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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.
用 Codex 或 Claude 帮你安装 复制这段 Prompt，粘贴到 Codex、Claude 或其他助手里，让它检查 Skill 页面并帮你完成安装。
基于 SOC 职业分类
| 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. |
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/GTKpkgs.emacs29-gtk3 / pkgs.emacs29-pgtkAdd 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).
Behavioral guidelines to reduce common LLM coding mistakes. Surface assumptions, avoid overcomplication, make surgical changes, define verifiable success criteria. Use when writing, reviewing, or refactoring code. Derived from Andrej Karpathy's observations on LLM coding pitfalls.
Run temporary/ad-hoc binaries on NixOS using nix-shell -p and patchelf. Use when a prebuilt binary won't run on NixOS, or when you need a tool ephemerally without permanent installation.
Configure and build Elisp projects using Eldev. Manage dependencies from package archives, Git repos, and local sources. Run tests with ERT, byte-compile projects, launch interactive Emacs sessions with project context, and manage loading modes. Use when user mentions eldev, an Eldev file, or an Elisp project with dependencies.
Prepare an Emacs Lisp package for MELPA submission — source file conventions, code style, and submission checklist. Use when user wants to submit a package to MELPA or asks about MELPA requirements.
Create and maintain agent skills using Org-mode as the source format. Add entries to SKILLS.org with EXPORT_SKILL_* properties, then export to markdown. Use when user wants to create, write, or build a new skill in an Org-mode-based skills repo.
Ultra-compressed communication mode. Cuts token usage ~75% by speaking like caveman while keeping full technical accuracy. Supports intensity levels: lite, full (default), ultra. Use when user says "caveman mode", "talk like caveman", "use caveman", "less tokens", "be brief", or invokes /caveman. Also auto-triggers when token efficiency is requested.