src/nixos-anywhere.sh: add shebang

It's not strictly needed because Nix wraps it with
writeShellApplication, but it makes it easier for the script to be
standalone and use the right syntax highlighter in shells.

Invoke it with bash in the README because of the Nix build sandbox
missing /usr/bin/env

Author: zimbatm

README.md

@@ -46,7 +46,7 @@ nix run github:numtide/nixos-anywhere -- root@yourip --flake github:your-user/yo
 The parameter passed to `--flake` should point to your nixos configuration
 exposed in your flake (`nixosConfigurations.your-system` in the example above).
 
-<!-- `$ ./src/nixos-anywhere.sh --help` -->
+<!-- `$ bash ./src/nixos-anywhere.sh --help` -->
 ```
 Usage: nixos-anywhere [options] ssh-host
 

src/nixos-anywhere.sh

@@ -1,3 +1,6 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
 showUsage() {
   cat <<USAGE
 Usage: nixos-anywhere [options] ssh-host