Authoring Commands

You’ll probably be using your own commands more than off-the-shelf commands. Right now, since Docker Hub is the only source for commands, making and publishing commands is as easy as any Docker container. After you’ve gone through this once, you might be surprised at how quickly you can make and update Cmd.io commands. It’s literally build, push, use.

Up to this point, we haven’t needed to use or install Docker. For the time being, you’ll need Docker to make Cmd.io commands. We highly recommend Docker for Mac if you’re running macOS.

You’ll also need a Docker Hub account and be sure to login with Docker (docker login).

Commands based on existing utilities

The recommended way to build commands from existing open source utilities is to install them via a package manager. To keep your experience snappy, and since Cmd.io may enforce a size limit on images, we highly encourage you to use Alpine Linux for all command containers.

Alpine combines the small size of Busybox (~5MB) with a large package index optimized for small disk footprints. You can search for packages based on name or based on contents. If you can’t find a package for a utility, you can try using ubuntu-debootstrap, which is a minimal Ubuntu image with apt-get. However it starts at ~90MB and easily bloats from there.

Here is jq for Alpine v3.4, so we can make a container for it with a simple Dockerfile that uses the apk package tool:

FROM alpine:3.4
RUN apk add --update --no-cache jq
ENTRYPOINT ["/usr/bin/jq"]

The directives used in this example are nearly all that make sense to use for Cmd.io commands, but here is the full Dockerfile reference.

Now we can build this with Docker, assuming we’re in the directory with the Dockerfile. Immediately after building, we can push to Docker Hub. Replace progrium with your Docker ID.

$ docker build -t progrium/jq .
...
$ docker push progrium/jq
...

At this point you can now install this command on Cmd.io like before. If you push new versions of the image to Docker Hub, Cmd.io will pull it just before the next run.

Commands based on scripts

Making a container for a script is not that different from making it for an existing utility. You’ll want to install the interpreter and any other utilities the script depends on the same way as before. But you’ll also be adding your script and making it the entrypoint.

Create a file called netpoll and make sure it’s executable with chmod +x netpoll. Inside it, put this Bash script that uses netcat to poll an address and port for roughly 10 seconds or until the port accepts a connection. If it connects it returns. If it times out it returns non-zero. A rather handy little script.

#!/bin/bash
for retry in $(seq 1 ${TIMEOUT:-10}); do
  nc -z -w 1 "$1" "$2" && break
done

In the same directory create a Dockerfile like this:

FROM alpine:3.4
RUN apk add --update bash netcat-openbsd
COPY ./netpoll /bin/netpoll
ENTRYPOINT ["/bin/netpoll"]

Build, push, and install with Cmd.io. Let’s say I installed it as netpoll. I can run it against cmd.io port 22 and it returns immediately with status 0. Run against cmd.io port 23 and it blocks for at least 10 seconds before giving up and returning status 1.

$ ssh alpha.cmd.io netpoll cmd.io 22; echo $?
0
$ ssh alpha.cmd.io netpoll cmd.io 23; echo $?
1

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