AFAICT, there is no difference between --ignore-cluster and --skip-checks, other than --skip-checks is
a subcommand flag, right?. Maybe we can consider replacing --ignore-cluster entirely with just --skip-checks in a future PR?

Currently,

# this works
$  bin/linkerd install config --ignore-cluster

# this doesn't
$  bin/linkerd install config --skip-checks

# this serves no purposes, even though the flag is available
# the command still fails
$  bin/linkerd install control-plane --ignore-cluster

# this works
$  bin/linkerd install control-plane --skip-checks

# this works
$  bin/linkerd install --ignore-cluster

# this doesn't
$  bin/linkerd install --skip-checks

@ihcsim Thanks for all this testing!

Re: the install parent flags getting pushed up into the global flags when using a subcommand, yeah, it's how cobra works. i agree with you that it would be preferable to be more clear on what constitutes a truly global flag vs. a flag that is specific to install and its subcommands. i looked at repeating cmd.Flags().AddFlagSet() in the subcommands as you mentioned, but i think the current implementation expresses the functionality more explicitly, as install parent flags that are shared with subcommands.

re: --skip-checks vs. --ignore-cluster, i agree it's a bit confusing. have a look at #2719 (comment) for the motivation. /cc @adleong

--skip-checks is specific to the linkerd install control-plane subcommand, which requires a control plane namespace to exist to successfully be applied via kubectl.
--ignore-cluster applies to install and all subcommands, and is meant to override an existing control plane installation.

re:

# this serves no purposes, even though the flag is available
# the command still fails
$  bin/linkerd install control-plane --ignore-cluster

... that command succeeds if the control plane namespace exists, similar in behavior to linkerd install.

this discussion probably implies we could at least do a better job documenting these commands in the help output. let me know what you think.

Sorry for jumping into this discussion after this has already been merged, but I just want to make sure I understand correctly what we landed on here.

I would expect that --skip-checks and --ignore-cluster are both flags on install control-plane that do different things:

• --skip-checks skips the check that the controller namespace exists which can be useful for grabbing the control plane yaml even if your cluster is not ready for it to be applied. I would also expect this to skip other checks that verify that install config ran successfully like checking that CRDs, ClusterRoles, and ClusterRoleBindings were created.
• --ignore-cluster disregards the contents of the linkerd config configmap and can be used to completely override an existing control-plane instead of preserving existing config settings.

Furthermore, I would expect these flags to also be available on linkerd install as well since linkerd install includes linkerd install control-plane.

I would not expect either of those flags to be available on linkerd install config. What do they do for that command?

@adleong All permutations of subcommands and flags:

linkerd install --ignore-cluster
linkerd install config --ignore-cluster
linkerd install control-plane --ignore-cluster

# don't check for namespace existence (adding RBAC and CRD checks is a good idea)
linkerd install control-plane --skip-checks 

Having --ignore-cluster available on install and all subcommands makes consistent the idea that "linkerd install" doesn't work after installation. This makes the user experience consistent post-installation:

$ bin/go-run cli install [config|control-plane]
Linkerd has already been installed on your cluster in the linkerd namespace. Please run upgrade if you'd like to update this installation. Otherwise, use the --ignore-cluster flag.

I'm not sure of the use case for linkerd install --skip-checks. That flag checks for namespace existence, and fails if the namespace (and perhaps eventually CRDs, etc) doesn't exist. I think that's specific to linkerd install control-plane.

Gotcha, this all makes sense to me. Thanks for clarifying!

Filed #2782 to track followup RBAC/CRD checks.