> source ./venv/bin/activate

To this day I'm not quite sure why the venv developers decided that sourcing was a good idea; all it does can be effectively replaced with

    #!/bin/sh
    export VIRTUAL_ENV="path to venv"
    export PATH="$VIRTUAL_ENV/bin:$PATH"
    unset PYTHONHOME
    exec "$SHELL"

Just run this script to get into an "activated" shell. To deactivate, just press Ctrl+D. If you're really fancy, you can replace the last line with

    exec "${@:-$SHELL}"

to run a command directly in the activated environment (and then deactivate it immediately).