gosubst

README

gosubst

Here's the deal: I just wanted to do some simple command-line templating of configuration files that are used as input to a lot of cloud service tools. YAML files, confs, .envs, lots of container definitions, orchestration manifests, etc. envsubst does not get the job done, and I want something that just sits on the command line, is sturdy, and allows me to share configuration publicly (or even internally) without exposing secrets.

That is gosubst.

Installation

Easy:

$ go install github.com/hews/gosubst

If you don't have a working Go environment, it's only a little harder:

$ export VERSION=v0.2.0; export OS=linux; # ... or "darwin" (MacOS), or "windows" ...
$ curl -LJ \
  https://github.com/hews/gosubst/releases/download/${VERSION}/gosubst.${OS} \
  -o gosubst
$ chmod +x gosubst
$ mv gosubst /usr/local/bin

See releases for what options will work.

However, if you use MacOS, it's as easy as can be:

$ brew tap hews/tap && brew update
# > ==> Tapping hews/tap
# > ...
$ brew install gosubst
# > ==> Installing gosubst from hews/tap
# > ...

Use

$ echo '{{ printf "This is a %s!" "${TEST}" }}' | TEST=test gosubst
# > This is a test!

This is meant to be a minimal and dead simple replacement for envsubst. Though it does not implement a superset of envsubst exactly, it is used for the same purpose, and any file being used with envsubst can be very easily changed to work with gosubst.

Like envsubst it reads from STDIN and writes to STDOUT (both with pipes, and in interactive mode), and expands environmental variables in the form ${ENVVAR}. Unlike envsubst, there is no way to filter or print the variables that are used, and variables in the form $ENVVAR are ignored.

After variable expansion, however, comes the fun part! The input is treated like a Go template, and the context for the calling process is injected into it. This context includes some shell variables, details about the process, and debugging flags.

A full suite of functions is available to use in templating via Sprig! Finally, there is an available function sh("...") that hands off to sh -c '...', so that we can nest shell commands into the template.

And there you go! See the /examples directory for examples.

Some Q&A, in which I lob myself softballs and knock 'em outta the park...

Are we concerned about the code running fast and efficiently?

Not at all. Speed is not of the essence.

Are we worried about getting or handling errors?

Nope. If we get an error we just spit it out.

Well it seems like this was slapped together. It's not particularly robust, and it's got a bunch of loopholes.

You munge data with the tools you have, not the tools you wish you had. When I have a specific problem I'll fix it. You have a problem? You fix it. Easy-peasy.

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Copyright (c) 2020 Philip Hughes
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