The above command returns at max 20 entries. We can request more by (typing it).

     • To find documents based on criteria

       db.<collection_name>.find({"key": value}) db.<collection_name>.find({"key": value, "key":value })

     • To count the documents in the returned collection

       db.<collection_name>.find({"key":value }).count()

     • To pretty print the result of find query

       db.<collection_name>.find({"key":value }).pretty()

     • To find one document, this one document can be any random document in the collection.

       db.<collection_name>.findOne()

   • Projections in find query

     db.<collection_name>.find({<query>}, {<projections>})

     • Example

        db.account.find(
              { "name": "Joe" },
              { "age": 1, "address": 0, "_id": 0 }
           )

     • In the above example, age is projected and address is not projected in the result.

       • 1 -> include the field

       • 0 -> exclude the field

     • Use only 1s or 0s donot mix and match, exception being _id field which is always projected, if not explicitly mentioned to be excluded.

3. Insert Command - To insert document/s in to a collection

   • To insert one document

     db.<collection_name>.insert()

     • Example

        db.accounts.insert({
              "_id":ObjectId("61d982f17d3abc1d237445b3"),
              "name" : "John Doe",
              "age":25
        })

     - In the above example "_id" field is specified which needs to be unique in the accounts collection, if not specified mongodb will automatically generate one.

   • To insert many documents at once

     db.<collection_name>.insert([<document1>, <document2> ...])

     • Example

        db.accounts.insert([
        {
              "_id":ObjectId("61d982f17d3abc1d237445b3"),
              "name" : "John Doe",
              "age":25
        },
        {
              "_id":ObjectId("61d982f17d3abc1d237445b4"),
              "name" : "Kirsten Smith",
              "age":26
        } 
        ])

     • When inserting duplicate documents, insertion fails on occurence of first duplicate document, hence preventing insertion of remaining valid (non-duplicating) entries. This can be prevented by using "ordered" flag as follows,

         db.accounts.insert([
        {
              "_id":ObjectId("61d982f17d3abc1d237445b3"),
              "name" : "John Doe",
              "age":25
        },
        {
              "_id":ObjectId("61d982f17d3abc1d237445b4"),
              "name" : "Kirsten Smith",
              "age":26
        } ], 
        **{ 
           "ordered":false
        }**
        )

     • Setting ordered flag as false makes documents insertion into collection in unordered manner. This helps when inserting duplicate documents with same "_id" value, so that failure will occur only for duplicate documents and insertion for non duplicate documents will be successful.

4. Update Command - To update document/s in a collection

   • To update one document.

   db.<collection_name>.update({<restriction>}, {<update operation>})

   • Example

   db.accounts.update(
      {
         "name": "Joe"
      },
      {
         "$set":{
            "name": "Jason"
         }
      }
   )

   • To update many document.

   db.<collection_name>.updateMany({<restriction>}, {<update operation>})

   • Example

   db.accounts.updateMany(
      {
         "age": 17
      },
      {
         "$inc":{
            "age": 1
         }
      }
   )

   • The above update operators i.e., $set and $inc can take more than one fields to update.

   • To add an element to an existing array in document use $push opeerator.

   • Read more about update operators.

5. Delete Command - To delete document/s from a collection

   • To delete one document

   db.<collection_name>.deleteOne({<restriction>})

   • Example

     db.accounts.deleteOne({"name": "Joe"})

   • To delete many document

   db.<collection_name>.deleteMany({<restriction>})

   • Example

      db.accounts.deleteMany({"name": "Joe"})

6. Drop Collection

   db.<collection_name>.drop()

7. db

• This command prints the current db selected.