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)

Geek Humor

One of my co-workers retired back in the Spring, but still stops by to visit on occasion. He understood the importance of software source control, but had a few struggles with it. As he was leaving after his most recent visit, we had this brief exchange

  • Co-Worker: You know, I left on March 15 and haven’t had any tree conflicts since then.
  • Me: That’s great! No branches falling in the back yard?
  • Co-Worker: Well… at least, they weren’t trying to merge.

It’s difficult to get much geekier than when you’re making source control puns.

Problem: chmod is ignored in the Git Bash prompt

So here’s a strange one that had me baffled for a bit – the chmod command is pretty much a null operation from the Git Bash prompt (MingW64). This initially showed up on a script for launching a Docker container, but as nearly as I can tell, it happens for any shell script.

So, we have a simple script that prints out “Hello World!”.

[email protected] MINGW64 ~/test
$ cat foo
echo Hello World!

Simple enough. Now the thing is, I want to make this script executable. Now this particular Bash implementation will let me run ./foo and it’ll execute, but my real use case (running a Docker container) is going to have a somewhat longer name. Just as a matter of convenience, I’d like to to type just the first few characters, press tab, and have the filename expanded. And besides, your executable files should always be marked as executable.

[email protected] MINGW64 ~/test
$ ls -l
total 2
-rwxr-xr-x 1 blair 197121 28 Oct 18 00:20 bar*
-rw-r--r-- 1 blair 197121 18 Oct 18 00:10 foo

[email protected] MINGW64 ~/test
$

OK, this is an easy fix, I just need to run chmod and set the execute bit to on, right?

[email protected] MINGW64 ~/test
$ ls -l
total 2
-rwxr-xr-x 1 blair 197121 28 Oct 18 00:20 bar*
-rw-r--r-- 1 blair 197121 18 Oct 18 00:10 foo

[email protected] MINGW64 ~/test
$ chmod 744 foo
[email protected] MINGW64 ~/test
$ ls -l
total 2
-rwxr-xr-x 1 blair 197121 28 Oct 18 00:20 bar*
-rw-r--r-- 1 blair 197121 18 Oct 18 00:10 foo

The execute bit didn’t change. Maybe I need to use the u+x syntax instead?

$ chmod u+x foo
[email protected] MINGW64 ~/test
$ ls -l
total 2
-rwxr-xr-x 1 blair 197121 28 Oct 18 00:20 bar*
-rw-r--r-- 1 blair 197121 18 Oct 18 00:10 foo

Still no luck. So why is bar marked as executable? What’s the difference between these two scripts? The answer turns out to be one line of code:

[email protected] MINGW64 ~/test
$ chmod u+x foo
[email protected] MINGW64 ~/test
$ cat bar
#!/bin/sh
echo Hello World!

Do you see that first line, where it says “#!/bin/sh”. That’s how Bash knows what interpreter to pass the script to. It also turns out, in this particular implementation, that’s how Bash knows the file contains an executable script instead of just text.

So we modify foo, and get this result:

[email protected] MINGW64 ~/test$ cat foo
#!/bin/sh
echo Hello World!
[email protected] MINGW64 ~/test
$ ls -l
total 2
-rwxr-xr-x 1 blair 197121 28 Oct 18 00:20 bar*
-rwxr--r-- 1 blair 197121 18 Oct 18 00:10 foo*

(Image credit: Screenshot by ThatBlairGuy)

git error: Permission to user-B/repo.git denied to user-A

I have two GitHub accounts: UserA and UserB. Over time I’ve been switching to working with UserB, but the switchover was a bit difficult.

I created a test repository on GitHub at https://github.com/UserB/test

On the local system, from the command prompt

cd \git
git clone https://[email protected]/UserB/test
cd test
# make some changes to README.md, add a new foo.txt
git add *
git commit -m "Banana!" # In real life, you'll probably want a more useful comment.

And that’s where the train went off the rails…

C:\git\test>git push
remote: Permission to UserB/test.git denied to UserA.
fatal: unable to access 'https://[email protected]/UserB/test/': The requested URL returned error: 403

So git’s saying that even though I expressly got this as UserB, it still thinks I’m UserA

Google came back with lots of stuff about making sure you have the right SSH key (apparently the cool kids do everything over SSH).

A few search results make reference to the Windows Credential Manager. Apparently the Windows version of Git hooks into that somehow. What’s the Windows Credential Manager? Well, from the name, it sounds like something that might be used for storing userids and passwords.

