4

Selecting a Python interpreter using a shebang doesn't seem to work in some cases.

I'm trying to use my python3.9 interpreter in a shebang (#!/...) on MacOS. See the Python-file below called hello.py:

#!/nix/store/i46k148mi830riq4wxh49ki8qmq0731k-python3-3.9.2-env/bin/python3.9 print("Hello world")

Now, I'll make sure this file is executable, and try to execute it:

$ chmod +x ./hello.py $ ./hello.py ./hello.py: line 2: syntax error near unexpected token `"Hello world"' ./hello.py: line 2: `print("Hello world")'

That's weird. Did I mistype something? Let me try to execute it with my interpreter directly to see if it works:

$ /nix/store/i46k148mi830riq4wxh49ki8qmq0731k-python3-3.9.2-env/bin/python3.9 ./hello.py Hello world

That seems to work fine. Why did the shebang fail? Let me try with a different interpreter:

#!/usr/bin/python3 print("Hello world") 
$ ./hello.py Hello world

And now it works? That's interesting.

Questions:

  • Why does the /nix/store-path not seem to work in the shebang?
  • Are there some limitations on shebang expressions on MacOS that I'm not aware of? For example a maximum length, or certain characters like . or - that are not permitted?

My system:

  • MacOS Big Sur 11.2.1 (20D74)
  • bash --version: GNU bash, version 4.4.23(1)-release (x86_64-apple-darwin17.7.0)

Edit: Executing ./hello.py from zsh instead of bash actually works (when using the nix-shebang). So that might hint at this being an issue specific to bash.

Improve this question
6
  • 1
    Most Unix systems have a 127-character limit to the shebang line, so this shouldn't be too long. Commented Apr 15, 2021 at 1:06
  • 1
    I'm able to reproduce this. The script is being run with bash as its interpreter, rather than python. Commented Apr 15, 2021 at 1:24
  • 1
    Bash has a bunch of logic to try to emulate thing the execve() call ought to do when that call fails. That logic has a quite long history of bugs; it's not unbelievable to me that this would be one of them. Commented Apr 15, 2021 at 1:29
  • 1
    More interesting is /usr/bin/python -c 'import subprocess; subprocess.call(["./test.py"], shell=False)', giving an Exec format error. This implies to me that the OS does have a problem with that shebang, and thus that we're relying on fallback behavior of the active shell. Commented Apr 15, 2021 at 1:32
  • 1
    github.com/NixOS/nixpkgs/issues/65351 Commented Apr 15, 2021 at 1:34

1 Answer 1

Reset to default
6

MacOS doesn't support shebangs pointing at scripts -- this causes an Exec format error.

Both bash and zsh try, with varying degrees of success, to work around execve() failures. However, the better fix is to use upstream nixpkgs' ability -- as introduced at https://github.com/NixOS/nixpkgs/pull/93757 -- to generate a wrapper that doesn't trigger the bug.

This wrapper will not look like:

#!/nix/store/i46k148mi830riq4wxh49ki8qmq0731k-python3-3.9.2-env/bin/python3.9

...but instead will look more like:

#!/nix/store/74shlfgb18717ixjlpivpxd7iqcyhyn5-bash-4.4-p23/bin/bash /nix/store/i46k148mi830riq4wxh49ki8qmq0731k-python3-3.9.2-env/bin/python3.9

...which, somewhat surprisingly, works as intended.

Improve this answer
Sign up to request clarification or add additional context in comments.

Comments

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.