[unsafe] install script does not include a truncation guard

[owlshrimp]

The install script generally recommended, for example

curl -fsSL https://deno.land/install.sh | sh

does not include a guard to prevent execution of an incomplete script. (see: https://deno.land/x/install@v0.3.3/install.sh)

This means that if the internet connection fails during download, incomplete code will be executed, or the install script will only be partially run. This can have disastrous results for the end user's computer, depending on where the install script is truncated.

There are many ways to alleviate this risk, for example the method employed by the asahi linux installer:

# SPDX-License-Identifier: MIT

# Truncation guard
if true; then
    set -e

    if [ ! -e /System ]; then
        echo "You appear to be running this script from Linux or another non-macOS system."
        echo "Asahi Linux can only be installed from macOS (or recoveryOS)."
        exit 1
    fi

    export LC_ALL=en_US.UTF-8
    export LANG=en_US.UTF-8
    export PATH="/usr/bin:/bin:/usr/sbin:/sbin:$PATH"

    if ! curl --no-progress-meter file:/// >/dev/null 2>&1; then
        echo "Your version of cURL is too old. This usually means your macOS is very out"
        echo "of date. Installing Asahi Linux requires at least macOS version 13.5."
        exit 1
    fi

    export VERSION_FLAG=https://cdn.asahilinux.org/installer/latest
    export INSTALLER_BASE=https://cdn.asahilinux.org/installer
    export INSTALLER_DATA=https://github.com/AsahiLinux/asahi-installer/raw/prod/data/installer_data.json
    export INSTALLER_DATA_ALT=https://alx.sh/installer_data.json
    export REPO_BASE=https://cdn.asahilinux.org
    export REPORT=https://stats.asahilinux.org/report
    export REPORT_TAG=alx-prod

    #TMP="$(mktemp -d)"
    TMP=/tmp/asahi-install

    echo
    echo "Bootstrapping installer:"

    if [ -e "$TMP" ]; then
        mv "$TMP" "$TMP-$(date +%Y%m%d-%H%M%S)"
    fi

    mkdir -p "$TMP"
    cd "$TMP"

    echo "  Checking version..."

    PKG_VER="$(curl --no-progress-meter -L "$VERSION_FLAG")"
    echo "  Version: $PKG_VER"

    PKG="installer-$PKG_VER.tar.gz"

    echo "  Downloading..."

    curl --no-progress-meter -L -o "$PKG" "$INSTALLER_BASE/$PKG"
    if ! curl --no-progress-meter -L -O "$INSTALLER_DATA"; then
        echo "    Error downloading installer_data.json. GitHub might be blocked in your network."
        echo "    Please consider using a VPN if you experience issues."
        echo "    Trying workaround..."
        curl --no-progress-meter -L -O "$INSTALLER_DATA_ALT"
    fi

    echo "  Extracting..."

    tar xf "$PKG"

    echo "  Initializing..."
    echo

    if [ "$USER" != "root" ]; then
        echo "The installer needs to run as root."
        echo "Please enter your sudo password if prompted."
        exec caffeinate -dis sudo -E ./install.sh "$@"
    else
        exec caffeinate -dis ./install.sh "$@"
    fi
fi

This installer wraps the entire script in a dummy "if true" so that if a matching "fi" isn't reached at the end, execution aborts before it begins.