OK, so how do I invoke it? Dunno. Let’s try the search box on the START menu. Aha! Two entries. One for “Credential Manager” and one for “Manage Windows Credentials.”

So let’s try the first one. Hey! This looks promising:

About halfway down the list, there’s one labeled “git:https://github.com” Let’s expand that.

Oh, looky there! Username and password.

Now what I did was to remove the entry and then push again. I was prompted to enter a userid and password. I still had to type the password at the command prompt, but IT STUCK.

C:\git\test>git push
Counting objects: 3, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 304 bytes | 0 bytes/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To https://github.com/UserB/test
5e74c47..cf2ca13 master -> master

I probably could have clicked “Edit” and changed the userid and password, and just kept going, but I didn’t notice the “Edit” right away.

It looks as though you might actually be able to have multiple entries for git:https://github.com, but I haven’t tried that yet.

(Public domain photo from PublicDomainPictures.net)

What is an SDK vs. a JDK vs. a JRE?

The Content Management System we use at the office is written in Java, so on the development systems, we need to be careful to match the JDK version to the JRE version used by the CMS vendor. Periodically someone new will join the team, and they end up wondering what a JDK is and how is that different from a JRE?

So let’s break this down. Generally speaking, an SDK or Software Development Kit. Basically, it’s the set of compilers, debuggers, and other tools used for creating software on a given platform.

That’s basically what the JDK is.  The Java Development Kit contains compilers, debuggers and other tools for creating Java applications.

While a software (or Java) development kit allows you to create software, some software platforms also require a runtime environment. The JRE, or Java Runtime Environment is the support software that allows a Java application to execute.

(Photo from Pexels.com, used under Creative Commons Zero.)

Dropbox

So, I woke up this morning to an email from Troy Hunt, or rather, a message from his Have I Been Pwned? service. It seems that my account was one of the 68,648,009 compromised in the Dropbox breach.

From the sound of things, there’s some mixed news. The bad news is, at the time of the breach, four years ago, many passwords were still being stored as SHA-1 (MD5) hashes. The good news is that they appear to have been salted hashes and the hash values weren’t included in the breach.

Dropbox did send out an alert a few days ago saying that they had reset passwords for anyone who hadn’t updated their password in the past four years (guilty!). The email said it was done as a precaution, but didn’t go into detail about what it was a precaution again. To find that out, you had to click through and read a blog post.

I’m probably OK. My password probably wasn’t as secure as it might have been, but thankfully, the lack of salt values for the SHA1 passwords should make them quite difficult to break. And perhaps most importantly, I’ve never used that same password anywhere else.

(But yes, I changed my password to something a bit more secure. It’s now 40 random characters generated by KeePass.)

Some important takeaways:

  1. Change your Dropbox password.
  2. Don’t use the same password in more than one place.
    • Consider a password manager. I’m mostly happy with KeePass, but also hear good things about LastPass.
  3. Consider turning on two-factor authentication.
  4. Consider also signing up with Have I Been Pwned?
  5. Why are you still reading this? Go change your Dropbox password!

(Photo from Pexels, free for non-commercial use.)

Stopping Credit Card Offers

It seems like the mail includes a good number of credit card offers. Various companies seem to send offers for their special Visa or Mastercard affinity programs almost daily. It only takes a moment to run them through the shredder, and the fees for sending them help keep the lights on at the Post Office. Besides, figuring out how to stop them takes too long, right?

But Thursday’s daily email included something special! A credit card offer with the magic phrase, “You can choose to stop receiving “prescreened” offers of credit from this and other companies by calling toll-free 1-888-567-8688.” Now I’m suspicious of pretty much any offer that comes in as a spam email (These turkeys weren’t even supporting the Post Office!) but a Google search is pretty painless.

The second result was the US Federal Trade Commission’s “Prescreened Credit and Insurance Offers” page. So what do you know? That phone number’s legit.

So it turns out there are actually three options:

  1. Call 888-567-8688 and you can stop (most) credit card offers for five years.
  2. You can also stop (most) credit card offers by visiting https://www.optoutprescreen.com/, clicking the “Click Here to Opt-In or Opt-Out” button.
  3. Or you can go for the gold by visiting that same link, printing out the form at the end, and mailing it to
    Opt­Out Department
    P.O. Box 2033
    Rock Island, IL 61204­-2033

    .

Total time spent: less than five minutes.

(Image via pixabay)