<Prereq> element

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The <Prereq> element assigns a prerequisite to <Get>, <Set>, and <Discover> operations.

Syntax

Parent element: <Get>, <Set>, and <Discover>

The following attribute is supported:
Table 1. Attribute
Attribute Required Type Description
id no string Specifies an unique identifier
The following child elements are supported:
Table 2. Child elements
Child element Required Description
<Command> no Command
<Argument> no Command-line arguments
<Stdin> no Arguments supported by the <Stdin> element
<ErrMessage> no Message to print if prerequisites fail
Note: The <Command> element must be defined for each prerequisite: at the <ParameterDef> level, at the <CfgMethod> level, or in a <PrereqDef> element.

Usage

Prereqs are commands that condition the processing of a <Get>, <Set>, and <Discover> operation for parameters that use this <Get>, <Set>, and <Discover> operations. Parameters for which a prereq command fails (nonzero return code) are ignored, and the error message defined in the prerequisite is displayed.

The <Prereq> element assigns a prerequisite to the parent <Get>, <Set>, or <Discover> operation. The prerequisite is either defined locally under the <Prereq> element, or inherited from a higer-level <Prereq> or <PrereqDef> element that has a matching id attribute.

A parameter has all the prerequisites defined locally under the <ParameterDef> element. Also, the prerequisite has the properties defined in the configuration method of the parameter, if a configuration method is used. The consequence is that if a prerequisite is defined in a <CfgMethod> element, all the <ParameterDef> elements that use the configuration method will automatically have that prerequisite (although some of those elements might redefine the prerequisite locally).

The <Command>, <Argument>, <Stdin>, and <ErrMessage> elements that define a prerequisite for a given operation are searched in this order:
  • Under the <Prereq> subelement of the relevant operation of the <ParameterDef> element.
  • If the <ParameterDef> element has a cfgmethod attribute, under the <Prereq> subelement that has a matching id of the relevant operation of the configuration method.
  • Under the <PrereqDef> element of the catalog that has a matching id.

Example

The following example defines a prerequisite that checks that the netaddr and netaddr6 parameters are applied on the same system on which they were captured:

<ParameterDef name="netaddr" type="string" targetClass="device" cfgmethod="attr">
  <Set type="permanent">
    <Prereq>
      <Command>[[ `/usr/bin/uname -f` = %p[nodeId] ]]</Command>
      <ErrMessage>Parameter cannot be applied to a different node</ErrMessage>
    </Prereq>
  </Set>
</ParameterDef>

<ParameterDef name="netaddr6" type="string" targetClass="device" cfgmethod="attr">
  <Set type="permanent">
    <Prereq>
      <Command>[[ `/usr/bin/uname -f` = %p[nodeId] ]]</Command>
      <ErrMessage>Parameter cannot be applied to a different node</ErrMessage>
    </Prereq>
  </Set>
</ParameterDef>

In this example, the test is run twice: once for the netaddr parameter, and once for the netaddr6 parameter. This dual processing is because each parameter has its own prerequisite with its own <Command> element. See , artex_catalog_elem_PrereqDef.html for an example that requires only one run of the test.

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