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+Writing HTML with m4
+
+[20]
+
+   HOWTOs
+
+   by Bob Hepple
+   on March 1, 1998
+
+   It's amazing how easy it is to write simple HTML pages--and the
+   availability of WYSIWYG (what you see is what you get) HTML editors
+   like Netscape Gold lulls one into a mood of "don't worry, be happy".
+   However, managing multiple, inter-related pages of HTML rapidly gets
+   very difficult. I recently had a slightly complex set of pages to put
+   together, and I started thinking, "there has to be an easier way."
+
+   I immediately turned to the WWW and looked up all sorts of tools--but
+   quite honestly I was rather disappointed. Mostly, they were what I
+   would call "typing aids"--instead of having to remember arcane
+   incantations like <a href="link"7gt;text</a> text, you are given a
+   button or a magic keychord like alt-ctrl-j which remembers the syntax
+   and does all the typing for you.
+
+   Linux to the rescue--since HTML is built as ordinary text files, the
+   normal Linux text management tools can be used. This includes revision
+   control tools such as rcs and the text manipulation tools like awk,
+   Perl, etc. These tools offer significant help in version control and
+   managing development by multiple users as well as automating the
+   process of displaying information from a database (the classic grep
+   |sort |awk pipeline).
+
+   The use of these tools with HTML is documented elsewhere, e.g., Jim
+   Weirich's article in Linux Journal Issue 36, April 1997, "Using Perl to
+   Check Web Links". I highly recommend this article as yet another way to
+   really flex those Linux muscles when writing HTML.
+
+   What I will cover here is work I've done recently using the
+   pre-processor m4 to maintain HTML. The ideas can very easily be
+   extended to the more general SGML case.
+   Using m4
+
+   I decided to use m4 after looking at various other pre-processors
+   including cpp, the C front-end, which is perhaps a little too
+   C-specific to be useful with HTML. m4 is a generic and clean macro
+   expansion program, and it's available under most Unices including
+   Linux.
+
+   Instead of editing *.html files, I create *.m4 files with my favourite
+   text editor. These files look something like the following:
+m4_include(stdlib.m4)
+_HEADER(`This is my header')
+<P>This is some plain text<P>
+_HEAD1(`This is a main heading')
+<P>This is some more plain text<P>
+_TRAILER
+
+   The format is just HTML code, but you can include files and add macros
+   rather like in C. I use a convention that my new macros are in capitals
+   and start with an _ character to make them stand out from HTML language
+   and to avoid name-space collisions.
+
+   The m4 file is then processed as follows to create an .html file using
+   the command:
+m4 -P <file.m4 >file.html
+
+   This process is especially easy if you create a makefile to automate
+   these steps in the usual way. For example:
+.SUFFIXES: .m4 .html
+.m4.html:
+        m4 -P <$*.m4 >$*.html
+DEFault:        index.html
+*.html: stdlib.m4
+all:    default PROJECT1 PROJECT2
+PROJECT1:
+        (cd project2; make all)
+PROJECT2:
+        (cd project2; make all)
+
+   Some of the most useful commands in m4 are listed here with their cpp
+   equivalents shown in parentheses:
+     * m4_include: includes a common file into your HTML (#include)
+     * m4_define: defines an m4 variable (#define)
+     * m4_ifdef: a conditional (#ifdef)
+     * m4_changecom: change the m4 comment character (normally #)
+     * m4_debugmode: control error diagnostics
+     * m4_traceon/off: turn tracing on and off
+     * m4_dnl: comment
+     * m4_incr, m4_decr: simple arithmetic
+     * m4_eval: more general arithmetic
+     * m4_esyscmd: execute a Linux command and use the output
+     * m4_divert(i): This is a little complicated, so skip on first
+       reading. It is a way of storing text for output at the end of
+       normal processing. It will come in useful later, when we get to
+       automatic numbering of headings. It sends output from m4 to a
+       temporary file number i. At the end of processing, any text which
+       was diverted is then output, in the order of the file number i.
+       File number -1 is the bit bucket and can be used to comment out
+       chunks of comments. File number 0 is the normal output stream.
+       Thus, for example, you can use m4_divert to divert text to file 1,
+       and it will only be output at the end.
+
+   Sharing HTML Elements Across Several Pages
+
+   In many "nests" of HTML pages, each page shares elements such as a
+   button bar containing links to other pages like this:
+[Home]  [Next]  [Prev]  [Index]
+
+   This is fairly easy to create in each page. The trouble is that if you
+   make a change in the "standard" button-bar then you have the tedious
+   job of finding each occurrence of it in every file and manually making
+   the changes. With m4 we can more easily do this job by putting the
+   shared elements into an m4_include statement, just like C.
