[docker] How to know if docker is already logged in to a docker registry server

I'm not sure if I have already logged in to a docker registry in cmd line by using cmd: docker login. How can you test or see whether you are logged in or not, without trying to push?

The docker cli credential scheme is unsurprisingly uncomplicated, just take a look:

cat ~/.docker/config.json

{
  "auths": {
    "dockerregistry.myregistry.com": {},
    "https://index.docker.io/v1/": {}

This exists on Windows (use Get-Content ~\.docker\config.json) and you can also poke around the credential tool which also lists the username ... and I think you can even retrieve the password

. "C:\Program Files\Docker\Docker\resources\bin\docker-credential-wincred.exe" list

{"https://index.docker.io/v1/":"kcd"}

My AWS ECR build-script has:

ECR_HOSTNAME="${ACCOUNT_ID}.dkr.ecr.${REGION}.amazonaws.com"
TOKEN=$(jq -r '.auths["'$ECR_HOSTNAME'"]["auth"]' ~/.docker/config.json)
curl --fail --header "Authorization: Basic $TOKEN" https://$ECR_HOSTNAME/v2/

If accessing ECR fails, a login is done:

aws ecr get-login-password --region ${REGION} | docker login --username AWS --password-stdin https://$ECR_HOSTNAME

For this to work, a proper Docker credential store cannot be used. Default credentials store of ~/.docker/config.json is assumed.

For private registries, nothing is shown in docker info. However, the logout command will tell you if you were logged in:

 $ docker logout private.example.com
 Not logged in to private.example.com

(Though this will force you to log in again.)

You can do the following command to see the username you are logged in with and the registry used:

docker system info | grep -E 'Username|Registry'

I use one of the following two ways for this check:

1: View config.json file:

In case you are logged in to "private.registry.com" you will see an entry for the same as following in ~/.docker/config.json:

"auths": {
    "private.registry.com": {
        "auth": "gibberishgibberishgibberishgibberishgibberishgibberish"
    }
 }

2: Try docker login once again:

If you are trying to see if you already have an active session with private.registry.com, try to login again:

bash$ docker login private.registry.com
Username (logged-in-user):

If you get an output like the above, it means logged-in-user already had an active session with private.registry.com. If you are just prompted for username instead, that would indicate that there's no active session.

If you want a simple true/false value, you can pipe your docker.json to jq.

is_logged_in() {
  cat ~/.docker/config.json | jq -r --arg url "${REPOSITORY_URL}" '.auths | has($url)'
}

if [[ "$(is_logged_in)" == "false" ]]; then
  # do stuff, log in
fi

Use command like below:

docker info | grep 'name'

WARNING: No swap limit support
Username: <strong>jonasm2009</strong>

At least in "Docker for Windows" you can see if you are logged in to docker hub over the UI. Just right click the docker icon in the windows notification area:

On windows you can inspect the login "authorizations" (auths) by looking at this file: [USER_HOME_DIR].docker\config.json

Example: c:\USERS\YOUR_USERANME.docker\config.json

It will look something like this for windows credentials

{
"auths": {
    "HOST_NAME_HERE": {},
    "https://index.docker.io/v1/": {}
},
"HttpHeaders": {
    "User-Agent": "Docker-Client/18.09.0 (windows)"
},
"credsStore": "wincred",
"stackOrchestrator": "swarm"
}

Just checked, today it looks like this:

$ docker login
Authenticating with existing credentials...
Login Succeeded

NOTE: this is on a macOS with the latest version of Docker CE, docker-credential-helper - both installed with homebrew.

The answers here so far are not so useful:

• docker info no longer provides this info
• docker logout is a major inconvenience - unless you already know the credentials and can easily re-login
• docker login response seems quite unreliable and not so easy to parse by the program

My solution that worked for me builds on @noobuntu's comment: I figured that if I already known the image that I want to pull, but I'm not sure if the user is already logged in, I can do this:

try pulling target image
-> on failure:
   try logging in
   -> on failure: throw CannotLogInException
   -> on success:
      try pulling target image
      -> on failure: throw CannotPullImageException
      -> on success: (continue)
-> on success: (continue)

As pointed out by @Christian, best to try operation first then login only if necessary. Problem is that "if necessary" is not that obvious to do robustly. One approach is to compare the stderr of the docker operation with some strings that are known (by trial and error). For example,

try "docker OPERATION"
if it failed: 
    capture the stderr of "docker OPERATION"
    if it ends with "no basic auth credentials": 
        try docker login
    else if it ends with "not found":
        fatal error: image name/tag probably incorrect
    else if it ends with <other stuff you care to trap>:
        ...
    else:
        fatal error: unknown cause

try docker OPERATION again
if this fails: you're SOL!