Add variable to have just require a program to be present on the system

[nogweii]

As a command runner, it would be convenient to be able to specify the list of programs necessary for my Justfile to run. Something like the following example:

set required-programs := ["kubectl", "helm", "helmfile"]

bootstrap: && helm-charts
    kubectl apply -f foo.yaml

helm-charts:
    helmfile apply --file bar.yaml

status:
    helm ls -A

And if I didn't have those programs installed, just would error out before running a recipe with a helpful message saying that I need to install, say, helm & helmfile.

If this is added, I can imagine a need for [require-optional] (with a better name though) if people wanted to provide a recipe to install all the missing programs.

This is basically automatically doing #2109 and reporting an error. If that function is added, this could be solved by creating a recipe that does this. However, I still think it's worthwhile to include this feature.

[casey]

You could do this with a recipe:

require-dependencies:
  helm --version || echo "please install helm"

status: require-dependencies
    helm ls -A

This is more verbose, since you have to add that recipe as a dependency, but more flexible, since you can add that recipe as a dependency only to those recipes which require the binaries your checking for.

I think actually the way to support this is #2178, which would allow defining a recipe to be run before any other recipe in the justfile, which would allow for checking any kind of precondition, including whether or not particular binaries are installed.