XProc 3.0: operating system steps

This specification describes the p:os-info and p:os-exec XProc steps. A machine-readable description of these steps may be found in steps.xpl.

Familarity with the general nature of [XProc 3.0] steps is assumed; for background details, see [XProc 3.0 Steps].

The p:os-info step returns information about the operating system on which the processor is running.

<p:declare-step type="p:os-info">
     <p:output port="result" content-types="application/xml" primary="true"/>
</p:declare-step>

The step returns a c:result element with attributes describing properties of the system. It should include the following properties:

The c:result element can contain zero or more c:environment elements that identify the environment variables available. These elements have a mandatory name and value attribute, reflecting name and value of the environment variable. Implementations should include all available operating system environment variables.

The exact set of properties returned is implementation-defined.

The p:os-exec step runs an external command passing the input that arrives on its source port as standard input, reading result from standard output, and errors from standard error.

<p:declare-step type="p:os-exec">
     <p:input port="source" sequence="true" content-types="any"/>
     <p:output port="result" primary="true" content-types="any"/>
     <p:output port="errors" content-types="any"/>
     <p:output port="exit-status" content-types="application/xml"/>
     <p:option name="command" required="true" as="xs:string"/>     
     <p:option name="args" select="()" as="xs:string*"/>           
     <p:option name="cwd" as="xs:string?"/>                        
     <p:option name="result-content-type" select="'text/plain'" as="xs:string"/>
     <p:option name="errors-content-type" select="'text/plain'" as="xs:string"/>
     <p:option name="path-separator" as="xs:string?"/>             
     <p:option name="failure-threshold" as="xs:integer?"/>         
     <p:option name="serialization" as="map(xs:QName,item()*)?"/>  
</p:declare-step>

The p:os-exec step executes the command passed on command with the arguments passed on args. The processor does not interpolate the values of the command or args (for example, expanding references to environment variables). It is a dynamic error (err:XC0033) if the command cannot be run.

If cwd is specified, then the current working directory is changed to the value of that option before execution begins. It is a dynamic error (err:XC0034) if the current working directory cannot be changed to the value of the cwd option. If cwd is not specified, the current working directory is implementation-defined.

If the path-separator option is specified, every occurrence of the character identified as the path-separator character that occurs in the command, args, or cwd will be replaced by the platform-specific path separator character. It is a dynamic error (err:XC0063) if the path-separator option is specified and is not exactly one character long.

The source port is declared to accept a sequence so that it can be empty. If no document appears on the source port, then the command receives nothing on standard input. If a document does arrive on the source port, it will be passed to the command as its standard input. It is a dynamic error (err:XD0006) if more than one document appears on the source port of the p:os-exec step. The serialization option is used to control the serialization of the document before passing it to the standard input. If a document has a “serialization” document property, the effective value of the serialization options is the union of the two maps, where the entries in the “serialization” document property take precedence. Serialization is described in [XProc 3.0].

The content of standard output is read and processed as described in p:load with result-content-type taken as value for p:load's content-type option. The created document is returned on result.

The content of standard error is read and processed as described in p:load with errors-content-type taken as value for p:load's content-type option. The created document is returned on errors.

The exit-status port always returns a single c:result element which contains the system exit status that the process returned. The specific exit status values returned by a process invoked with p:os-exec are implementation-dependent.

If a failure-threshold value is supplied, and the exit status is greater than that threshold, then the p:os-exec step must fail. It is a dynamic error (err:XC0064) if the exit code from the command is greater than the specified failure-threshold value. This failure, like any step failure, can be captured with a p:try.

These steps can raise dynamic errors.

[Definition: A dynamic error is one which occurs while a pipeline is being evaluated.] Examples of dynamic errors include references to URIs that cannot be resolved, steps which fail, and pipelines that exhaust the capacity of an implementation (such as memory or disk space). For a more complete discussion of dynamic errors, see Dynamic Errors in XProc 3.0: An XML Pipeline Language.

If a step fails due to a dynamic error, failure propagates upwards until either a p:try is encountered or the entire pipeline fails. In other words, outside of a p:try, step failure causes the entire pipeline to fail.

The following errors can be raised by these steps: