How to detect if docker run succeeded programmatic

Question

I'm writing a very simple bash script to quickly check that my container still builds and starts correctly and that the app inside responds to requests.

Sometimes docker run fails, e.g. because the port I'm trying to bind the container to is already allocated. But when this happens docker run's exit code is still 0 so I can't use the exit code. How can I check programmatically that the container got started correctly?

The solutions I'm considering are:

• parse the output for errors
• docker ps to see if the container is running

but these both seem a little overkill and ugly. Am I missing a better way to check whether docker run succeeded?

Answer 1

As suggested by Abel Muiño in comments, this may have been fixed in more recent Docker versions (I'm currently running 0.9.1).

But, if you're temporarily stuck like me with an older version, I did find a decent workaround to check if the container started by using docker inspect.

docker inspect returns a JSON object with a lot of info about the container, and in particular whether the container is currently running or not. The -f flag lets you easily extract the bits needed:

docker inspect -f {{.State.Running}} $CONTAINER_ID

or

docker inspect -f "{{.State.Running}}" $CONTAINER_ID

will return true or false.

Note that you probably want to sleep 1 (or more) between starting the container and checking if it is up. If there's something wrong with your setup it's possible that it would appear as 'running' for a very short time before actually exiting.

Answer 2

To avoid parsing anything, you could use docker top, which returns 1 if the container is not running:

id=$(docker run mycontainer)
if ! docker top $id &>/dev/null
then
    echo "Container crashed unexpectedly..."
    return 1
fi

Answer 3

We could use docker exec $id true 2>/dev/null || echo not running.

This command does not write to stdout, as "docker top" does. It writes to stderr when the container is not running, the same message as "docker top".