+
+   Let's also automate the naming of pages by putting the following lines
+   into an include file called button_bar.m4:
+m4_define(`_BUTTON_BAR',
+ <a href="homepage.html">[Home]</a>
+ <a href="$1">[Next]</a>
+ <a href="$2">[Prev]</a>
+ <a href="indexpage.html">[Index]</a>)
+
+   and then these lines in the document:
+m4_include button_bar.m4
+_BUTTON_BAR(`page_after_this.html',
+ `page_before_this.html')
+
+   The $1 and $2 parameters in the macro definition are replaced by the
+   strings in the macro call.
+   Managing HTML elements that often change
+
+   It is troublesome to have items change in multiple HTML pages. For
+   example, if your e-mail address changes, you need to change all
+   references to it to the new address. Instead, with m4 you can put a
+   line like the following in your stdlib.m4 file:
+m4_define(`_EMAIL_ADDRESS', `MyName@foo.bar.com')
+
+   and then just put _EMAIL_ADDRESS in your m4 files.
+
+   A more substantial example comes from building strings with multiple
+   components, any of which may change as the page is developed. If, like
+   me, you develop on one machine, test out the page and then upload to
+   another machine with a totally different address, then you could use
+   the m4_ifdef command in your stdlib.m4 file (just like the #ifdef
+   command in cpp). For example:
+m4_define(`_LOCAL')
+...
+m4_define(`_HOMEPAGE',
+ m4_ifdef(`_LOCAL',
+ `//127.0.0.1/~YourAccount',
+ `http://ISP.com/~YourAccount'))
+m4_define(`_PLUG', `<A HREF="http://www.ssc.com/linux/">
+<IMG SRC="_HOMEPAGE/gif/powered.gif"
+ALT=<"[Linux Information]"> </A>')
+
+   Note the careful use of quotes to prevent the variable _LOCAL from
+   being expanded. _HOMEPAGE takes on different values according to
+   whether the variable _LOCAL is defined or not. This definition can then
+   ripple through the entire project as you build the pages.
+
+   In this example, _PLUG is a macro to advertise Linux. When you are
+   testing your pages, use the local version of _HOMEPAGE. When you are
+   ready to upload, remove or comment out the _LOCAL definition in this
+   way:
+m4_dnl m4_define(`_LOCAL')
+
+   ... and then re-make.
+   Creating New Text Styles
+
+   Styles built into HTML include things like <EM> for emphasis and <CITE>
+   for citations. With m4 you can define your own new styles like this:
+m4_define(`_MYQUOTE',
+ <BLOCKQUOTE><EM>$1</EM></BLOCKQUOTE>)
+
+   If, later, you decide you prefer <STRONG> instead of <EM>, it is a
+   simple matter to change the definition. Then, every _MYQUOTE paragraph
+   falls into line with a quick make.
+
+   The classic guides to good HTML writing say things like "It is strongly
+   recommended that you employ the logical styles such as <EM>...</EM>
+   rather than the physical styles such as <I>...</I> in your documents."
+   Curiously, the WYSIWYG editors for HTML generate purely physical
+   styles. Using the m4 styles may be a good way to keep on using logical
+   styles.
+   Typing and Mnemonic Aids
+
+   I don't depend on WYSIWYG editing (having been brought up on troff) but
+   all the same I'm not averse to using help where it's available. There
+   is a choice (and maybe it's a fine line) to be made between:
+<BLOCKQUOTE><PRE><CODE>Some code you want to display.
+</CODE></PRE></BLOCKQUOTE>
+
+   and:
+_CODE(Some code you want to display.)
