older-emacs-via-nix

name	older-emacs-via-nix
description	Pin nixpkgs revisions to build and use older Emacs versions (27, 28, 29) for testing eldev projects. Use when user needs an older Emacs for testing, or mentions emacs27/28/29 and nix.

Older Emacs via Nix

The current nixos-unstable channel only ships emacs (the latest, e.g. 30.2) and emacs30. Older majors (emacs28, emacs29) are removed once they fall out of nixpkgs's active set. To get them, pin a revision:

Want	nixpkgs branch	Version shipped
Emacs 29	nixos-24.05	29.4
Emacs 28	nixos-23.11	28.2
Emacs 27	nixos-22.05	27.2

One-shot build — write a tiny .nix expression to /tmp and nix-build it:

cat > /tmp/emacs29.nix <<'EOF'
let
  pkgs = import (fetchTarball {
    url = "https://github.com/NixOS/nixpkgs/archive/nixos-24.05.tar.gz";
  }) {};
in pkgs.emacs29
EOF

nix-build /tmp/emacs29.nix --no-out-link

nix-build prints the store path (e.g. /nix/store/...-emacs-29.4). The binary is at <store-path>/bin/emacs. Omit --no-out-link to register a GC root.

For a reproducible pin (fetchTarball follows the branch tip, so the hash drifts over time):

let
  pkgs = import (fetchTarball {
    url = "https://github.com/NixOS/nixpkgs/archive/<commit-sha>.tar.gz";
    sha256 = "<nix-prefetch-url-output>";
  }) {};
in pkgs.emacs29

Get the sha256:

nix-prefetch-url --unpack https://github.com/NixOS/nixpkgs/archive/<commit-sha>.tar.gz

Headless variants (substitute the attribute name, faster builds for --batch):

• pkgs.emacs29-nox — no X/GTK
• pkgs.emacs29-gtk3 / pkgs.emacs29-pgtk

Add to shell.nix for multiple Emacs versions together:

{ pkgs ? import <nixpkgs> {} }:

let
  oldPkgs = import (fetchTarball {
    url = "https://github.com/NixOS/nixpkgs/archive/nixos-24.05.tar.gz";
  }) {};
in
pkgs.mkShell {
  buildInputs = [ pkgs.emacs oldPkgs.emacs29 /* eldev, etc. */ ];
}

Both land on PATH; disambiguate by full path or by EMACS= (see the eldev skill's Targeting a specific Emacs binary section).