[Python] How does one enter a Python virtualenv when executing a bashscript?

If one defines which version of python to use in a bash script, it would be

export PYTHON = "/path/python/python-3.5.1/bin/python"


But for Python virtualenv's, one executes these commands in the command line

cd /path/pathto/virtualenv
source activate
cd another_directory


How does one "enter" a Python virtualenv in a bash script? What is the standard approach here?

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