cli-output-conventions

Overview

Per clig (opens in new window):

"Send output to stdout. The primary output for your command should go to stdout. Anything that is machine readable should also go to stdout."

"Send messaging to stderr. Log messages, errors, and so on should all be sent to stderr."

Tests can only assert on output that is stable, separated, and documented. This skill defines those conventions; sister skills (bats-testing, tui-snapshot-tester) test against them.

When to use

• Authoring or reviewing a CLI's output (any new flag, command, or error message).
• Writing tests for a CLI: this skill tells you WHAT to assert on; bats / pytest etc. tell you HOW.
• A CLI's tests are flaky / brittle - usually the underlying output isn't deterministic. Fix the output, not the assertion.

Convention 1 - Exit codes

Per clig (opens in new window):

"Return zero exit code on success, non-zero on failure."

"Map non-zero codes to important failure modes for script integration."

Exit code | Meaning
----------+----------------------------
0         | Success
1         | General error (catch-all)
2         | Misuse (bad flag, bad usage)
3         | Resource not found
4         | Permission denied
5         | Network / external failure
... documented per CLI

Test pattern:

@test "exit 0 on success" {
  run mycli list
  [ "$status" -eq 0 ]
}

@test "exit 2 on bad flag" {
  run mycli --nonexistent
  [ "$status" -eq 2 ]
}

Don't test [ "$status" -ne 0 ] - assert the specific code. Otherwise refactors silently change the contract.

Convention 2 - stdout vs stderr

Per clig (opens in new window): "This separation ensures piped commands receive only data, not messages."

# Good: data on stdout, message on stderr
$ mycli list 2>/dev/null
alice
bob

$ mycli list >/dev/null
fetched 2 users in 0.3s

Test pattern (bats):

@test "list emits names on stdout, status on stderr" {
  run --separate-stderr mycli list
  [ "$status" -eq 0 ]
  [ "$output" = $'alice\nbob' ]
  [[ "$stderr" == *"fetched 2 users"* ]]
}

run --separate-stderr (Bats 1.5+) splits the streams; without it, $output mixes both.

Convention 3 - Machine-readable mode

Per clig (opens in new window):

"Display output as formatted JSON if --json is passed."

"If human-readable output breaks machine-readable output, use --plain to display output in plain, tabular text format for integration with tools like grep or awk."

"Encourage your users to use --plain or --json in scripts to keep output stable."

$ mycli list --json | jq '.[] | .name'
"alice"
"bob"

$ mycli list --plain
alice  active   2026-04-15
bob    active   2026-04-20

Test pattern: assert against --json, never against the human-formatted default. Human output is allowed to evolve; JSON is the contract.

@test "list --json contract" {
  run mycli list --json
  assert_success
  echo "$output" | jq -e '.[0] | has("name") and has("status")'
}

Convention 4 - Determinism

Stable output for tests requires:

• No timestamps in default output (only with --verbose or human-mode banner).
• Stable ordering - sort lists by a canonical key, not insertion order.
• No random IDs in output unless documented.
• Locale-independent number formatting in --json / --plain (no thousand separators; ISO 8601 dates).

# Bad: randomized order, locale-dependent date
$ mycli list
bob   2026/04/20
alice 2026/04/15

# Good: sorted, ISO 8601
$ mycli list --plain
alice 2026-04-15
bob   2026-04-20

Test pattern: golden-file comparison with sorted output.

Conventions 5-8 - color/TTY, verbosity, --help/--version, documenting the contract

The remaining four conventions and their bats test patterns are in references/conventions-5-8.md.

Anti-patterns

Limitations

• Legacy CLIs. Adopting this skill against an existing CLI requires a deprecation cycle for output changes.
• Verbose / debug modes are inherently non-deterministic. Don't snapshot them; assert presence of key markers.
• Cross-shell quoting. Bats runs in Bash; PowerShell tests via Pester need their own assertions.
• Color contract is OS-dependent. Windows terminal emulators have different ANSI support; test on the target platforms.

References

• clig (opens in new window) - Command Line Interface Guidelines: exit codes, stdout / stderr, --json / --plain, NO_COLOR, -q / --verbose, --help / --version.
• https://no-color.org/ - NO_COLOR informal standard.
• ISO 8601 - date / time format for --json output.
• bats-testing - Bash-based assertion runner that consumes this contract.
• tui-snapshot-tester - layout-level snapshots; this skill covers text-level contract.