+
+   In this case, you would define _CODE like this:
+m4_define(`_CODE',
+<BLOCKQUOTE><PRE><CODE>$1</CODE></PRE></BLOCKQUOTE>)
+
+   Which version you prefer is a matter of taste and convenience although
+   the m4 macro certainly saves some typing. Another example I like to
+   use, since I can never remember the syntax for links, is:
+m4_define(`_LINK', <a href="$1">$2</a>)
+
+   Then, instead of typing:
+<a href="URL_TO_SOMEWHERE">Click here to get to SOMEWHERE
+</a>
+
+   I type:
+_LINK(`URL_TO_SOMEWHERE', `Click here to get to SOMEWHERE')
+
+   Automatic Numbering
+
+   m4 has a simple arithmetic facility with two operators m4_incr and
+   m4_decr. This facility can be used to create automatic numbering,
+   perhaps for headings, for example:
+m4_define(_CARDINAL,0)
+m4_define(_H, `m4_define(`_CARDINAL',
+ m4_incr(_CARDINAL))<H2>_CARDINAL.0 $1</H2>')
+_H(First Heading)
+_H(Second Heading)
+
+   This produces:
+<H2>1.0 First Heading</H2>
+<H2>2.0 Second Heading</H2>
+
+   Automatic Date Stamping
+
+   For simple date stamping of HTML pages, I use the m4_esyscmd command to
+   maintain an automatic timestamp on every page:
+This page was updated on m4_esyscmd(date)
+
+   which produces:
+This page was last updated on Fri May 9 10:35:03 HKT 1997
+
+   Generating Tables of Contents
+
+   Using m4 allows you to define commonly repeated phrases and use them
+   consistently. I hate repeating myself because I am lazy and because I
+   make mistakes, so I find this feature an absolute necessity.
+
+   A good example of the power of m4 is in building a table of contents in
+   a big page. This involves repeating the heading title in the table of
+   contents and then in the text itself. This is tedious and error-prone,
+   especially when you change the titles. There are specialised tools for
+   generating a table of contents from HTML pages, but the simple facility
+   provided by m4 is irresistible to me.
+   Simple To Understand TOC
+
+   The following example is a fairly simple-minded table of contents
+   generator. First, create some useful macros in stdlib.m4:
+m4_define(`_LINK_TO_LABEL',
+ <A HREF="#$1">$1</A>)
+m4_define(`_SECTION_HEADER',
+ <A NAME="$1"><H2>$1</H2></A>)
+
+   Then define all the section headings in a table at the start of the
+   page body:
+m4_define(`_DIFFICULTIES',
+ `The difficulties of HTML')
+m4_define(`_USING_M4', `Using
+ <EM>m4</EM>')
+m4_define(`_SHARING', `Sharing HTML
+ Elements Across Several Pages')
+
+   Then build the table:
+<UL><P>
+ <LI> _LINK_TO_LABEL(_DIFFICULTIES)
+ <LI> _LINK_TO_LABEL(_USING_M4)
+ <LI> _LINK_TO_LABEL(_SHARING)
+<UL>
+
+   Finally, write the text:
+ ...
+_SECTION_HEADER(_DIFFICULTIES)
+...
+
+   The advantages of this approach are twofold. If you change your
+   headings you only need to change them in one place, and the table of
+   contents is then automatically regenerated. Also, the links are
+   guaranteed to work.
+   Simple To Use TOC
+
+   The table of contents generator that I normally use is a bit more
+   complex and requires a bit more study, but it is much easier to use. It
+   not only builds the table, but it also automatically numbers the
+   headings on the fly--up to four levels of numbering (e.g., section
+   3.2.1.3), although this can be easily extended. It is very simple to
+   use as follows:
+    1. Where you want the table to appear, call Start_TOC.
+    2. At every heading use _H1(`Heading for level 1') or _H2(`Heading for
+       level 2') as appropriate.
+    3. After the last line of HTML code (probably </HTML>), call End_TOC.
+
+   The code for these macros is shown in [21]Listing 1. One restriction is
+   that you should not use diversions (i.e., m4-divert) within your text,
+   unless you preserve the diversion to file 1 used by this TOC generator.
+   Simple Tables
+
+   Other than Tables of Contents, many browsers support tabular
+   information. Here are some funky macros as a short cut to producing
+   these tables. First, an example (see Figure 1) of their use:
+<CENTER>
+_Start_Table(BORDER=5)
+_Table_Hdr(,Apples, Oranges, Lemons)
+_Table_Row(England, 100,250,300)
+_Table_Row(France,200,500,100)
+_Table_Row(Germany,500,50,90)
+_Table_Row(Spain,,23,2444)
+_Table_Row(Danmark,,,20)
+_End_Table
+</CENTER>
+
+   Writing HTML with m4
+
+   Figure 1. Example Table
+   m4 Gotchas
+
+   Unfortunately, m4 needs some taming. A little time spent on
+   familiarisation will pay dividends. Definitive documentation is
+   available (for example, in the Emacs info documentation system) but,
+   without being a complete tutorial, here are a few tips based on my
+   experiences.
