Check if image:tag combination already exists on docker hub

As part of a bash script, I want to check if a particularly docker image:tag combination exists on docker hub. Also, it will be a private repository.

i.e. the pseudocode would be like:

tag = something
if image:tag already exists on docker hub:
    Do nothing
else
    Build and push docker image with that tag

Update: Docker-Free solution see below

Using Docker

This is the solution I use with gitlab using the docker:stable image.

Login

docker login -u $USER -p $PASSWORD $REGISTRY

Check whether it's there:

docker manifest inspect $IMGNAME:$IMGTAG > /dev/null ; echo $?

docker will return 0 on success or 1 on failure.

If you get a warning: Update Docker or enable experimental client-features:

Set the environment variable DOCKER_CLI_EXPERIMENTAL to enabled (See Matěj's answer below)

Alternatively adjust the config (original answer):

echo '{"experimental": "enabled"}' > ~/.docker/config.json

This will also overwrite your config. If that is not an option you need to do that manually or use jq, sed or whatever you have available.

Update: If you don't have access to a docker-daemon, e.g. because you are building a docker image using kaniko within a docker, you can use the registry-api scripts provided by harbor. Note that they are python2.

Please try this one

function docker_tag_exists() {
    curl --silent -f -lSL https://index.docker.io/v1/repositories/$1/tags/$2 > /dev/null
}

if docker_tag_exists library/nginx 1.7.5; then
    echo exist
else 
    echo not exists
fi

Update:

In case of usage Docker Registry v2 (based on this):

# set username and password
UNAME="user"
UPASS="password"

function docker_tag_exists() {
    TOKEN=$(curl -s -H "Content-Type: application/json" -X POST -d '{"username": "'${UNAME}'", "password": "'${UPASS}'"}' https://hub.docker.com/v2/users/login/ | jq -r .token)
    curl --silent -f --head -lL https://hub.docker.com/v2/repositories/$1/tags/$2/ > /dev/null
}

if docker_tag_exists library/nginx 1.7.5; then
    echo exist
else 
    echo not exists
fi