alternatives Resource

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Use the alternatives resource to configure command alternatives in Linux using the alternatives or update-alternatives packages.

New in Chef Infra Client 16.0.

Syntax

The full syntax for all of the properties that are available to the alternatives resource is:

alternatives 'name' do
  link           String # default value: "/usr/bin/LINK_NAME"
  link_name      String # default value: 'name' unless specified
  path           String
  priority       String, Integer
  action         Symbol # defaults to :install if not specified
end

where:

• alternatives is the resource.
• name is the name given to the resource block.
• action identifies which steps Chef Infra Client will take to bring the node into the desired state.
• link, link_name, path, and priority are the properties available to this resource.

Actions

The alternatives resource has the following actions:

:auto

Set an alternative up in automatic mode with the highest priority automatically selected.

:install

Install an alternative on the system including symlinks. (default)

:nothing

This resource block does not act unless notified by another resource to take action. Once notified, this resource block either runs immediately or is queued up to run at the end of a Chef Infra Client run.

:refresh

Refresh alternatives.

:remove

Remove an alternative and all associated links.

:set

Set the symlink for an alternative.

Properties

The alternatives resource has the following properties:

link

Ruby Type: String | Default Value: /usr/bin/LINK_NAME

The path to the alternatives link.

link_name

Ruby Type: String | Default Value: The resource block's name

The name of the link to create. This will be the command you type on the command line such as ruby or gcc.

path

Ruby Type: String

The absolute path to the original application binary such as /usr/bin/ruby27.

priority

Ruby Type: String, Integer

The priority of the alternative.

Common Resource Functionality

Chef resources include common properties, notifications, and resource guards.

Common Properties

The following properties are common to every resource:

compile_time

Ruby Type: true, false | Default Value: false

Control the phase during which the resource is run on the node. Set to true to run while the resource collection is being built (the compile phase). Set to false to run while Chef Infra Client is configuring the node (the converge phase).

ignore_failure

Ruby Type: true, false, :quiet | Default Value: false

Continue running a recipe if a resource fails for any reason. :quiet will not display the full stack trace and the recipe will continue to run if a resource fails.

retries

Ruby Type: Integer | Default Value: 0

The number of attempts to catch exceptions and retry the resource.

retry_delay

Ruby Type: Integer | Default Value: 2

The delay in seconds between retry attempts.

sensitive

Ruby Type: true, false | Default Value: false

Ensure that sensitive resource data is not logged by Chef Infra Client.

Notifications

notifies

Ruby Type: Symbol, 'Chef::Resource[String]'

A resource may notify another resource to take action when its state changes. Specify a 'resource[name]', the :action that resource should take, and then the :timer for that action. A resource may notify more than one resource; use a notifies statement for each resource to be notified.

If the referenced resource does not exist, an error is raised. In contrast, subscribes will not fail if the source resource is not found.

A timer specifies the point during a Chef Infra Client run at which a notification is run. The following timers are available:

:before

Specifies that the action on a notified resource should be run before processing the resource block in which the notification is located.

:delayed

Default. Specifies that a notification should be queued up, and then executed at the end of a Chef Infra Client run.

:immediate, :immediately

Specifies that a notification should be run immediately, for each resource notified.

The syntax for notifies is:

notifies :action, 'resource[name]', :timer

subscribes

Ruby Type: Symbol, 'Chef::Resource[String]'

A resource may listen to another resource, and then take action if the state of the resource being listened to changes. Specify a 'resource[name]', the :action to be taken, and then the :timer for that action.

Note that subscribes does not apply the specified action to the resource that it listens to - for example:

file '/etc/nginx/ssl/example.crt' do
  mode '0600'
  owner 'root'
end

service 'nginx' do
  subscribes :reload, 'file[/etc/nginx/ssl/example.crt]', :immediately
end

In this case the subscribes property reloads the nginx service whenever its certificate file, located under /etc/nginx/ssl/example.crt, is updated. subscribes does not make any changes to the certificate file itself, it merely listens for a change to the file, and executes the :reload action for its resource (in this example nginx) when a change is detected.

If the other resource does not exist, the subscription will not raise an error. Contrast this with the stricter semantics of notifies, which will raise an error if the other resource does not exist.

A timer specifies the point during a Chef Infra Client run at which a notification is run. The following timers are available:

:before

Specifies that the action on a notified resource should be run before processing the resource block in which the notification is located.

:delayed

Default. Specifies that a notification should be queued up, and then executed at the end of a Chef Infra Client run.

:immediate, :immediately

Specifies that a notification should be run immediately, for each resource notified.

The syntax for subscribes is:

subscribes :action, 'resource[name]', :timer

Guards

A guard property can be used to evaluate the state of a node during the execution phase of a Chef Infra Client run. Based on the results of this evaluation, a guard property is then used to tell Chef Infra Client if it should continue executing a resource. A guard property accepts either a string value or a Ruby block value:

• A string is executed as a shell command. If the command returns 0, the guard is applied. If the command returns any other value, then the guard property is not applied. String guards in a powershell_script run Windows PowerShell commands and may return true in addition to 0.
• A block is executed as Ruby code that must return either true or false. If the block returns true, the guard property is applied. If the block returns false, the guard property is not applied.

A guard property is useful for ensuring that a resource is idempotent by allowing that resource to test for the desired state as it is being executed, and then if the desired state is present, for Chef Infra Client to do nothing.

The following properties can be used to define a guard that is evaluated during the execution phase of a Chef Infra Client run:

not_if

Prevent a resource from executing when the condition returns true.

only_if

Allow a resource to execute only if the condition returns true.

Examples

The following examples demonstrate various approaches for using the alternatives resource in recipes:

Install an alternative:

alternatives 'python install 2' do
  link_name 'python'
  path '/usr/bin/python2.7'
  priority 100
  action :install
end

Set an alternative:

alternatives 'python set version 3' do
  link_name 'python'
  path '/usr/bin/python3'
  action :set
end

Set the automatic alternative state:

alternatives 'python auto' do
  link_name 'python'
  action :auto
end

Refresh an alternative:

alternatives 'python refresh' do
  link_name 'python'
  action :refresh
end

Remove an alternative:

alternatives 'python remove' do
  link_name 'python'
  path '/usr/bin/python3'
  action :remove
end