Work the Shell - Displaying Image Directories in Apache, Part III
In last month's column, we built our directory display script to the point where you could get a smart listing that showed your image files (offering links to any other file type), and we allowed thumbnails to be displayed too.
The latter trick is done by letting the Web browser do the work. If you specify a height or width that's different from the actual image size, Web browsers automatically scale the image to fit the specified dimensions. Even better, if you specify only one dimension, it scales proportionally to fit.
Let me explain that just a wee bit more, because it's critical to this particular scripting project. If you have an image that's 250x250 pixels and you'd like to display a 75x75 thumbnail, the best practice is to specify both height=“75” and width=“75”, of course. The problem is, what if the image is actually 250x317 and you want to reduce it to exactly 75 pixels wide. How tall should it be?
You could do the math, of course, but it's much nicer to let the browser do the work for you automatically, which happens if you specify only width=“75” or use a full HTML statement:
<img src="my250x317.png" width="75" />
Doing that scales it, and you end up with an image that's exactly 75x95 pixels in size. However, if you always constrain one dimension, things can break. What if the image is actually 250x1100, because it's a very tall graphic? Now the thumbnail is going to break the entire layout, because the scaled version of it is 330 pixels wide, quite a bit more than the 75x75 target box for the image!
That's why an ideal script would figure out which of the dimensions is larger, and then constrain that one to the size of the box we seek, letting the other scale proportionally automatically, thanks to the Web browser. And, that's exactly what we'll do!
Big Important Caveat: I realize there's a significant performance penalty for letting the browser scale images—the entire full-size image has to be downloaded, even though you're seeking a smaller version. If it was a problem, you could use a tool such as ImageMagick to scale the images and create thumbnail graphics that were displayed instead, probably dropping them into a cache and creating new ones on the fly as needed. But honestly, don't you have a high-bandwidth Internet connection, and does an additional second or two of load time really matter?
Last month, we created the darn useful script function figuresize, which, when given a graphic image, returned height and width parameters when those could be calculated. The resultant main loop in the script ended up looking like this:
for name in * do if [ ! -z "$(file -b $name|grep 'image data')" ] then figuresize $name if [ ! -z "$height" ] ; then echo "<img src=$name alt=$name height=50 />" echo "<br />$name ($height x $width)<br />" else echo "<img src=$name alt=$name height=50 />" echo "<br />$name<br />" fi else echo "<a href=$name>$name</a><br /><br />" fi done
If you read the code closely, it's really not doing anything smart with the height and width parameters, just displaying them in the output. Instead, let's turn that into a test to figure out which is larger. Before I do that though, we need to make some rudimentary improvements to the loop so the output is more attractive:
for name in * do if [ ! -z "$(file -b $name|grep 'image data')" ] then figuresize $name if [ ! -z "$height" ] ; then echo "<a href=$name><img src=$name border=0" echo "alt=$name height=$size " echo "align="absmiddle" />" echo "$name ($height x $width)</a>" else echo "<a href=$name><img src=$name border=0" echo "alt=$name height=$size" echo "align="absmiddle" />" echo "$name</a>" fi else echo "<a href=$name>$name</a><br />" fi echo "<hr />" done
The result of running this improved script (where images are clickable, there's a horizontal rule between entries and so forth) is shown in Figure 1.
Now, let's look at how to make the script even smarter:
if [ ! -z "$height" ] ; then if [ $height -gt $width ] ; then dimensionlabel="height" else dimensionlabel="width" fi
Can you see what I've done here? This lets us figure out which of the two dimensions of the graphic is larger and then set the dimensionlabel to that particular dimension. Here's the result:
echo "<img src=$name $dimensionlabel=$size />"
Dave Taylor has been hacking shell scripts for over thirty years. Really. He's the author of the popular "Wicked Cool Shell Scripts" and can be found on Twitter as @DaveTaylor and more generally at www.DaveTaylorOnline.com.
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
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With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide