Feature request: KUBECTL_EXTERNAL_DIFF support

[abentley-ssimwave]

kubectl diff and kubectl neat are both nice improvements, but they're even better together.

The problem with kubectl diff is that it can obscure actual changes through the inclusion of a bunch of fields that are not relevant-- the very ones that kubectl-neat will trim.

kubectl diff supports a KUBECTL_EXTERNAL_DIFF environment variable. It's possible to use that variable to use kubectl neat to perform the diff through a reasonably simple script:

#!/bin/bash
set -euo pipefail

files="$(cat <(ls $1) <(ls $2) | sort | uniq)"
retcode=0
for file in $files; do
    diff -uN --label old/$file <(kubectl-neat -f $1/$file) --label new/$file <(kubectl-neat -f $2/$file) || retcode=$?
done
exit $retcode

But providing it as standardized functionality would help more people and avoid rewriting the above script, and ensure any edge cases are handled.

KUBECTL_EXTERNAL_DIFF does not support arguments, so it does not support subcommands. Supporting it would likely involve reading the basename of argument 0 and determining whether to behave as kubectl-neat or diff based on that. kubectl-neat could then be symlinked to that alternate name.

A diff subcommand could also be provided. This would act as a wrapper for kubectl diff, supplying an appropriate KUBECTL_EXTERNAL_DIFF value.