commit 3883e36abcf5a0df0f8521378f08305cb4b89059
Author: James Swineson <github@public.swineson.me>
Date:   Tue Apr 9 15:51:15 2019 +0800

    initial commit:

diff --git a/menhera.sh b/menhera.sh
new file mode 100755
index 0000000..7bf4604
--- /dev/null
+++ b/menhera.sh
@@ -0,0 +1,146 @@
+#!/bin/bash
+set -Eeuxo pipefail
+
+# config
+WORKDIR="/tmp/menhera"
+ROOTFS="https://images.linuxcontainers.org/images/debian/stretch/amd64/default/20190406_05:24/rootfs.squashfs"
+
+# internal global variables
+OLDROOT="/"
+NEWROOT=""
+
+# https://stackoverflow.com/a/3232082/2646069
+confirm() {
+    # call with a prompt string or use a default
+    read -r -p "${1:-Are you sure? [y/N]} " response
+    case "$response" in
+        [yY][eE][sS]|[yY]) 
+            true
+            ;;
+        *)
+            false
+            ;;
+    esac
+}
+
+sync_filesystem() {
+    echo "Syncing..."
+    sync
+    sync
+}
+
+prepare_environment() {
+    echo "Loading kernel modules..."
+    modprobe overlay
+    modprobe squashfs
+
+    echo "Creating workspace in '${WORKDIR}'..."
+    # workspace
+    mkdir -p "${WORKDIR}"
+    mount -t tmpfs tmpfs "${WORKDIR}"
+
+    # new rootfs
+    mkdir -p "${WORKDIR}/newroot"
+    # readonly part of new rootfs
+    mkdir -p "${WORKDIR}/newrootro"
+    # writeable part of new rootfs
+    mkdir -p "${WORKDIR}/newrootrw"
+    # overlayfs workdir
+    mkdir -p "${WORKDIR}/overlayfs_workdir"
+
+    echo "Downloading temporary rootfs..."
+    wget "${ROOTFS}" -O "${WORKDIR}/rootfs.squashfs"
+}
+
+mount_new_rootfs() {
+    echo "Mounting temporary rootfs..."
+    mount -t squashfs "${WORKDIR}/rootfs.squashfs" "${WORKDIR}/newrootro"
+    mount -t overlay overlay -o rw,lowerdir="${WORKDIR}/newrootro",upperdir="${WORKDIR}/newrootrw",workdir="${WORKDIR}/overlayfs_workdir" "${WORKDIR}/newroot"
+
+    NEWROOT="${WORKDIR}/newroot"
+}
+
+install_software() {
+    echo "Installing OpenSSH Server into new rootfs..."
+    DEBIAN_FRONTEND=noninteractive chroot "${NEWROOT}" apt-get update -y
+    DEBIAN_FRONTEND=noninteractive chroot "${NEWROOT}" apt-get -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" install -y ssh
+}
+
+copy_config() {
+    echo "Copying important config into new rootfs..."
+    cp -ax "${OLDROOT}/etc/resolv.conf" "${NEWROOT}/etc"
+    cp -axr "${OLDROOT}/etc/ssh" "${NEWROOT}/etc"
+    cp -ax "${OLDROOT}/etc/{passwd,shadow}" "${NEWROOT}/etc"
+    cp -axr "${OLDROOT}/root/.ssh" "${NEWROOT}/root"
+
+    chroot "${NEWROOT}" chsh -s /bin/bash root
+}
+
+swap_root() {
+    echo "Swapping rootfs..."
+    # prepare future mount point for our old rootfs
+    mkdir -p "${WORKDIR}/newroot/mnt/oldroot"
+    mount --make-rprivate /
+
+    # swap root
+    pivot_root "${WORKDIR}/newroot" "${WORKDIR}/newroot/mnt/oldroot"
+
+    OLDROOT="/mnt/oldroot"
+    NEWROOT="/"
+
+    # move mounts
+    for i in dev proc sys run; do mount --move "${OLDROOT}/$i" "${NEWROOT}/$i"; done
+    mount -t tmpfs tmpfs "${NEWROOT}/tmp"
+}
+
+clear_processes() {
+    echo "Restarting SSH daemon..."
+    systemctl restart ssh
+
+    echo "Restarting systemd..."
+    systemctl daemon-reexec --no-block
+    sleep 15
+
+    echo "Killing all programs still using the old root..."
+    fuser -kvm "${OLDROOT}" -15
+    sleep 15
+    fuser -kvm "${OLDROOT}" -9
+    sleep 3
+
+    echo "Disabling swap..."
+    swapoff -a
+}
+
+unmount_old_rootfs() {
+    echo "Unmounting old rootfs..."
+    umount "${OLDROOT}"
+}
+
+# main procedure
+
+if [[ $EUID -ne 0 ]]; then
+   echo "This script must be run as root" 
+   exit 1
+fi
+
+echo -e "We will start a temporary RAM system as your recovery environment."
+echo -e "Note that this script will kill programs and umount filesystems without prompting."
+echo -e "Please confirm:"
+echo -e "\tYou have closed all programs you can, and backed up all important data"
+echo -e "\tYou can SSH into your system as root user"
+confirm || exit -1
+
+sync_filesystem
+
+prepare_environment
+mount_new_rootfs
+install_software
+copy_config
+swap_root
+
+echo -e "If you are connecting from SSH, please create a second session to this host and confirm you can get a shell."
+echo -e "After your confirmation, we are going to kill the old SSH server."
+confirm || exit -1
+
+clear_processes
+unmount_old_rootfs
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