While you can create Launch Agent files with text editors, another way is to use online tools such as Launchd Plist Generator. At this site, you can supply the label you'd like to use, followed by specifying the full path to a program in the Program Arguments field, and then setting conditions for launching the program before saving the generated plist in your username > Library > LaunchAgents folder. (You'll need to create this folder if it does not exist.)
Note that program files that end in .app in OS X are actually folders, so to launch these apps you will need to specify the executable file within their folders. For instance, the following is the full path to the Calculator application at its default location in OS X, and how it should look as the Program Argument value in a launch agent file: /Applications/Calculator.app/Contents/MacOS/calculator.
Granted this approach might seem complicated, but you can use it for running scripts and programs in the background, making it exceptionally useful for backups, maintenance, and other custom routines.
Direct and Indirect launching
The above approaches can be used to open applications directly, but you might consider having them open a script that will in turn launch the desired applications. Even though using a script adds complexity, by doing so you can set and forget the scheduling routine, and then simply update the script to change what is opened and how it is opened.
Scripting can sound intimidating, and for shell scripts and even AppleScript it can be daunting to get right. However, Apple's Automator program makes it relatively simple to create basic workflows. You can then save these workflows as small applications, and then use the above methods to open those on the schedule or condition you set.
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