Without doubt, the most complex shell one-liner I have done

August 25, 2018

I was putting some photos on my website and created this insanely complex one-liner without even really intending to. It's an aspect of the Unix shell philosophy to incrementally build up greater and greater complexity on a line until it does what you want. Often, that consists of incrementally building a string that will be run or added to a file. In this particular case, I wanted to add photos to my website, and I wanted to automate the generation of html. So what did I do? I looped through all the photo files, adding the name of each one into a script that generated the necessary html text, and then added those lines to my webpage, in this case "photos.html"

for i in /var/www/photos/1/*; do echo "<p><a href=\"$(echo $i|sed 's:/var/www/::g')\">$(echo $i|sed 's:/var/www/photos/1/::g')</a>" >> ../renomadwebsite/photos.html; echo "<p>$(identify -verbose $i|grep ImageDescription|sed 's/^.*exif:ImageDescription: //g')</p>" >> ../renomadwebsite/photos.html; done 

Let's step through this. First, I'll break it down a little - a for loop with two lines. At this point we can see that we loop through all the files in a directory, and run two commands on them. To help you better understand, the first line creates html to refer to a photo, and the second line uses a command from ImageMagick called "Identify" to pull out text information from the photo.

for i in /var/www/photos/1/*
  do 
    echo "<p><a href=\"$(echo $i|sed 's:/var/www/::g')\">$(echo $i|sed 's:/var/www/photos/1/::g')</a>" >> ../renomadwebsite/photos.html
    echo "<p>$(identify -verbose $i|grep ImageDescription|sed 's/^.*exif:ImageDescription: //g')</p>" >> ../renomadwebsite/photos.html
  done 

Let's take apart the first line. The one unusual thing going on is that I have nested commands using the $() shell operator. You can see that I am setting up a hyperlink starting at href=. But then you see I have some commands inside the parentheses. What I am doing there is using the sed command to replace the path information on the file with an empty string, so it has the exactly correct syntax. I do a very similar thing a little further on, where I remove the entire path and leave only the file name. At the end of the line, I append the output to the photos.html file. Getting ahead a little bit, I do a very similar thing in the next line.

echo "<p><a href=\"$(echo $i|sed 's:/var/www/::g')\">$(echo $i|sed 's:/var/www/photos/1/::g')</a>" >> ../renomadwebsite/photos.html

I should emphasize that this isn't meant to represent a finished and general program. It's a one-off artifact that gets thrown away at the end. In Unix, you use the tools available to build the things you need. Certainly, it's possible to build this out into something more generally usable, but then you lose flexibility. It's like the difference between having a workshop full of tools and a do-it-all-machine. When you know how to use the tools, you can do anything. The do-it-all-machine, on the other hand, is wonky and doesn't always do it just how you wanted, and is hard to fathom and fix.

Contact me at renaissance.nomad (at) google.com