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link Resource

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To suggest a change, edit the link.rb file and submit a pull request to the Chef Infra Client repository.

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Use the link resource to create symbolic or hard links.

A symbolic link–sometimes referred to as a soft link–is a directory entry that associates a file name with a string that contains an absolute or relative path to a file on any file system. In other words, it’s a file that contains a path that points to another file. A symbolic link creates a new file with a new inode that points to the inode location of the original file.

A hard link is a directory entry that associates a file with another file in the same file system. In other words, multiple directory entries to the same file. A hard link creates a new file that points to the same inode as the original file.

Syntax

A link resource block creates symbolic or hard links. For example, to create a hard link from /tmp/file to /etc/file:

link '/tmp/file' do
  to '/etc/file'
  link_type :hard
end

Because the default value for link_type is symbolic, and because properties that are not specified in the resource block will be assigned their default values, the following example creates a symbolic link:

link '/tmp/file' do
  to '/etc/file'
end

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

link 'name' do
  group            String, Integer
  link_type        String, Symbol # default value: :symbolic
  owner            String, Integer
  target_file      String # default value: 'name' unless specified
  to               String
  action           Symbol # defaults to :create if not specified
end

where:

  • link 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.
  • group, link_type, owner, target_file, and to are the properties available to this resource.

Actions

The link resource has the following actions:

:create
(default) Create a link. If a link already exists (but does not match), update that link to match.
:delete
Delete a link.
: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.

Properties

The link resource has the following properties:

group
Ruby Type: String, Integer

A group name or ID number that identifies the group associated with a symbolic link.

link_type
Ruby Type: String, Symbol | Default Value: :symbolic
Allowed Values: :hard, :symbolic

The type of link: :symbolic or :hard. On Windows, :symbolic will create a junction point if the target is a directory.

mode
Ruby Type: Integer, String | Default Value: 777

If mode is not specified and if the file already exists, the existing mode on the file is used. If mode is not specified, the file does not exist, and the :create action is specified, Chef Infra Client assumes a mask value of '0777' and then applies the umask for the system on which the file is to be created to the mask value. For example, if the umask on a system is '022', Chef Infra Client uses the default value of '0755'.

The behavior is different depending on the platform.

UNIX- and Linux-based systems: A quoted 3-5 character string that defines the octal mode that is passed to chmod. For example: '755', '0755', or 00755. If the value is specified as a quoted string, it works exactly as if the chmod command was passed. If the value is specified as an integer, prepend a zero (0) to the value to ensure that it is interpreted as an octal number. For example, to assign read, write, and execute rights for all users, use '0777' or '777'; for the same rights, plus the sticky bit, use 01777 or '1777'.

Microsoft Windows: A quoted 3-5 character string that defines the octal mode that is translated into rights for Microsoft Windows security. For example: '755', '0755', or 00755. Values up to '0777' are allowed (no sticky bits) and mean the same in Microsoft Windows as they do in UNIX, where 4 equals GENERIC_READ, 2 equals GENERIC_WRITE, and 1 equals GENERIC_EXECUTE. This property cannot be used to set :full_control. This property has no effect if not specified, but when it and rights are both specified, the effects are cumulative.

owner
Ruby Type: String, Integer

The owner associated with a symbolic link.

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

An optional property to set the target file if it differs from the resource block’s name.

to
Ruby Type: String

The actual file to which the link is to be created.


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.

Properties

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 link resource in recipes:

Create symbolic links

The following example will create a symbolic link from /tmp/file to /etc/file:

link '/tmp/file' do
  to '/etc/file'
end

Create hard links

The following example will create a hard link from /tmp/file to /etc/file:

link '/tmp/file' do
  to '/etc/file'
  link_type :hard
end

Delete links

The following example will delete the /tmp/file symbolic link and uses the only_if guard to run the test -L command, which verifies that /tmp/file is a symbolic link, and then only deletes /tmp/file if the test passes:

link '/tmp/file' do
  action :delete
  only_if
'test -L /tmp/file'
end

Create multiple symbolic links

The following example creates symbolic links from two files in the /vol/webserver/cert/ directory to files located in the /etc/ssl/certs/ directory:

link '/vol/webserver/cert/server.crt' do
  to '/etc/ssl/certs/ssl-cert-name.pem'
end

link '/vol/webserver/cert/server.key' do
  to '/etc/ssl/certs/ssl-cert-name.key'
end

Create platform-specific symbolic links

The following example shows installing a filter module on Apache. The package name is different for different platforms, and for the Red Hat Enterprise Linux family, a symbolic link is required:

include_recipe 'apache2::default'

case node['platform_family']
when 'debian'
  ...
when 'suse'
  ...
when 'rhel', 'fedora'
  ...

  link '/usr/lib64/httpd/modules/mod_apreq.so' do
    to      '/usr/lib64/httpd/modules/mod_apreq2.so'
    only_if 'test -f /usr/lib64/httpd/modules/mod_apreq2.so'
  end

  link '/usr/lib/httpd/modules/mod_apreq.so' do
    to      '/usr/lib/httpd/modules/mod_apreq2.so'
    only_if 'test -f /usr/lib/httpd/modules/mod_apreq2.so'
  end
end
...

For the complete recipe, see https://github.com/onehealth-cookbooks/apache2/blob/68bdfba4680e70b3e90f77e40223dd535bf22c17/recipes/mod_apreq2.rb.

Create Windows junction/reparse points

This example demonstrates how to create a directory junction/reparse point. In this example, C:\\destination will be a junction/reparse point to the C:\\source directory.

directory 'C:/source'

link 'C:/destination' do
  link_type :symbolic
  to 'C:/source'
end
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