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 books for the most technical people on the planet. Newly available books include:
- Agile Product Development by Ted Schmidt
- Improve Business Processes with an Enterprise Job Scheduler by Mike Diehl
- Finding Your Way: Mapping Your Network to Improve Manageability by Bill Childers
- DIY Commerce Site by Reven Lerner
Plus many more.
- Unikernels, Docker, and Why You Should Care
- Server Hardening
- diff -u: What's New in Kernel Development
- Controversy at the Linux Foundation
- 22 Years of Linux Journal on One DVD - Now Available
- Non-Linux FOSS: Snk
- Giving Silos Their Due
- Don't Burn Your Android Yet
- What's New in 3D Printing, Part III: the Software