A sandbox's startup script is run in your environment during the initial launch of a sandbox session. It can contain Bash commands to install software, configure your environment, or anything else you'd like to do!
To edit the startup script, click the gears on the left sidebar, and edit the appropriate field. For example, here's one to install the SQLAlchemy Python library:
Your script can contain multiple lines, for example:
mkdir ~/workspace/fooecho Hello > ~/workspace/foo/bar