Add TIL on CI with Nix and Bash

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+---
+
+title: DIY bare bones CI server with Bash and Nix
+
+date: 2020-11-12 3
+
+layout: post
+
+lang: en
+
+ref: diy-bare-bones-ci-server-with-bash-and-nix
+
+---
+
+With a server with Nix installed (no need for NixOS), you can leverage its build
+isolation for running CI jobs by adding a [post-receive][post-receive] Git hook
+to the server.
+
+In most of my project I like to keep a `test` attribute which runs the test with
+`nix-build -A test`. This way, a post-receive hook could look like:
+
+```shell
+#!/usr/bin/env bash
+set -Eeuo pipefail
+set -x
+
+LOGS_PREFIX="/data/static/ci-logs/libedn"
+mkdir -p "$LOGS_DIR"
+LOGFILE="${LOGS_DIR}/$(date -Is)-$(git rev-parse master).log"
+exec &> >(tee -a "${LOGFILE}")
+
+unset GIT_DIR
+CLONE="$(mktemp -d)"
+git clone . "$CLONE"
+pushd "$CLONE"
+
+finish() {
+  printf "\n\n>>> exit status was %s\n" "$?"
+}
+trap finish EXIT
+
+nix-build -A test
+```
+
+We initially (lines #5 to #8) create a log file, named after *when* the run is
+running and for *which* commit it is running for. The `exec` and `tee` combo
+allows the output of the script to go both to `stdout` *and* the log file. This
+makes the logs output show up when you do a `git push`.
+
+Lines #10 to #13 create a fresh clone of the repository and line #20 runs the
+test command.
+
+After using a similar post-receive hook for a while, I now even generate a
+simple HTML file to make the [logs available][ci-logs] through the browser.
+
+[post-receive]: https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks
+[ci-logs]: https://ci.euandreh.xyz/
+
+## Upsides
+
+No vendor lock-in, as all you need is a server with Nix installed.
+
+And if you pin the Nixpkgs version you're using, this very simple setup yields
+extremely sandboxed runs on a very hermetic environment.
+
+## Downsides
+
+Besides the many missing shiny features of this very simplistic CI, `nix-build`
+can be very resource intensive. Specifically, it consumes too much memory. So if
+it has to download too many things, or the build closure gets too big, the
+server might very well run out of memory.