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IdEst |
ID3 Editing and Scripting Tool |
Sergey Poznyakoff |
IdEst – ID3 Editing and Scripting Tool (split by node): 
Idest offers a scripting facility, which makes it possible
to extend its functionality beyond the basic operations, described in
previous chapters. Scripts must be written in Scheme, using
‘Guile’, the
GNU’s Ubiquitous Intelligent Language for Extensions. For
information about the language, refer to Revised(5)
Report on the Algorithmic Language Scheme. For a detailed
description of Guile and its features, see
Overview in The Guile Reference Manual.
The scripting mode is enabled when the option
--script (-S) is given in the command line. This
option stops further option processing, so any other idest
command line options must be given before it. The argument
to this option specifies the name of the script file:
$ idest --script list.scm *.mp3
You can omit the ‘.scm’ suffix, as idest will try it
automatically (see below).
When this option is given, the following operations are performed:
This step is omitted if the program is given the --no-init-files (-N) option.
When a startup file is loaded, the list of files which were to be tried after it is passed to it as arguments. This allows for chain-loading all files in the list using the following code:
(let load-loop ((name-list (cdr (command-line))))
(if (not (null? name-list))
(let ((name (car name-list)))
(load-loop (cdr name-list))
(if (file-exists? name)
(primitive-load name)))))
%load-path.
The default load path is formed as follows:
version-site-dir . package-site-dir guile-site-dir %load-path
where the components are as follows:
The standard Guile load path (see Build Config in The Guile Reference Manual).
This directory is selected at compile time using the rules below.
Its value is returned by the (%idest-guile-site-dir) primitive:
The reason for using this directory is described in http://www.gnu.org.ua/software/gint/#guile-site-dir.
If guile-site-dir coincides with the standard %site-dir,
this part is omitted, because the latter is always present in the
%load-path.
This is the directory for installing version-independent
idest files. It is formed as follows:
guile-site-dir/idest
This value is returned by the (%idest-package-site-dir) primitive.
This is the directory for installing version-dependent
idest files. It is formed as follows:
package-site-dir/2.1
This value is returned by the (%idest-version-site-dir) primitive.
The load path can be modified using the --load-path (-P) and --prepend-load-path (-p) command line options. Both options take as argument a list of directory names, separated by colons. The --load-path option adds these directories to the tail of the load path list. The --prepend-load-path option adds them to the head of the load path list.
The script is loaded via primitive-load-path (see primitive-load-path in The Guile Reference Manual), so
idest will consult the %load-extensions list and
try suffixes from that list as described in
%load-extensions in The Guile Reference Manual).
The script can access command line arguments via the usual
command-line function (see command-line in The Guile Reference Manual). It
can also modify the argument list (e.g. by removing its command
line options). It must not, however, modify ‘argv[0]’. Any
changes it does to the argument list become visible to
idest. The only requirement is that the modified argument
list consist of the script name (as argv[0]) and input file names.
The main function must be declared as:
It takes two arguments. The file argument supplies the name of
the file being processed. The frames argument is a list of ID3
frames read from that file. Each element of frames is a pair,
with the frame name in its car and an association list of
frame properties in its cdr.
The properties are identified by property names, which are Scheme symbols. The following property names are defined:
Value of this frame, as a string.
Frame description. It is a string, verbosely describing the frame. For example, the description of ‘TRCK’ frames is ‘Track number/position in set’.
These are the same descriptions that are output with the --describe option (see describe).
Unsupported or partially-supported frames contain only this property.
Its value is a list of frame fields. Each field is represented by a
triplet ‘(ord type value)’, where ord is the
ordinal number of that field in frame, type is its type (integer) and
value is its value. If type is one of numeric types,
value is the numeric value converted to string (as per
number->string). If type is a string type, value
contains the string in the appropriate encoding. Otherwise,
value holds the field value as a binary string. Each byte in
such a string is represented by two hexadecimal digits. For example,
‘AB\n’ is represented as ‘41420A’.
More properties are defined at a per-frame basis to represent
frame qualifiers. They are named after corresponding qualifiers
as listed in --list-frames output (see describe). For
example, for ‘comment’ (‘COMM’) frames:
A three-letter code of the language in which the text is written.
Content descriptor.
The mode in which input files are open is controlled by the
idest-readonly variable:
This is a boolean variable that indicates whether idest-main
can modify tag frames. If its value is #t, the function return
value will be ignored, and input files will be opened in read-only
mode. This is the default.
If idest-readonly is ‘#f’ the idest-main function
should return the new list of frames. If it returns an empty list,
all existing frames will be deleted. If the function chooses not
to modify any frames, it must return #f.
The two following sections show how to write script files. The
sample scripts they discuss can be found in subdirectory
examples of the idest distribution.
IdEst – ID3 Editing and Scripting Tool (split by node):
This document was generated on March 11, 2017 using makeinfo.
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