Bash Script That Automates A Software Install
The following should work for Snagit (and, with adaptions, for any application which is delivered in a .dmg and must be dragged to /Applications manually). It will not work for package installers.

#!/bin/bash

cd ~/Desktop

# get image from site
curl -O http://download.techsmith.com/snagitmac/enu/Snagit.dmg

# mount image
hdiutil attach Snagit.dmg

# copy app, remove old version first if installed
if [[ -d /Applications/Snagit.app ]]; then
    sudo rm -rf /Applications/Snagit.app
fi
sudo cp -r /Volumes/Snagit/Snagit.app /Applications/Snagit.app

# unmount and remove image
hdiutil detach /Volumes/Snagit
mv Snagit.dmg ~/.Trash/