Improve venv script

[Irubataru]

Based on feedback from Alessandro

• I prefer /usr/bin/env in the shebang. AFAIK it is more portable across different Unix OS.
• This is minor, but lately I changed mind about if [[ -n "${VIRTUAL_ENV}" ]]. It is for sure correct, but I find if [[ "${VIRTUAL_ENV}” != '' ]] simply more direct and easier to read, avoiding some brain gymnastic.
• I would use set -u in scripts, too. I mean, your editor might be advanced enough to spot typos, but accessing unset variables in Bash is something I got bitten by a couple of times. Sure, nounset option has drawbacks e.g. in functions interfaces/design but it is easy to go around them with parameter expansion.
• These three lines look odd to me. Maybe I miss the point, but is your mkdir doing anything at all?
• Another voice probably outside the box: I never really used getopt. I do not really see a reason for using getopt. Everything it offers (than I am aware of and ever needed) is easily implementable in bash. And getopt versions on BSD, Debian, GNU and so on are different. I simply avoid this headache. What you have from line 243 on is basically a parser of the arguments already and I implement there all my parsing myself.
• Furthermore, the eval on line 242 is breaking parsing of possible command line arguments with spaces in them, isn’t it? I am not saying you will ever ending having a space in a filename… ;) What is that eval for?
• You have this pattern repeated some times and you might delegate it to a function that takes the function to call as first argument and forward the remaining argument to the function to be called. Proof of concept.
• I am also not sure what you would like to have here. Maybe something to discuss in next email.