How to periodically check for the current date from within a program?

I would like to write a tiny calendar-like application for someone as a birthday present (to be run on Ubuntu). All it should do is display a separate picture each day, so whenever it's invoked it should check the date and select the appropriate picture from the collection I would provide, but also, in case it just keeps running, it should switch to the next picture when the next day begins.

The date-checking on invocation isn't the problem; my 开发者_StackOverflow社区question pertains to the second case: how can I have the program notice the beginning of the next day? My clumsy approach would be to make it check the current date at regular intervals and let it change the displayed picture once there was a change in date, but that strikes me as very roundabout and not particularly elegant.

In case any of you have got some idea of how I could accomplish this, please don't hesitate to reply. I would aim to write the application in either Perl or Python, so suggestions concerning those two languages would be most welcome, but any other suggestions would be appreciated as well.

Thanks a lot for your time!

The answer to this could be very system dependant. Controlling the time at which your program is executed is likely to be system dependant. On all *nix type systems, I would use cron. Assuming for a moment that you are using a *nix system, the answer then depends on what the program actually does.

If it only needs to select an image, then I would suggest that it not be run continuously, but terminates itself after selecting it, and is then run again the next day (there are a lot of tutorials on how to setup cron).

If, however, it has some form of UI and it is likely (read possible) to keep running for several days, then you can follow two approaches:

1. Create your program as it is, to poll periodically for the current time, and do a date delta comparison. Python timedelta objects could help here. This is pretty much your inelegant approach.
2. The other solution would be to send it a signal from cron when you do wish it to update. This process would mean that you would have to make it signal aware, and respond to something like USR1. The Python docs talk to this, but you can find many tutorials on the web. This approach also works quite nicely for daemonised apps.

I'm sure there are many other approaches too, but those are the ones that come to mind for a quickish and nastyish app.

Did you think about scheduling the invoke of your script?

For me, the best approach is this:

1.Have two options to run the script:

• run_script
• run_script --update

2.Schedule the update run in some task scheduler (for example Cron) to be executed daily.

3.When you would want to check the image for current day, simply run the script without update option.

If you would like me to extend any part of these, simply ask about it.