Enables shebang usage of nix shell. All arguments with `#! nix` get
added to the nix invocation. This implementation does NOT set any
additional arguments other than placing the script path itself as the
first argument such that the interpreter can utilize it.
Example below:
```
#!/usr/bin/env nix
#! nix shell --quiet
#! nix nixpkgs#bash
#! nix nixpkgs#shellcheck
#! nix nixpkgs#hello
#! nix --ignore-environment --command bash
# shellcheck shell=bash
set -eu
shellcheck "$0" || exit 1
function main {
hello
echo 0:"$0" 1:"$1" 2:"$2"
}
"$@"
```
fix: include programName usage
EDIT: For posterity I've changed shellwords to shellwords2 in order
not to interfere with other changes during a rebase.
shellwords2 is removed in a later commit. -- roberth
I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests
- Concepts is harder to understand, the documentation makes a good
unit vs functional vs integration distinction, but when the
integration tests are just two subdirs within `tests/` this is not
clear.
- Source filtering in the `flake.nix` is more complex. We need to
filter out some of the dirs from `tests/`, rather than simply pick
the dirs we want and take all of them. This is a good sign the
structure of what we are trying to do is not matching the structure
of the files.
With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests
functional/
installer/
nixos/
```
