Constraints

Relational rules between options: when something must be present, when things can coexist, when they cannot.

Conditional required

:required_if

Required when another option holds a specific value:

option :format, type: :string, choices: ["json", "table"]
option :output, type: :string, required_if: [format: "json"]

error: --output is required when --format is 'json'

Multiple conditions: any match triggers the requirement.

option :region, type: :string, required_if: [env: "prod", env: "staging"]

Use :required_if_all when every condition must hold instead of any:

option :confirm, type: :boolean, required_if_all: [env: "prod", force: true]
# error: --confirm is required when --env is 'prod' and --force is true

:required_unless

Required unless any of the named options is present. Accepts an atom or a list:

option :config, type: :string, required_unless: :inline
option :config, type: :string, required_unless: [:inline, :stdin]

error: --config is required unless --inline, --stdin is provided

Use :required_unless_all to require the option unless all of the named options are present:

option :out, type: :string, required_unless_all: [:a, :b]
# error: --out is required unless --a, --b are all provided

Per-option relations

:conflicts_with

Declares that two options cannot be set together:

option :json, type: :boolean, conflicts_with: :yaml
option :json, type: :boolean, conflicts_with: [:yaml, :toml]

error: --json cannot be used with --yaml

:requires

The inverse: setting one option implies another must also be set:

option :user,   type: :string, requires: :password
option :deploy, type: :boolean, requires: [:env, :region]

error: --deploy requires --env

Groups

Group a set of options under a named constraint.

Mutually exclusive

At most one of the group's options can be set:

group :format, mutually_exclusive: true do
  option :json, type: :boolean
  option :csv,  type: :boolean
  option :yaml, type: :boolean
end

error: options --json, --yaml are mutually exclusive (group: format)

Co-occurring

All or none of the group's options must be set:

group :auth, co_occurring: true do
  option :username, type: :string
  option :password, type: :string
end

error: options --username, --password must be used together (group: auth)

Required (one-of)

At least one of the group's options must be set:

group :source, required: true do
  option :file, type: :string
  option :stdin, type: :boolean
end

error: one of --file, --stdin is required (group: source)

Constraints combine: mutually_exclusive: true, required: true means exactly one member must be set.

Picking the right tool

• One option conditionally required based on another's value: :required_if.
• One option required unless another is present: :required_unless.
• Two or three specific options that must or must not coexist: :conflicts_with / :requires on a single option.
• A larger set of options with one constraint (format flags, auth flags): group with mutually_exclusive or co_occurring.
• Anything with its own logic: a cross-param validate function.

See Validation for the latter.