enter chroot

enter.sh

#!/bin/sh
set -eu

# make sure we're root
if [ "$(id -u)" != 0 ]; then
  sucmd="su -c"
  if command -v doas >/dev/null; then
    sucmd="doas"
  elif command -v sudo >/dev/null; then
    sucmd="sudo"
  fi

  exec $sucmd env HOME="$HOME" USERNAME="$(id -u -n)" "$0" "$@"
fi

# and make sure we're in a separate mount namespace
if [ "$(readlink /proc/$$/ns/mnt | cut -d: -f2)" = "$(readlink /proc/1/ns/mnt | cut -d: -f2)" ]; then
  exec unshare -m "$0" "$@"
fi

bindpoints="/etc/resolv.conf"
command=""
user="$USERNAME"

while getopts "be:u:c:" opt; do
  case $opt in
    'b') bindpoints="
      $bindpoints
      $HOME/aports
      $HOME/packages
      $HOME/.abuild
      " ;;
    'e') bindpoints="$bindpoints $OPTARG" ;;
    'u') user="$OPTARG" ;;
    'c') command="$OPTARG" ;;
    *) echo "unknown $opt" ;;
  esac
done
shift $(( $OPTIND - 1 ))

dest="$1"
shift

if [ ! -d "$dest" ]; then
  echo "no such directory: $dest"
  exit 1
fi

if [ ! -x "$dest"/bin/sh ]; then
  echo "$dest does not contain executable /bin/sh"
  exit 1
fi

mount -t proc proc "$dest"/proc
mount -t sysfs sysfs "$dest"/sys
mount -t tmpfs tmpfs "$dest"/tmp
# some mini kernels don't have devtmpfs
if grep -q devtmpfs /proc/filesystems; then
  mount -t devtmpfs devtmpfs "$dest"/dev
else
  mount -t tmpfs tmpfs "$dest"/dev

  if [ -x "$dest"/sbin/mdev ]; then
    echo "devtmpfs not supported - running 'mdev -s' instead"
    chroot "$dest" /sbin/mdev -s
  else
    echo "devtmpfs not supported - devices need to be created manually"
  fi
fi
mount -t devpts devpts "$dest"/dev/pts
mount -t tmpfs tmpfs "$dest"/dev/shm

for bindpoint in $bindpoints; do
  if [ -f "$bindpoint" ]; then
    touch "$dest"/"$bindpoint"
  else
    mkdir -p "$dest"/"$bindpoint"
  fi

  mount --bind "$bindpoint" "$dest"/"$bindpoint"
done

# if running on chromeos, fixup symlink exec
# https://goo.gl/8xICW6
# https://github.com/dnschneid/crouton/commit/5cb7ad05
if [ -e "/run/chrome" ]; then
  if mount -n -t securityfs -o nodev,noexec,nosuid securityfs /sys/kernel/security; then
    policies=/sys/kernel/security/chromiumos/inode_security_policies
    if [ -d "$policies" ]; then
      for policy in "$policies/allow_"*; do
        printf "$(realpath "$dest")" > "$policy"
      done
    fi
    umount /sys/kernel/security
  fi
fi

if [ "$command" ]; then
  exec chroot "$dest" su $user -c "$command"
elif [ "$#" -gt 0 ]; then
  exec chroot "$dest" su $user -c "$*"
else
  exec chroot "$dest" login -f $user
fi