Unable to open Mongo shell using Docker run but exec - mongodb

Question

What I want to is open mongo shell in terminal.
When I run mongo container using run with mongo command,
I got an error.
$ docker run -it --name mongodb mongo:latest mongo
MongoDB shell version v4.2.1
connecting to: mongodb://127.0.0.1:27017/?compressors=disabled&gssapiServiceName=mongodb
2019-12-01T10:01:18.524+0000 E QUERY [js] Error: couldn't connect to server 127.0.0.1:27017, connection attempt failed: SocketException: Error connecting to 127.0.0.1:27017 :: caused by :: Connection refused :
connect#src/mongo/shell/mongo.js:341:17
#(connect):2:6
2019-12-01T10:01:18.526+0000 F - [main] exception: connect failed
2019-12-01T10:01:18.526+0000 E - [main] exiting with code 1
But when I run mongo first and exec command later, I can open mongo shell nicely.
$ docker run -d --name mongodb mongo:latest
0296856a6e614667ad7cb81cac104d2704369d8d98f9c5dfdc8724dd5c74591a
$ docker exec -it mongodb mongo
MongoDB shell version v4.2.1
connecting to: mongodb://127.0.0.1:27017/?compressors=disabled&gssapiServiceName=mongodb
Implicit session: session { "id" : UUID("02a0086f-35f8-4c8c-a615-9769743b22d4") }
MongoDB server version: 4.2.1
Welcome to the MongoDB shell.
(...)
> show dbs
admin 0.000GB
config 0.000GB
local 0.000GB
Why I can't open mongo shell using single line run command?
I guess that command try to mongo shell start before mongod ready. But I saw few docuements start mongo shell like this. Please let me know why this happend.

Answer 1

When you docker run imagename command, that command runs instead of the normal command the image would run (the CMD from its Dockerfile). When you docker exec containername command, that launches an additional command in the same container. So your docker run example launches the mongo shell instead of running a new database, but by default it tries to connect to a database on localhost, that is, in the same container.
For interacting with databases, my general recommendation would be to install the client tools you need locally and use them directly: run mongo on your host pointing at whatever port you published. You don't need Docker at all (or the root-level privileges it implies) just to make simple client calls. docker exec is in many ways equivalent to ssh'ing as root into the container, and it wouldn't be my preferred path to interacting with a database.
If you do want to docker run a client, you'd have to make it communicate with a server container, using the normal mechanisms for this.
docker network create mongo
docker run -d --net mongo -p 27017:27017 --name mongodb mongo:latest
docker run -it --net mongo mongo:latest \
mongo --host mongodb