Reconcile all Carvel tools to behave identically when no arguments are given

[pivotaljohn]

Describe the problem/challenge you have
Reported first as: carvel-dev/kbld#184

For each command-line tool in the Carvel suite, they behave differently from others when no arguments are given.

tool	displays help	displays error message	exit status
imgpkg	❌	✅	1
kapp	✅	✅	1
kbld	❌	❌ (displays "Succeeded")	0
kwt	✅	✅	1
vendir	❌	✅	1
ytt	❌	❌	0

Describe the solution you'd like
For all the tools to behave identically, unless there's a clear reason to be different.

Seems like the most useful behavior would be to:

• display the help text,
• report an error message, and
• exit with a non-zero exit status.

Anything else you would like to add:
[Additional information that will assist in solving the issue.]

[cppforlife]

both ytt and kbld do not display any error, because it theory its valid to give them to input and have them return no output. other commands do not have such behaviour because they always require some input.

[pivotaljohn]

both ytt and kbld do not display any error, because it theory its valid to give them to input and have them return no output. other commands do not have such behaviour because they always require some input.

I don't understand the theory. In the ytt case, it's my understanding that in order to accept input, the --file flag must be given.

It's the difference between:

$ echo "foo: bar" | ytt
$

and

$ echo "foo: bar" | ytt -f-
foo: bar
$

and the same kind of thing seems to apply to kbld as well:

$ echo "image: nginx" | kbld
Succeeded

and

$ echo "image: nginx" | kbld -f-
resolve | final: nginx -> index.docker.io/library/nginx@sha256:644a70516a26004c97d0d85c7fe1d0c3a67ea8ab7ddf4aff193d9f301670cf36
---
image: index.docker.io/library/nginx@sha256:644a70516a26004c97d0d85c7fe1d0c3a67ea8ab7ddf4aff193d9f301670cf36
metadata:
  annotations:
    kbld.k14s.io/images: |
      - origins:
        - resolved:
            tag: latest
            url: nginx
        url: index.docker.io/library/nginx@sha256:644a70516a26004c97d0d85c7fe1d0c3a67ea8ab7ddf4aff193d9f301670cf36

Succeeded

Seems like it would be possible for a user to mistakenly believe that piping STDOUT to ytt/kbld would result in them processing it as input (without a flag). With their current behavior, there's no indication that the input is being ignored.

What am I missing?

[cppforlife]

you are right that -f is needed for input, which means that this could only happen when no flags are given. that may happen if you are writing wrappers (in bash, etc) that allow to deal with 0+ files -- im thinking of it as func ytt(files ...File) with 0 files being valid. there is of course a question whether we should disallow 0 files case entirely (which if we do, i would expect would result in error stating 1 or more files is needed)

[pivotaljohn]

im thinking of it as func ytt(files ...File) with 0 files being valid.

This. This is the bit of context I bet I was missing. Yeah... I can absolutely see it from that perspective.

there is of course a question whether we should disallow 0 files case entirely (which if we do, i would expect would result in error stating 1 or more files is needed)

At first blush, the examples I gave above seem to me to be compelling evidence to make the 0 files case an error case.