Can I track an RSS feed with a shell script?
More than once, readers have written to me, asking if it was possible to track an RSS feed from a Weblog or news site with a shell script. Sounds kinda wacky, but in fact, it's a very good use of a shell script, as the following rather extensive entry -- including source code! -- demonstrates. If you're a bit confused by the following, you might want to consider picking up a copy of my best-selling Wicked Cool Shell Scripts.
The following article originally appeared at MacDevCenter and is reprinted with permission.
Tapping RSS with Shell Scripts
If you're like me, you want to keep up with the latest news and information. Shell scripts help me do just that. In this article I'll show you how I wrote a shell script that watches the news at Slashdot.org and automatically shows me the latest story headlines every time I launch a Terminal application.
First Things First
Before any shell script work begins, the first step is to figure out the URL of the RSS page on Slashdot.
The Slashdot home page doesn't make it particularly easy to find, but the very bottom line, the very rightmost link, is "rss", and the URL behind that link is http://slashdot.org/index.rss.
To look at it from within the Terminal, I'm going to utilize the powerful curl application, piping the output to head to ensure that I'm not drowned in output:
Yes, this looks fairly scary as output goes, I admit, but with a little help from the grep utility, this can quickly become a lot more user-friendly. In this case, let's just pull out the lines that are tagged as either the <title> or the <description>:
Not bad. In fact, that's really almost all we need. So let's turn this into a shell.
To turn this command line into a shell script is a breeze: just open up your favorite Terminal command-line editor (I use vi but I've been trapped in Unix since 1980 so it's already subverted my neural pathways. You might prefer pico or even BBEdit or similar) Whichever you choose, type in the following, a standard shell script preamble:
This tells the operating system that when this particular file is executed, it should be given to the shell (sh) to be run. Then let's create a variable that contains the URL:
Now we can reference
This script produces the output already seen, so let's make two tweaks
to it so it's more useful. First off, the first three lines of output,
the Slashdot title and description, never change so it'd be just as
easy to strip them out of the output. This can be done a variety of
ways, but I'm going to turn to the
Notice the trailing backslash here: rather than have our command pipe stretch longer and longer, the backslash (which must be the very last character on the line) let's me wrap the command to multiple lines and make it generally more readable.
We're getting close to trying the script. The only other tweak worth
making is to strip out the
The XML tags are effectively stripped out, except the
This shows the top two stories (4 lines = two titles + two descriptions). Not bad. Not beautiful, but certainly functional for a first script.
I always spend way too much time fine-tuning scripts to get just the output I want, so let's continue working on this to ensure that the output is more readable, shall we? It's so easy, you'll be amazed:
The results, piped through head again:
The problem now is that the
Headlines, As Many As You Want
The obvious solution is to add a command flag that lets you specify how
many headlines you want: multiply it by two and you'll know what value
Now I can specify that I only want the top headline, the newest entry
on the Slashdot site, by simply specifying '
That's pretty cool, I think. I could tweak it forever, but let's stop
here and see how to turn this into a Unix command just like
Turning It Into a Command
There are two ways to turn a shell script into a command: create an alias or make the script executable and ensure it's in your PATH. To create an alias, if you're using Bash, an alias can be created like this:
Then you can see the headlines by just typing
To make the shell script itself executable, first make sure you've saved it in a directory that's in your PATH by typing:
You can see that my PATH includes /Users/dt/bin - that's where I save this script and similar. Once it's in the right place, you'll need to make it executable by using the chmod command:
Optionally, you could rename the script to be a bit more friendly, of course.
Finally, Having It Auto-Execute Upon Terminal Launch
If you're running the Bash shell, which you probably are if you're in Panther, then it's a breeze: move to your home directory and append an invocation of the script to your .bash_login file:
Make extra sure that you use two
Now the next time you start up a Terminal application window, you'll see:
It's also worth noting that this use of shell scripts to parse and format XML has more applications. For example, go to http://www.casino-bookstore.com/ and have a close look at the "Latest Gambling News" box: it's using almost an identical script to keep track of the gambling news XML feed from about.com. Another example? Go to http://www.healthy-bookstore.com/ and look at the medicinenet news feed. Again, it's using curl and sed to turn the XML data into HTML data.
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