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.

Document properties

The resulting document has no properties apart from content-type. In particular, it has no base-uri.

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="source-is-xml" select="true()" as="xs:boolean"/>
     <p:option name="result-is-xml" select="true()" as="xs:boolean"/>
     <p:option name="wrap-result-lines" select="false()" as="xs:boolean"/>
     <p:option name="errors-is-xml" select="false()" as="xs:boolean"/>
     <p:option name="wrap-error-lines" select="false()" as="xs:boolean"/>
     <p:option name="path-separator" as="xs:string?"/>             
     <p:option name="failure-threshold" as="xs:integer?"/>         
     <p:option name="arg-separator" select="' '" as="xs:string"/>  
     <p:option name="serialization" as="map(xs:QName,item()*)?"/>  
</p:declare-step>

The values of the command, args, cwd, path-separator, and arg-separator options must be strings.

The values of the source-is-xml, result-is-xml, errors-is-xml, and fix-slashes options must be boolean.

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 value of the args option is a string. In order to support passing more than one argument to a command, the args string is broken into a sequence of values. The arg-separator option specifies the character that is used to separate values; by default it is a single space It is a dynamic error (err:XC0066) if the arg-separator option is specified and is not exactly one character long.

The following examples of p:os-exec are equivalent. The first uses the default arg-separator:

<p:exec command="someCommand" args="arg1 arg2 arg3"/>

The second specifies an alternate separator:

<p:exec command="someCommand" args="arg1,arg2,arg3"
	arg-separator=","/>

If one of the arguments contains a space (e.g., a filename that contains a space), then you must specify an alternate separator.

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. If source-is-xml is true, the value of the serialization option is used to convert the input into serialized XML which is passed to the command, otherwise the XPath string-value of the document is passed. Serialization is described in [XProc 3.0].

The standard output of the command is read and returned on result; the standard error output is read and returned on errors. In order to assure that the result will be an XML document, each of the results will be wrapped in a c:result element.

If result-is-xml is true, the standard output of the program is assumed to be XML and will be parsed as a single document. If it is false, the output is assumed not to be XML and will be returned as escaped text.

If wrap-result-lines is true, a c:line element will be wrapped around each line of output.

<c:line>
    string
</c:line>

It is a dynamic error (err:XC0035) to specify both result-is-xml and wrap-result-lines.

The same rules apply to the standard error output of the program, with the errors-is-xml and wrap-error-lines options, respectively.

If either of the results are XML, they must be parsed with namespaces enabled and validation turned off, just like p:document.

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.

Document properties

No document properties are preserved.

This step 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 this step: