Avoid unbound variable error when nounset is enabled

[robsonpeixoto]

If you are using asdf-direnv with asdf-elixir and try to execute the command asdf direnv shell elixir $version -- echo "hi" will raise the error "unbound variable" because the direnv uses set -u by default.

I got this error when I tried to setup this hook on ~/.asdfrc to always install the local.hex and local.rebar:

post_asdf_install_elixir = asdf direnv shell elixir $version -- mix do local.hex --force, local.rebar --force

The hack is: always use an empty string when the variable is not defined.

Besides, it is possible to add a set +u at the beginning of the file and set -u at the end, but set set -u at the end can cause an unexpected side effect for another script.

[Stratus3D]

Thanks @robsonpeixoto ! I think this is the right solution.