Use older package version with Nix

gistfile1.txt

This works with the package manager [Nix](https://nixos.org/nix/), available on NixOS and working on most linux distros and macOS (theoretically also BSD and Windows).

1. search for package definition on GitHub, for example ansible

https://github.com/NixOS/nixpkgs/search?p=1&q=ansible&unscoped_q=ansible

it is here https://github.com/NixOS/nixpkgs/blob/master/pkgs/tools/admin/ansible/default.nix

2. search the history for the last commit with your preferred version, for example 2.5

https://github.com/NixOS/nixpkgs/blob/dcf5fd5afcced6e8e89d8dd3b7ff6eda86cd2958/pkgs/tools/admin/ansible/default.nix

3. create an interactive shell with that package version available, with nix-shell

specify the commit id in the URL, here dcf5fd5

```
[davidak@X230:~]$ nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/dcf5fd5.tar.gz -p ansible_2_5
...
[nix-shell:~]$ ansible --version
ansible 2.5.15
  config file = None
  configured module search path = ['/home/davidak/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /nix/store/4crxnkz6zvkl2j8nxcfpnxr1yjaba86m-python3.7-ansible-2.5.15/lib/python3.7/site-packages/ansible
  executable location = /nix/store/4crxnkz6zvkl2j8nxcfpnxr1yjaba86m-python3.7-ansible-2.5.15/bin/ansible
  python version = 3.7.3 (default, Mar 25 2019, 20:59:09) [GCC 7.4.0]
```
when you exit the shell, the package is no longer in your PATH. so your system remains clean. the package and it's specific dependencies will get removed from the disk with next `nix-collect-garbage -d`
 

when the version you need was never packaged, you might be able to use overrides, when the dependencies are still the same

like actually done here https://github.com/NixOS/nixpkgs/blob/dcf5fd5afcced6e8e89d8dd3b7ff6eda86cd2958/pkgs/tools/admin/ansible/default.nix#L8-L16

or you can package it locally, in your configuration. use the existing package definition as base and modify as needed

i for example have a personal package that no one else needs, so i have it in my own git instead of upstreaming it into nixpkgs

https://github.com/davidak/nixos-config/blob/master/packages/satzgenerator.nix

this is a python application. the package definition is actually quiet simple