gem executables can't be executed by /bin/sh or /bin/bash on macOS

[rzane]

Environment

OS: macOS 14.7.2
ARCH: arm64
SHELL: /bin/zsh

Reproduction

I'm installing Traveling Ruby like this:

$ curl -L https://github.com/YOU54F/traveling-ruby/releases/download/rel-20241122/traveling-ruby-20241122-3.2.6-osx-arm64-full.tar.gz | tar xzf -

I can install and execute a command line utility:

$ bin/gem install rspec
$ bin/rspec --version
RSpec 3.13
  - rspec-core 3.13.2
  - rspec-expectations 3.13.3
  - rspec-mocks 3.13.2
  - rspec-support 3.13.2

But, the gem executable can't be executed by /bin/sh or /bin/bash (or, at least I think that's the issue):

$ /bin/sh -c 'bin/rspec --version'
bin/rspec: line 9: require: command not found
bin/rspec: line 11: Gem.use_gemdeps: command not found
bin/rspec: line 13: version: command not found
bin/rspec: line 15: str: command not found
bin/rspec: line 17: syntax error near unexpected token `('
bin/rspec: line 17: `  str = str.b[/\A_(.*)_\z/, 1]'

$ /bin/bash -c 'bin/rspec --version' 
(same thing)

$ bin/ruby -e 'system "bin/rspec --version"'
(same thing)

It does work if I specify the interpreter explicitly:

$ sh -c 'bin/ruby bin/rspec --version'
RSpec 3.13
  - rspec-core 3.13.2
  - rspec-expectations 3.13.3
  - rspec-mocks 3.13.2
  - rspec-support 3.13.2

[rzane]

I did a little more digging here. Turns out, a script cannot be used as an interpreter in a shebang line on macOS.

Here's an explanation and possible solution for this issue: NixOS/nixpkgs#93757