Now that we have told Apache how to handle SSIs, we can start to use them in our files. Once again, only those files with .shtml suffixes will be parsed by Apache's server-side include mechanism, so make sure to save your files with an .shtml suffix, rather than with an .html suffix.
As we saw above, server-side includes look like HTML comments. This means any server-side includes not parsed by Apache will be invisible to the end user. Even if the SSI is passed unmodified to the user's browser (because of an error or a misconfigured server), there won't be any problems or oddities in what the user sees.
One of the most common uses of SSIs is to indicate when a document was modified. This is useful when a page is updated regularly; a typical example might be a news service or events calendar.
Here is a file indicating its latest modification date. We print the date with the SSI #echo command, which prints the value of an SSI variable. SSI variables include environment variables, plus several others defined by Apache. In this example, we look at LAST_MODIFIED, which contains the date and time of when the file was changed:
<HTML> <Head><Title>I was modified</Title></Head> <Body> <H1>I was modified</H1> <P>I was last modified on <!--#echo var="LAST_MODIFIED" --> </P> </Body> </HTML>
If you have followed the instructions so far, retrieving this page should indicate when it was last modified. Remember to save the file with an extension of .shtml—while writing this column, I spent some time trying to figure out why one particular SSI wasn't working. As it turns out, the problem was with the file extension, not my server.
The date printed by #echo might look nice to programmers, but it is a little daunting for most people. Non-programmers would prefer a slightly more familiar date and time format.
Fortunately, Apache allows us to modify the way in which dates are displayed. C programmers are probably familiar with the strftime function, which allows for the creation of many different time and date strings by using characters preceded by percent signs (%). For example, “%A” gives us the name of a day, “%B” returns the name of a month, “%d” gives the day of the month and “%Y” returns the four-digit year. Thus by specifying “%A, %d %B %Y”, we can get a string that looks like “Wednesday, February 22 1998”.
Here is an example of setting the date to American format, first using the “config” SSI, and then using the “echo” SSI to display the results in our new format.
<HTML> <Head><Title>Testing</Title></Head> <Body> <H1>Testing</H1> <!--#config timefmt="%m/%d/%y" --> <P>In America, I was changed on <!--#echo var="LAST_MODIFIED"></P> </Body> </HTML>
Already, we can see a pattern in how server-side includes are defined and used. They consist of a keyword and then one or more attribute,value pairs, just as in this example:
<HTML> <Head><Title>Testing</Title></Head> <Body> <H1>Testing</H1> <!--#config timefmt="%m/%d/%y" --> <P>In America, I was changed on <!--#echo var="LAST_MODIFIED" --></P> <!--#config timefmt="%d/%m/%y" --> <P>In Europe, I was changed on <!--#echo var="LAST_MODIFIED" --></P> </Body> </HTML>
Printing a file's modification date is fine if you are running a continuously updated news service, but it does not have many other applications. By contrast, I find the file-inclusion SSI functions are extremely useful when designing sites.
The syntax is quite simple, as you can see from this example:
<HTML> <Head><Title>A basic template</Title></Head> <!--#include virtual="/fragments/header.htmlf" --> <P>This is the text of my page, sandwiched between two server-side includes.</P> <!--#include virtual="/fragments/footer.htmlf" --> </Body> </HTML>
Here, we use #include, with a single argument named “virtual”. Apache replaces the contents of this SSI with the contents of the named file. This might not seem all that useful, but consider how easy this makes it to create a site with a uniform look. The header.htmlf fragment could contain the standard <Body> tag, defining text and background colors, as well as putting a menu bar across the top of the page. By the same token, the footer.htmlf fragment could contain a copyright notice, smaller menu bar or information about the server.
What is the advantage? When you decide to add a new button to the menu bar or when the site's sponsors move to a new address, you only need to modify a single file. The changes propagate automatically through the rest of the site. This is easier than changing each individual page, and more efficient than creating the page with a CGI program. Just as you can avoid programming errors by putting repeated instructions into a subroutine, so too you can avoid typos and other potential problems by putting repeated information into HTML fragments that are imported with the “include” SSI.
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- Problems with Ubuntu's Software Center and How Canonical Plans to Fix Them
- Concerning Containers' Connections: on Docker Networking
- A Project to Guarantee Better Security for Open-Source Projects
- Where's That Pesky Hidden Word?
- Firefox Security Exploit Targets Linux Users and Web Developers
- My Network Go-Bag
- Doing Astronomy with Python
- Build a “Virtual SuperComputer” with Process Virtualization
- diff -u: What's New in Kernel Development