9.16. Using external preprocessor

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An external preprocessor is a stand-alone program that lets the programmer easily extend the language. The simplest available external preprocessor is the HEB preprocessor that lets a programmer embed ScriptBasic into HTML. This preprocessor is written in ScriptBasic and converts HTML embedded ScriptBasic code into pure ScriptBasic.

When ScriptBasic recognizes that an external preprocessor is to be started it does not read the source file but starts the external preprocessor and uses the file created by that. The external preprocessor should be an executable program that gets at least two arguments: the name of the file to preprocess and the file name to create; containing the preprocessed text.

The external preprocessor exits with the code zero in case it was executing successfully and should exit with the code 1 if an error occurred and the output file can not be used. This is the usual UNIX exit code convention for processes.

There can be more than one external preprocessors applied to a program. In case there are more than one preprocessors applied to a file then ScriptBasic starts each of them one after the other each getting the output file of the previous to work on.

The preprocessors should be configured in the configuration file before ScriptBasic can execute any of them. Each preprocessor should have a symbolic name. This is the choice of the person who configured the ScriptBasic installation. You can name a preprocessor as you like, it will not alter the behavior.

For example the HTML embedded basic preprocessor is named heb in the sample configuration file. To use this preprocessor you can use the option `-p' on the command line following with the name of the preprocessor:

# scriba -p heb myheb.bas

You can also assign file extensions to preprocessors. The sample configuration file contains the line:

epreproc$heb heb

This means that the extension heb is assigned to the preprocessor heb. Well, this is a stupid example because the extension is the same as the symbolic name of the preprocessor, but usually this is the convention. The extension stands after the $ sign and the symbolic name of the preprocessor is the value of the configuration line. Thus

epreproc$extension preprocessor_symbolic_name

You can assign the same preprocessor to many extensions, for example:

epreproc$heb heb
epreproc$htb heb
epreproc$hb heb

can exist in the same configuration file. Whenever ScriptBasic processes a source file with the extension hb it will start the HEB external preprocessor.

A file may have multiple extensions. These are worked up from left to right. For example the command:

# scriba sample.heb.sql.bas

will start ScriptBasic starting first the preprocessor assigned to the extension heb, then the preprocessor assigned to the extension sql and finally the preprocessor assigned to the extension bas. If there is no preprocessor assigned to some of the extensions then they eventually will not be executed. This is not an error. (Note that there is no sql preprocessor currently for ScriptBasic, this is an artificial example.)

This behavior lets you to chain preprocessors.

External preprocessors have to be configured. You have to tell ScriptBasic what program to use for the specific preprocessing. To do this you have to use the configuration line:

eprep$exe$heb /usr/bin/scriba heber.bas

This configuration line tells ScriptBasic that the executable of the preprocessor having the symbolic name heb is scriba heber.bas. This is not actually the name of an executable. This is the start of the command line. The first element of it should be the name of the executable. The next elements are the command line options that precede the input and the output file name. Using this configuration line ScriptBasic will start another issue of ScriptBasic with the command line:

#/usr/bin/scriba heber.bas sample.heb.bas outputfile

where sample.heb.bas is the HTML embedded BASIC code we actually want to execute; `outputfile' is the file that the preprocessor has to create. When this process finishes ScriptBasic reads the generated output and executes the program.

Of course you can use any other executable as preprocessors. You can write preprocessors in C, C++, Perl or in any other language that can be started on the command line, is capable reading and writing file and is able to signal error via exit code. Note that ScriptBasic exists with the last non caught error code.

Before ScriptBasic is able to start the external preprocessor it has to calculate the name of the output file that the preprocessor has to write. Each preprocessor has to be configured to use a temporary directory to store the preprocessed file. To specify the directory you have to use the configuration line:

eprep$dir$heb /etc/temp/heb/

This means that the temporary directory for the files created by the preprocessor with the symbolic name "heb" is the directory `/etc/temp/heb/'. When a file is to be preprocessed with this preprocessor ScriptBasic calculates a unique file name based on the name of the source file and asks the external preprocessor to put the result into that file in this directory. Finally the command line that ScriptBasic starts for the example above is:

#/usr/bin/scriba heber.bas sample.heb.bas /etc/temp/heb/EACAPAOALAEAHAHAAANAJAOALAGAKAPA

The algorithm used to calculate the unique file name is the same as for the cache file name.

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