Take timed or scripted screenshots from Terminal — via Apple (man page mirror by Keith Smiley)

The easy way

1. The example waits 5 seconds (-T 5), captures only the main display (-m), skips the camera sound (-x) and saves to delayed-shot.png on the desktop. Pick a file name that doesn't exist yet, or the old file is replaced.
2. Other handy options: -i interactive selection, -c to the clipboard, -P open in Preview, -R x,y,w,h a fixed rectangle, -o no window shadow, -v record video.
3. Run screencapture -help for the full list.

Command

Run in Terminal (Applications › Utilities).

screencapture -m -T 5 -x ~/Desktop/delayed-shot.png

Without -m, a full-screen capture on a Mac with several displays saves one file per screen, so you'd list one file name per display (or use -D 2 to pick a specific display). The first time, macOS may ask you to allow Terminal to record the screen in System Settings > Privacy & Security. To undo, delete the screenshot file(s) you created.