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

Question

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 tryi

Answer 1

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 :
        ...
    else:
        fatal error: unknown cause

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