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You can't sudo echo and redirect from the non-sudo shell like that.

    echo 3 | sudo tee /proc/sys/vm/drop_caches

    sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'

Also fixed your typo in /proc...

Also try:

     sudo sysctl -w vm.drop_caches=3

Or more simply, use

   su -c 'echo 3 > /proc/sys/vm/drop_caches'

echo 3 | sudo tee /proc/sys/vm/drop_caches

thanks. copy pasting from the github via my phone, and should have taken the extra few mins

No worries, overall a very useful summary comment.