feat: add ndjson output format with field selection to list commands

[vikashkumar2020]

Changes

Fixes: #2853
Add --output ndjson and --fields to every tkn * list command so that
automation scripts and AI agents can consume resource lists incrementally and
request only the fields they need.

--output ndjson — newline-delimited JSON streaming

Instead of a single JSON array, each resource is emitted as one JSON object
per line (NDJSON / JSON Lines). Consumers can begin processing the first record
before the last one arrives, and never need to buffer an entire response in
memory.

tkn pipelinerun list --output ndjson
tkn taskrun list    --output ndjson -n staging
tkn pipeline list   --output ndjson -A

--fields — dot-path field projection

Restricts each NDJSON object to the requested fields, specified as a
comma-separated list of dot-separated paths. Shared path prefixes are merged
into a single nested object; unknown paths are silently ignored.

# Only name + status condition for every PipelineRun
tkn pipelinerun list --output ndjson \
  --fields metadata.name,status.conditions

# Combine with --all-namespaces
tkn taskrun list --output ndjson -A \
  --fields metadata.name,metadata.namespace,status.startTime

Implementation

New helper in pkg/formatted/ndjson.go:

Symbol	Purpose
PrintNDJSON(w, obj, fields)	Converts any runtime.Object list to NDJSON via runtime.DefaultUnstructuredConverter, then streams one line per item
pickFields(src, fields)	Dot-path projection — builds a new map with only the requested paths
getNestedField / setNestedField	Recursive helpers for reading/writing dot-separated paths

Each of the nine list commands gains:

• Fields []string in its ListOptions / listOptions struct
• --fields flag wired to that slice
• A case output == "ndjson": branch (in switch {} form to satisfy gocritic) that calls formatted.PrintNDJSON

The --output flag is the existing cliopts.NewPrintFlags flag — no new top-level flag required.

Commands covered

Command	Package
tkn pipeline list	pkg/cmd/pipeline
tkn pipelinerun list	pkg/cmd/pipelinerun
tkn task list	pkg/cmd/task
tkn taskrun list	pkg/cmd/taskrun
tkn customrun list	pkg/cmd/customrun
tkn eventlistener list	pkg/cmd/eventlistener
tkn triggertemplate list	pkg/cmd/triggertemplate
tkn triggerbinding list	pkg/cmd/triggerbinding
tkn clustertriggerbinding list	pkg/cmd/clustertriggerbinding

(tkn bundle list reads from an OCI registry, not a Kubernetes API, and is
intentionally excluded.)

Tests

pkg/formatted/ndjson_test.go covers:

• All-fields output produces one valid JSON object per list item
• Multi-field dot-path projection includes only the requested keys
• Top-level field selection (no dot) works
• Unknown paths are silently ignored; known paths are still present
• Empty list produces empty output

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Run the code checkers with make check
• Regenerate the manpages, docs and go formatting with make generated
• Commit messages follow commit message best practices

See the contribution guide
for more details.

Release Notes

All `tkn * list` commands now support `--output ndjson` (one JSON object per
line, streamable) and `--fields` (comma-separated dot-path projection, e.g.
`metadata.name,status.conditions`) to reduce token usage for automation and
AI-agent workflows.

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[Copilot]

Pull request overview

This PR adds a new NDJSON (--output ndjson) streaming output mode plus optional field projection (--fields) to Tekton CLI tkn * list commands, enabling incremental consumption of Kubernetes list results and reducing payload size for automation workflows.

Changes:

