## `kn` User Interface Principles This document describes the conventions that are used for all `kn` commands and options. It is normative in the sense that any new feature that introduces or changes user-facing commands or options needs to adhere to the given principles. Also, the given rules apply to plugins as well. Especially any plugin that wants to be promoted to the [knative/client-contrib](https://github.com/knative/client-contrib) plugin repository has to adhere to these rules. ### Command Structure The general format for kn and plugin commands is ``` kn [ ...] [ ` [1](#foot-1), where [](#noun) is often the name of a resource (e.g. `service`) but can also refer to other concepts (e.g. `plugin` or `config`). This first noun forms a command group for all the operations you might want to do with that kind of resource. Sometimes there can be deeper hierarchies with multiple nouns (`kn .... `) when it makes sense to structure complex concepts. A good example is `kn source ` which is used like in `kn source ping create`. `kn` commands take only positional arguments that are used as [identifiers](#identifier). This identifier is often the name of a resource which identifies the resource uniquely for the current or given namespace. Top-level commands concerning the operation of `kn` itself, like `help` and `version` are also okay. ### Noun For resource-related commands, the kind itself used as a command in singular and lowercase form. For example, `service` for `serving.knative.dev/service` or `trigger` for `eventing.knative.dev/trigger` are the commands for managing these resources respectively ### Verb For CRUD (create-retrieve-update-delete) operation the following verbs have to be used: - `describe` prints detailed information about a single resource that can contain data of dependent objects, too. - `list` prints summary information about all resources of a type. - `create` creates a resource. - `update` updates a resource. - `delete` deletes a resource. - `apply` for an idempotent "create-or-update", much like `kubectl apply` For a given resource, create and update should use the same arguments as much as possible and where it makes sense. Other domain-specific verbs are possible on a case-by-case basis for operations that go beyond basic CRUD operations. ### Identifier For the `CRUD` operations `describe`, `create`, `update`, `delete` the identifier is the resource's name and is required as a positional argument after the commands. For example it is the last argument that does not start with a flag prefix `-` or `--`. `list` operations can use a resource name to filter on the resource. Other identifiers can be plugin names or other entities' identifiers. For bulk operations also multiple identifiers can be provided. For example, a `delete` operation could use multiple resource names that should be deleted. ```bash kn service create foo --image gcr.io/things/stuff:tag ``` In this case, `foo` is positional, and provides the name of the service to create. ### Flags Flags are used for specifying the input for `kn` commands and can have different characteristics: - They can be _mandatory_ or _optional_ - Mandatory flags are mentioned in the `Use` attribute of a command like in `service NAME --image IMAGE` for `ServiceCommand` - Optional flags can have _default values_ - Flag values can be _scalars_, _binary_, _lists_ or _maps_ (see below for details) - Flags always have a long-form (starting with a double `--`) but can also have a shortcut (beginning with a single `-`) - Every flag has a help message attached - Flags can be specific to a command or can be globally applicable When adding new flags, the following recommendations should be considered: - Never add a global flag except for very good reasons - Group related flags together by using a common prefix, like `--label-revision` or `--label-service` so that they appear together in the help message (which is sorted alphabetically) - Don't add a short form without former discussions - Choose a name for the flag that is the same or close to the naming used in Knative serving itself like the corresponding CRD field or annotation name. As mentioned above, flag values can be of different types. The rules of how these values are modelled on the command line are given below. #### Scalar A scalar option is one which just takes a single value. This value can be a string or a number. Such an option is allowed to be given only once. If given multiple times, an error should be thrown. A scalar flag's value can have an inner structure, too. For example `--sink ksvc:myservice` uses a prefix `ksvc:` to indicate the targeted sink is a Knative Service. A colon (`:`) should be used as separators if values have a structure. Example: ``` # Scalar parameter "--image" for specifying an application image kn service create myservice --image docker.io/myuser/myimage ``` #### Binary Binary flags come in pairs and don't carry any value. The flag representing the `true` value is just the flag name without a value (e.g. `--wait`) whereas the flag for a `false` value is this name with a `no-` prefix (e.g. `--no-wait`) Example: ``` # Create a service an wait until deployed kn service create myservice --wait .... # Don't wait for the service to start kn service create myservice --no-wait ... ``` Such a binary option can be provided only once. Otherwise, an error has to be thrown. #### List List flag values can be provided in two flavours: - Within a single flag value as comma-separated list of key-value pairs (e.g. `--resource pod:v1,job:batch/v1`) - By providing the same option multiple times (e.g. `--resource pod:v1 --resource job:batch/v1`) The value itself can carry a structure where colons separate the parts (`:`), like in the examples above. Example: ``` # Create an ApiServer source for listening on Pod and Job resource events kn source apiserver create mysrc --resource pod:v1 --resource job:batch/v1 --sink ksvc:mysvc # Same as above, but crammed into a single option kn source apiserver create mysrc --resource pod:v1,job:batch/v1 --sink ksvc:mysvc ``` #### Maps - Within a single flag value as comma separated list of key-value pairs (e.g. `--env USER=bla,PASSWORD=blub`) - By providing the same option multiple times (e.g. `--env USER=bla --env PASSWORD=blub`) For update operations, to _unset_ a value, the key has a dash suffix (`-`) and no value part. For example, to _remove_ an environment variable named `USER` from a service "hello-world". If the same key is given multiple times on the command line, the latter definition overwrites the previous one. Example: ``` # Create a Service "hello-world" that sets USER and PASSWORD environment variables kn service create hello-world --env USER=bla --env PASSWORD=blub # Same as above kn service create hello-world --env USER=bla,PASSWORD=blub # Remove the USER envvar and add a HOME envvar to the service "hello-world" kn service update hello-world --env USER- --env HOME=/root # Same as above kn service update hello-world --env USER-,HOME=/root # Same as above, but the last HOME "/home" flag overwrites the previous one kn service update hello-world --env HOME=/root --env USER- --env HOME=/home ``` ### Shared flags Certain functionality is the same across command groups. For example, specifying resource requests and limits via flags can be done when managing services but also for sources. Those common functionalities should share the same conventions, syntax. Area to which this applies: - Resource limits - Output formats, i.e. the data formats supported by the `--output` option (which is reused from k8s' _cli-runtime_) - Sinks - .... _this section needs to be completed with the concrete specifications. tbd_ 1: Note that this differs from the `kubectl` model where this order is vice versa (`kubectl `)