+   Gotcha 1--Quotes
+
+   m4's quotation characters are the grave accent ` which starts the
+   quote, and the acute accent ' which ends it. It may help to put all
+   arguments to macros in quotes, for example:
+_HEAD1(`This is a heading')
+
+   The main reason for using quotes is to prevent confusion if commas are
+   contained in an argument to a macro, since m4 uses commas to separate
+   macro parameters. For example, the line _CODE(foo, bar) would put the
+   foo in the HTML output but not the bar. Use quotes in the line
+   _CODE(`foo, bar'), and it works properly.
+   Gotcha 2--Word Swallowing
+
+   The biggest problem with m4 is that some versions of it swallow key
+   words that it recognises, such as include, format, divert, file, gnu,
+   line, regexp, shift, unix, builtin and define. You can protect these
+   words by putting them in single quotes, for example:
+Smart people `include' Linux in their list
+of computer essentials.
+
+   The trouble is, this is both inconvenient and easy to forget.
+
+   A safer way to protect keywords (my preference) is to invoke m4 with
+   the -P or --prefix-builtins option. Then all built-in macro names are
+   modified so that they all begin with the prefix m4_ and ordinary words
+   are left as is. For example, using this option, one would write
+   m4_define instead of define (as shown in the examples in this article).
+   One hitch is that not all versions of m4 support this option--most
+   notably some PC versions under MS-DOS.
+   Gotcha 3--Comments
+
+   Comment lines in m4 begin with the # character--everything from the #
+   to the end of the line is ignored and output unchanged. If you want to
+   use # in the HTML page, you must quote it like this: `#'. Another
+   option (my preference) is to change the m4 comment character to
+   something exotic with a line like this:
+m4_changecom(`[[[[')
+
+   and not have to worry about # symbols in your text.
+
+   If you want to use comments in the m4 file but not have them appear in
+   the final HTML file, use the macro m4_dnl (dnl = Delete to New Line).
+   This macro suppresses everything until the next newline character.
+m4_define(_NEWMACRO, `foo bar')
+m4_dnl This is a comment
+
+   Yet another way to have source code ignored is the m4_divert command.
+   The main purpose of m4_divert is to save text in a temporary buffer for
+   inclusion in the file later--for example, in building a table of
+   contents or index. However, if you divert to "-1", it just goes to
+   limbo-land. This option is useful for getting rid of the whitespace
+   generated by the m4_define command. For example:
+m4_divert(-1) diversion on
+m4_define(this ...)
+m4_define(that ...)
+m4_divert diversion turned off
+
+   Gotcha 4--Debugging
+
+   Another tip for when things go wrong is to increase the number of error
+   diagnostics that m4 outputs. The easiest way to do this is to add the
+   following to your m4 file as debugging commands:
+m4_debugmode(e)
+m4_traceon
+...
+buggy lines
+...
+m4_traceoff
+
+   Conclusion
+
+   It should be noted that HTML 3.0 does have an include statement that
+   looks like this:
+<!--#include file="junk.html" -->
+
+   However, the HTML include has the following limitations:
+     * The work of including and interpreting the include is done on the
+       server-side before downloading and adds overhead as the server has
+       to scan files for include statements.
+     * Most servers (especially public ISPs) deactivate this feature
+       because of the large overhead.
+     * Include is all you get--no macro substitution, no parameters to
+       macros, no ifdef, etc., as with m4.
+
+   There are several other features of m4 that I have not yet exploited in
+   my HTML ramblings so far, such as regular expressions. It might be
+   interesting to create a "standard" stdlib.m4 for general use with nice
+   macros for general text processing and HTML functions. By all means
+   download my version of stdlib.m4 as a base for your own hacking. I
+   would be interested in hearing of useful macros, and if there is enough
+   interest, maybe a Mini-HOWTO could evolve from this article.
+
+   There are many additional advantages to using Linux to develop HTML
+   pages, far beyond the simple assistance given by the typical typing
+   aids and WYSIWYG tools. Certainly, I will go on using m4 until HTML
+   catches up--I will then do my last make and drop back to using pure
+   HTML. I hope you enjoy these little tricks and encourage you to
+   contribute your own.
+
+   Writing HTML with m4
+   Bob Hepple has been hacking at Unix since 1981 under a variety of
+   excuses and has somehow been paid for it at least some of the time.
+   It's allowed him to pursue another interest--living in warm, exotic
+   countries including Hong Kong, Australia, Qatar, Saudi Arabia, Lesotho
+   and (presently) Singapore. His initial aversion to the cold was learned
+   in the UK. Ambition--to stop working for the credit card company and
+   tax man and to get a real job. Bob can be reached at
+   bhepple@pacific.net.sg.
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