You have a ? in the URL, which has special meaning in the shell. As such, you have to quote the text:
open 'alfred://runtrigger/com.alfredapp.vero.gettingstarted.snippettrigger/sniptest/?argument=test'
You can also use the AppleScript from a shell script:
osascript -e 'tell application id "com.runningwithcrayons.Alfred" to run trigger "sniptest" in workflow "com.alfredapp.vero.gettingstarted.snippettrigger" with argument "test"'
LRZ reacted to vitor in How to launch workflow from shell script?
