Docker Tips

I’ve been working with Docker the past few months and all-in-all, I’ve been very pleased with the quality of the documentation. But, as with any other tool, there are always a few tricks to pick up, particularly when trying to script things out for an automated build or deployment. I’ve listed some of the more useful ones below and will update this post as I learn new ones.

Note: These are mainly oriented around running Docker in a Linux environment, as that’s where I’m currently using it.

How do I stop typing sudo all the time?

Docker runs as root, so when you’re working with an out-of-the-box installation, the docker command must be preceded by sudo. Since it may not be desirable for all Docker users to be able to execute commands as root, the installation creates a docker group. Members of the group may execute docker commands without elevated privileges.

To add users to the group, execute the command:
sudo usermod -aG docker <username>

How do I remove all stopped containers?

When a container is stopped, it remains loaded. You can remove it by issuing the command docker rm container_name, but that can be a hassle if you have a large number of containers loaded and they all have random names (a frequent occurrence when you’re first learning Docker).

You can remove all stopped containers by executing the command:
docker rm $(docker ps --quiet -a --filter status=exited)

(The –filter option prevents errors from attempting to remove containers which are currently running.)

You can also cause your containers to remove themselves automatically by including the –rm option on the docker run command line.

How do I know if a container is running?

To determine if a named container (e.g. “clever_leakey”) is currently running

containerID=$(docker ps --quiet --filter status=running --filter name=clever_leakey)

if $containerID is non-null, the named container is running. If it’s null, then the container is no longer running.

Do note however that there are other non-running states, e.g. paused, which will also return a null containerID for this test. As an alternative, to find only the containers which are stopped, use status=exited.

If the docker run command includes the –rm option, the container will be removed from memory.

(Image via openclipart under Creative Commans CC0 1.0 Universal)

Fixing Evernote’s “Could not add tray icon, error: An attempt was made to reference a token that does not exist.” message

I reinstalled Evernote a week or so back and every time I fired it up, a background window would also open containing the message “Could not add tray icon, error: An attempt was made to reference a token that does not exist.” Every time this happened, I’d dismiss the message and move on with what I was working on.

This routine got old pretty quickly so I did what any other geek would do and Googled for the message. Apparently the message has been around for a while, with the suggested fix being to reinstall Evernote. So I uninstalled Evernote, waited a few minutes, and then reinstalled it. Then I went back into the application and a background window opened with the same message.

This time, after closing both the pop-up and the main application window, I took a look in the system tray and discovered that Evernote’s “running in the background” icon was also missing. I also realized I’d never been prompted to run the installer as an administrator.

I run my computer differently than most people – the user account where I do my day-to-day work has reduced privileges. There’s a separate login for anything requiring elevated privileges, such as installing software. Most installers will either prompt you to either login as an administrator, or else they’ll install to an alternate location (generally somewhere in the %APPDATA% folder). I didn’t dig too deeply, but my best guess is that Evernote was doing the latter, but the system tray icon requires something to be installed with higher privileges.

In the end, I uninstalled Evernote again and this time made sure to re-install with admin privileges.

I haven’t seen the error message since.

(Public domain image, via pixabay)