skfold - a Docker image


Now skfold is also available as a Docker image.

In issue 465 of Perl Weekly, Gabor Szabo nicely points out:

The question though how can you make skafold easily distributable? Asking the users to install Perl and on top of that install skafold would probably limit the reach of the project.

This is totally right.

My answer is three-fold:

  1. I do not care too much about it. As most of the things I do, I’m not really into promoting it too much - heck, I did even automate the notification process on social media, which is as lame as you can think.

  2. Perl is an amazing tool, easily available in all platform where one is likely to do actual development on (usually, your laptop). Chances are it’s already there for a reason or another; if not, it’s usually very, very easy to install it, because most Linux distributions have it as a package, Mac OS X has it, and Windows is easily addressed (e.g. through Strawberry Perl). If it’s still a show-stopper for someone… I guess we fall back on point 1 above 😜

  3. I’m also offering a Docker-based alternative!

Getting the Docker image

The skfold Docker image is hosted in the skfold repository in GitHub. You can easily grab it:

export SFK_IMAGE=''
docker pull "$SKF_IMAGE"

although you will have to be logged in GitHub’s Docker registry to do this.

Other tags for the image are 0.1 and 0.1.0 (there’s also a date-specific tag that is only useful for tracking).

Using the image: the shell wrapper

The image is a wrapper around skfold, with a couple of added command-line options to ease its usage.

One is --wrapper:

$ docker run --rm "$SKF_IMAGE" --wrapper

*** WARNING: not remapping user <urist> to user id 0

skf() {
   : ${SKF_IMAGE:=""}
   docker run --rm \
      -v "$PWD:/mnt" \
      -v "$HOME/.skfold/defaults.json:/app/.skfold/defaults.json:ro" \
      "$SKF_IMAGE" "$@"

Ignore the warning… it’ OK at this stage (it’s also printed on the standard error, so you can avoid printing it pretty easily).

The result on standard output is a simple shell wrapper function that you can customize to your heart’s content to ease calling the program without dieing from typing.

By default:

  • it bind-mounts the current working directory as /mnt inside the container, which is also where the program will work. Although the image is executed as root, permissions are dropped (via suexec) early on, so that operations are run with the same user as the owner of the mounted directory;

  • it also assumes that you have your defaults.json file in the right directory, but probably not all modules - for this reason, it only bind-mounts file ~/.skfold/defaults.json. You can of course modify this line to mount your own ~/.skfold directory instead (see further on for an example).

If that is fine with you, you can eval the wrapper and get the wrapper function.

$ eval "$(docker run --rm "$SKF_IMAGE" --wrapper)"

*** WARNING: not remapping user <urist> to user id 0

Or put it in your shell inizialization file (e.g. ~/.bashrc if you use bash).

Using the image: the tarball

The skfold Docker image contains the modules distributed in skfold by default. If you want to extract them, e.g. to customize and/or add others, you can easily do this with option --tarball, whick outputs a tar file on the standard output:

$ docker run --rm "$SKF_IMAGE" --tarball | tar xC ~

This will create/overwrite ~/.skfold, so use with caution!

When you have your full-blown ~/.skfold directory, your wrapper shell function is probably better expressed as follows:

skf() {
   : ${SKF_IMAGE:=""}
   docker run --rm \
      -v "$PWD:/mnt" \
      -v "$HOME/.skfold:/app/.skfold:ro" \
      "$SKF_IMAGE" "$@"

A note on the warning…

Remember the warning we got before?

$ docker run --rm "$SKF_IMAGE"

*** WARNING: not remapping user <urist> to user id 0


This is suexec complaining because we are not bind-mounting anything on /mnt and it belongs to root inside the container (which has user identifier 0). As we were saying before, it can be ignored.

After you do the bind-mounting, though, this error message will disappear. Unless you’re root, of course 🤓. This also happens when you install the wrapper, because it takes care to do the bind-mounting.

Using the image: all the rest

Any other parameers combination is actually passed to the skfold executable inside the image, so nothing new here:

$ skf
# skf version 0.1.0 - more info with
# skf --usage|--help|--man
# Available modules:

$ skf --usage
       skf [--usage] [--help] [--man] [--version]

       skf --help-on <module>
       skf -h        <module>

       skf [-b|--base <dir>]
           [-l|--loglevel <level>]
           <target> <module> [<module options...>]

For technical reasons, the --man option will not invoke the pager out of the bat… it’s a small price to pay 😇


So there you have it: if you feel like not installing Perl, you will surely have Docker around!

And, again, I would not understand this prejudice against Perl

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