[mike@mwxm4]

README.md

@@ -28,6 +28,20 @@ version from `main.go`.
 `../t/corpus`, the same corpus `t/select.t` uses, and fails if the two
 implementations start disagreeing.
 
+## The first one
+
+`upd --update` needs an upd to run it. `loadupd.sh` fetches that first one -
+POSIX sh, curl or wget, no upd required:
+
+```sh
+./loadupd.sh              # into the current directory
+./loadupd.sh ~/bin        # into a directory of your choosing
+```
+
+It reads the newest release, picks the asset matching `uname -s`/`uname -m`,
+runs it once before putting it in place, and says if the directory is not in
+`$PATH`. `$UPD_REPO_URL` points it at a mirror. From there on upd takes over.
+
 ## Updating itself
 
 upd installs itself the same way it installs everything else - same forge

loadupd.sh

@@ -0,0 +1,111 @@
+#!/bin/sh
+# loadupd.sh - fetch the newest upd binary for this machine.
+#
+# The one thing upd cannot do for itself: `upd --update` needs an upd to run
+# it. Everything after this first one is upd's own job.
+#
+#   ./loadupd.sh              # into the current directory
+#   ./loadupd.sh ~/bin        # into a directory of your choosing
+#
+# Plain POSIX sh, and nothing beyond uname, sed, grep and one of curl or wget:
+# it has to run on the machines upd exists for, before upd is on them.
+set -e
+
+REPO=${UPD_REPO_URL:-https://git.micw.org/mike/upd}
+NAME=upd
+DEST=${1:-.}
+
+die() {
+	echo "loadupd: $*" >&2
+	exit 1
+}
+
+# https://host/owner/repo -> https://host/api/v1/repos/owner/repo
+api=$(echo "$REPO" | sed -e 's#/*$##' -e 's#\.git$##' \
+	-e 's#^\(https\{0,1\}://[^/]*\)/\(.*\)$#\1/api/v1/repos/\2#')
+case "$api" in
+*/api/v1/repos/*/*) ;;
+*) die "cannot read $REPO, expected https://host/owner/repo" ;;
+esac
+
+os=$(uname -s | tr 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' 'abcdefghijklmnopqrstuvwxyz')
+case "$os" in
+darwin | linux | freebsd) ;;
+*) die "unsupported system: $(uname -s)" ;;
+esac
+
+# The names are Go's, not uname's, because that is what the release assets are
+# called.
+case "$(uname -m)" in
+x86_64 | amd64) arch=amd64 ;;
+aarch64 | arm64) arch=arm64 ;;
+i386 | i686) arch=386 ;;
+armv7* | armv6*) arch=arm ;;
+*) die "unsupported architecture: $(uname -m)" ;;
+esac
+
+asset="$NAME-$os-$arch"
+
+# curl on an old machine may be the very thing that cannot speak TLS 1.2 any
+# more - which is half the reason upd exists - so wget gets its turn before we
+# give up.
+fetch() { # url outfile
+	if command -v curl >/dev/null 2>&1; then
+		curl -fsSL -o "$2" "$1" && return 0
+	fi
+	if command -v wget >/dev/null 2>&1; then
+		wget -q -O "$2" "$1" && return 0
+	fi
+	return 1
+}
+
+tmp=$(mktemp -d "${TMPDIR:-/tmp}/loadupd.XXXXXX") || die "cannot create a temporary directory"
+trap 'rm -rf "$tmp"' EXIT INT TERM
+
+echo "Looking for the newest release of $NAME ..."
+fetch "$api/releases/latest" "$tmp/release.json" ||
+	die "cannot reach $api
+  If the TLS handshake failed, this machine is too old to fetch it itself -
+  copy $asset over from somewhere else."
+
+tag=$(sed -n 's/.*"tag_name"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p' "$tmp/release.json" | head -n 1)
+[ -n "$tag" ] || die "no release found in $REPO"
+
+# One asset per line, then pick ours by name. The JSON arrives as one long
+# line, so the comma is what separates the fields.
+urls=$(tr ',' '\n' <"$tmp/release.json" |
+	sed -n 's/.*"browser_download_url"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p')
+url=$(echo "$urls" | grep "/$asset\$" | head -n 1)
+if [ -z "$url" ]; then
+	die "release $tag has no $asset (only: $(echo "$urls" | sed 's#.*/##' | tr '\n' ' '))"
+fi
+
+echo "Downloading $asset $tag ..."
+fetch "$url" "$tmp/$NAME" || die "download failed: $url"
+chmod 755 "$tmp/$NAME"
+
+# A truncated file or one built for the wrong platform shows up here, not on
+# first use.
+"$tmp/$NAME" --version >/dev/null 2>&1 || die "the downloaded binary does not run"
+
+[ -d "$DEST" ] || mkdir -p "$DEST" || die "cannot create $DEST"
+# Spelled out in full: "." says nothing in a message, and even less in the
+# comparison against $PATH below.
+dir=$(cd "$DEST" && pwd) || die "cannot use $DEST"
+target="$dir/$NAME"
+
+# Via a file alongside, so that replacing an upd that is currently running is
+# one atomic step and not half a copy.
+cp "$tmp/$NAME" "$target.new.$$" || die "no write permission in $dir"
+chmod 755 "$target.new.$$"
+mv "$target.new.$$" "$target" || {
+	rm -f "$target.new.$$"
+	die "cannot replace $target"
+}
+
+echo "$NAME $tag installed at $target"
+case ":$PATH:" in
+*":$dir:"*) ;;
+*) echo "Note: $dir is not in \$PATH." ;;
+esac
+echo "From here on: $NAME --update"