January « 2009 « Musings

Scripting HAL

Recent releases of Fedora and other GNU/Linux distributions include a Hardware Abstraction Layer (HAL) which is used to support device plug-and-play capabilities. In this post I will show you how your shell scripts can use HAL to retrieve device and system information.

The term HAL is overloaded as it used to refer both to a specification and the actual software which implements the specification. From an application developers viewpoint, HAL is way to enumerate the capabilities and features of hardware attached to a system and receive notification when something about the hardware changes.

First, a very quick overview of HAL. Each item of physical hardware in a computer is regarded as being an device object which identified by a Unique Device Identifier (UDI). Associated with each device object is a variable set of well-defined typed key-value pairs (or metadata) called device properties which describe what each device object represents together with its properties. Some device properties are derived from the actual physical hardware, some are merged from XML-formatted files, known as Device Information Files, and some are derived from the actual device configuration. Mandatory device properties are defined in the HAL specification.

A HAL

KSH93 Extended Patterns

Pattern matching is an important component of any modern shell. The ksh93 shell supports both regular expressions as well as what is called extended patterns. Extended patterns can be thought of as class or type of extended regular expressions. The purpose of this post is to explain, with some examples, how to use the power of extended patterns in your ksh93 scripts.

JavaScript E4X

In previous posts, I discussed the SpiderMonkey command line shell js and how to add support to it to enable full access (read, write, create, copy, delete, etc.) to the local filesystem via the File object and the NSPR library.

While rumaging around in the source code and documentation for js, I found that js partially supported the EX4 XML extension via a user configurable option.

This post looks at what it takes to load an XML document into js from your local filesystem, process it and write out the resulting document to your local filesystem using File objects and the E4X extension.

The ECMAScript for XML (E4X) (ECMA-357) specification adds native support for XML objects and XMLList objects to the JavaScript programming language. This standard was first published in 2004 and was based on XML extensions provided in the BEA (now Oracle) Weblogic Workshop product. These extensions were designed by Terry Lucas and John Schneider who led the ECMAScript for XML (E4X) initiative.

The basis idea behind E4X was that declarative languages such as XSL and XPATH are too complex for the average programmer to quickly learn and therefore a simpler way of accessing and manipulating XML documents was needed. Personally I do not agree with that assertion.

As an aside, currently Schneider is founder and CEO at AgileDelta which developed the Efficient XML binary format specification which I plan to write about in a future post. A W3C working group is currently developing the EXI specification which is based on the AgileDelta specification.

JavaScript File Object

As you are probably aware JavaScript engines such as SpiderMonkey typically do not allow access to the local filesystem for reasons of security. To enable developers to test the scripts from a command line, js includes the load() function which enables you to load one or more JavaScript scripts into the SpiderMonkey engine. However this is not sufficient for our purposes as no means is provided to write to the filesystem. Looking more closely at the source code, I noticed support for File objects. This support is not enabled by default however. It is not sufficient to simply recompile SpiderMonkey with this option enabled; you must also download and build the Netscape Portable Runtime (NSPR) library. This library provides a platform-neutral API for system level and libc-like functions, and is used by a number of Mozilla projects and other third party software developers. The current release is 4.7.3 and you can download it here.

There are some gotchas to building Spidermonkey with NSPR. First of all, you need to successfully build NSPR. The source code tarball for NSPR comes with the standard GNU autoconfigure tools. If you are on a 64-bit system, you need to execute configure with the -enable-64bit option; otherwise the build will quickly fail. You should then test the build by going to the test subdirectory, building the testsuite and executing it. You also need to modify SpiderMonkey’s Makefile.ref (I am assuming you are building SpiderMonkey 1.7 and not an earlier release) to include libnspr and the NSPR headers. Two compile time defines are needed. You can either define JS_HAS_FILE_OBJECT and JS_THREADSAFE in Makefile.ref or as command line arguments to make. After than you, should be able to successfully build SpiderMonkey with native File object support.

Now that we have js build with support for File objects, what can we do with it. Well, I guess we should start with the expected Hello World script. js> File.output.writeln(“Hello World”); Hello World true js> File.output.writeln(“Hello, world”); “OK” Hello, world OK js> File.output.writeln(“Hello, world”); “” Hello, world js>

Notice that true is outputted unless you append something else as shown above. Here is another short example which demonstrates how to list the properties of the instance File object for the current directory. js> dir = new File(‘.’); /home/fpm/js/. js> for ( i in dir ) print(i); length parent path name isDirectory isFile exists canRead canWrite canAppend canReplace isOpen type

Korn Shell 93 Stat Builtin

This post demonstrates how to write a loadable stat builtin for Korn shell 93.

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Scripting HAL

KSH93 Extended Patterns

JavaScript E4X

JavaScript File Object

Korn Shell 93 Stat Builtin

Musings

Translate

Archives

Scripting HAL

KSH93 Extended Patterns

JavaScript E4X

JavaScript File Object

Korn Shell 93 Stat Builtin

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