Work the Shell - Function Return Codes and Daylight Calculations
Don't worry, I won't burst into a song from Fiddler on the Roof, but sunrise and sunset times are very dependent on not only the time of year but also on your location.
After digging around quite a bit, it seems like Almanac.com is one of the easiest sites to work with, so that's what I'll use. A sunrise/sunset query to Almanac.com ends up with a URL of this form: http://www.almanac.com/astronomy/rise/zipcode/80302/2010-08-01.
You'll have to use date to calculate the current date in the proper format and hard-code the local zipcode into the function.
As with most sites, the HTML generated by the Almanac.com result is not parse-friendly, so I had to dig around for a while to figure out how to proceed. Here's what I came up with:
yourzip="80302" # set this to your local zip code request="http://www.almanac.com/astronomy/rise/zipcode" thedate="$(date +%Y-%m-%d)" url="$request/$yourzip/$thedate" curl --silent "$url" | grep rise_nextprev | \ cut -d\< -f28-30
You can see that the zipcode is indeed hard-coded, and notice how I use the $() notation to grab the date in YYYY-MM-DD format. Curl gives you the resultant HTML page, grep finds the one line you're interested in, and then cut chops out the following snippet:
td> 5:38 A.M.</td><td> 8:34 P.M.
There are a few more hoops to jump through, so you can pull out the hour and minute of sunrise and sunset separately (as you'll have to test that way). Here's the code I came up with:
raw="$(/usr/bin/curl --silent "$url" | \ grep rise_nextprev | cut -d\<-f28-30)" sunrise="$(echo $raw | cut -d\ -f2)" sunset="$(echo $raw | cut -d\ -f4)" srh=$(echo $sunrise | cut -d: -f1) srm=$(echo $sunrise | cut -d: -f2) ssh=$(echo $sunset | cut -d: -f1) ssm=$(echo $sunset | cut -d: -f2)
You could make it a bit faster by avoiding the intermediate calculations of sunrise and sunset, but on modern Linux systems, it should be a matter of milliseconds, so let's leave it just like that.
There's one more important tweak: sunset hour (ssh) needs to be on a 24-hour clock, as that's what you're getting from the date invocation shown earlier. It turns out that you can drop the cut subshell invocation into a calculation:
ssh=$(( $(echo $sunset | cut -d: -f1) + 12 ))
Yes, it works. It looks like I'm moving into LISP territory, but fortunately not!
To work properly, the script needs to do three tests:
Whether it's sunrise hour and greater than sunrise minute.
Whether it's greater than sunrise hour but less than sunset hour.
Whether it's sunset hour but less than sunset minute.
Here's how that looks as script:
if [ $hour -eq $srh -a $min -ge $srm ] ; then return 0 # special case of sunrise hour fi if [ $hour -gt $srh -a $hour -lt $ssh ] ; then return 0 # easy: after sunrise, before sunset fi if [ $hour -eq $ssh -a $min -le $ssm ] ; then return 0 # special case: sunset hour fi
Voilà! Kinda neat, if I say so myself.
My full implementation of isdaytime is available on the Linux Journal FTP server at ftp.linuxjournal.com/pub/lj/listings/issue198/10860.tgz.
Dave Taylor has been hacking shell scripts for a really long time, 30 years. 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.
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.
Free to Linux Journal readers.Register Now!
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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