Docker in Windows Moving A Mounting

Using docker in Windows is a bit like discovering that someone’s missed a vital memo. Everything nearly works as you might hope, but there are glitches all over the place.

I use bash as my shell for scripting various things we do with docker in Windows. This leads to two enormously annoying issues:

• I’ve found it very difficult to mount the working directory into the container I’ve just launched
• Giving a docker container a working directory seems to result in “The directory name is invalid”

Let’s look at what we want to do:

Let’s unpack what the above is trying to do. Note, I think the above command should work on a linux/MacOS bash, and there’s a potential variant of it that might work in PowerShell… yet, it won’t work on GitBash or similar bash shells in Windows.

• -v $(pwd):/var/myproject – I do this sort of thing a lot when dockerising tools to run on my project folder, so they’re portable across machines that don’t have everything installed – it’s trying to mount the current working directory to a nice neutral path /var/myproject on the target container
• -w /var/myproject – given I mounted my working directory, let’s make it the working directory of the running container
• someimage:latest – replaced with the actual image – e.g. node:10.6.1 or somesuch
• somecommand – whatever we’re running on the working directory – e.g. npm install or whatever

These Two Simple Tricks Will Blow Your Mind

You need to know why the above fails on Windows:

• $(pwd) returns the path in unix style – e.g. /c/foo/bar/ where Windows docker (annoyingly) requires volume mounts in native windows – e.g. C:/foo/bar/
• /var/myproject on the command line is treated as a local file no matter how you quote it, and the bash invocation refuses to let it pass to the docker runtime because it’s not a valid local path

Here’s the fix:

• $(pwd) => $(pwd -W) – the -W switch makes bash use the windows native path, not the unixey alternative
• /var/myproject => //var/myproject – this double slash makes the shell agree that this can’t be a local file, so it just passes it on!

It’s trivial when you know… but annoyingly not self-evident.

I know I’ll be coming back to this post to remind myself of how to fix the above. I hope it’s been useful for you too.