Mac: If you've been using OS X El Capitan for a while, you might have noticed that the old option to "Secure Empty Trash" is gone from the trash can's right-click menu. There's a pretty good reason why, but it's still possible to do it you don't mind digging into the command line.
As the name suggests, "Secure Empty Trash" was always a way to delete files securely, ensuring they can't get dug up again by data recovery software. According to Apple, it turns out that "Secure Empty Trash" doesn't reliably work on flash storage, like an Solid-State Drive, so Apple removed the option in El Capitan. SSDs don't store data in the same way as hard drives. As Macworld explains it, HDDs suffer wear with each write and erasure because they write to the same area each time, whereas SSD distributes that wear across the drive. So, with an SSD it's harder to ensure specific memory locations are erased because the data isn't reliably in the same place after each write.
Of course, not everyone running El Capitan has a SSD, but if you want to securely delete a file you'll need to dig into the command line. Note: this will delete a file for good, so make sure you know what you're doing and you select the correct file. Open up Terminal (Applications > Utilities) and type in (as the video above from OS X Daily points out, you can drag a file from Finder instead of typing in the path manually) :
srm -v /path/to/file
For example, if I wanted to securely delete a screenshot from desktop (for some reason), the command would look like this:
srm -v /Users/thorinklosowski/Desktop/Screen\ Shot\ 2015-11-24\ at\ 9.19.58\ AM.jpg
This will securely delete the file on a HDD, writing over the data seven times. If you have an SSD, you're better off enabling Apple's built-in encryption software, FileVault. Don't worry, with an SSD, you shouldn't notice too much of a difference in speed and performance.