• Introduces pkg/formatted.PrintNDJSON to stream Kubernetes list items as one JSON object per line, with optional dot-path field projection.
• Adds a --fields flag and an ndjson output branch to the supported list commands.
• Adds unit tests for NDJSON output and field projection behavior.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pkg/formatted/ndjson.go	Adds NDJSON list streaming + dot-path field projection helpers.
pkg/formatted/ndjson_test.go	Adds tests covering NDJSON output and field selection semantics.
pkg/cmd/pipeline/list.go	Adds --fields and --output ndjson handling for tkn pipeline list.
pkg/cmd/pipelinerun/list.go	Adds --fields and --output ndjson handling for tkn pipelinerun list.
pkg/cmd/task/list.go	Adds --fields and --output ndjson handling for tkn task list.
pkg/cmd/taskrun/list.go	Adds --fields and --output ndjson handling for tkn taskrun list.
pkg/cmd/customrun/list.go	Adds --fields and --output ndjson handling for tkn customrun list.
pkg/cmd/eventlistener/list.go	Adds --fields and --output ndjson handling for tkn eventlistener list.
pkg/cmd/triggertemplate/list.go	Adds --fields and --output ndjson handling for tkn triggertemplate list.
pkg/cmd/triggerbinding/list.go	Adds --fields and --output ndjson handling for tkn triggerbinding list.
pkg/cmd/clustertriggerbinding/list.go	Adds --fields and --output ndjson handling for tkn clustertriggerbinding list.

---

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[vikashkumar2020]

@divyansh42 @pradeepitm12 Can you please take a look at PR, i have fixed the review comments

[divyansh42]

Thanks @vikashkumar2020. I have added few suggestion, please let me know your thoughts on the same.

Also, the test file only covers the PrintNDJSON helper function. There are no unit tests verifying:

• The --output ndjson branch is correctly reached in each command's RunE
• --fields is properly parsed and passed through the options struct
• Error handling in commands that make their own ListV1 call (pipeline, task)

The existing test infrastructure for other commands (e.g., pipelinerun/list_test.go) uses fake clients — adding similar tests for the ndjson path would guard against regression in the wiring logic.

[Copilot]

Pull request overview

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Suppressed comments (2)

pkg/cmd/pipeline/list.go:122

• The generated command documentation has not been updated for these public flags: docs/cmd/tkn_pipeline_list.md:20-26 contains no --fields, and its --output help still lists formats without ndjson; the other affected list docs are likewise unchanged. Also extend the runtime output-format usage so ndjson is advertised, then run make generated and commit the generated Markdown/manpage changes.

	f.AddFlags(c)
	c.Flags().BoolVarP(&opts.AllNamespaces, "all-namespaces", "A", opts.AllNamespaces, "list Pipelines from all namespaces")
	c.Flags().BoolVarP(&opts.NoHeaders, "no-headers", "", opts.NoHeaders, "do not print column headers with output (default print column headers with output)")
	c.Flags().StringSliceVar(&opts.Fields, "fields", opts.Fields, "Comma-separated list of fields to include in output (e.g. metadata.name,status.startTime); only used with --output ndjson")

pkg/cmd/pipeline/list.go:122

• Issue #2853 demonstrates field selection with --output json, but this flag is read only by the ndjson branch, so --fields ... --output json succeeds while silently emitting every field. Apply projection to JSON output too, or reject that combination and explicitly resolve the narrower scope before closing the linked issue.

	c.Flags().StringSliceVar(&opts.Fields, "fields", opts.Fields, "Comma-separated list of fields to include in output (e.g. metadata.name,status.startTime); only used with --output ndjson")

[vikashkumar2020]

@divyansh42 addressed the comments

[Copilot]

🟡 Changes recommended

Field projection misses the linked JSON requirement, and typed-list buffering undermines NDJSON’s intended memory benefit.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (2)

pkg/cmd/pipeline/list.go:101

• This typed PipelineList round-trip drops fields unknown to the vendored Tekton API before NDJSON projection, unlike the existing JSON/YAML path that prints the server’s unstructured list. This makes newer CRD fields silently unavailable to --fields; retain and pass the dynamic client’s unstructured result instead.

				var pl *v1.PipelineList
				if err := actions.ListV1(pipelineGroupResource, cs, metav1.ListOptions{}, ns, &pl); err != nil {

pkg/cmd/task/list.go:90

• As in the Pipeline command, converting the dynamic response into the vendored TaskList silently removes server fields that this client version does not know about. NDJSON/--fields should operate on the dynamic client’s unstructured result so it preserves the same resource data as JSON/YAML output.

				var tl *v1.TaskList
				if err := actions.ListV1(taskGroupResource, cs, metav1.ListOptions{}, ns, &tl); err != nil {

• Files reviewed: 14/14 changed files
• Comments generated: 5
• Review effort level: Balanced