sanity preserving chroot(1)

.sh

#!/bin/sh
# sane_chroot: sanity-preserving chroot(1)
set -e

finish() {
  prefix="$(realpath "$prefix")"
  if [ "x$(unlock)" != "x0" ]; then
    return 0
  else
    echo "Shutting down..."
  fi

  found=1
  while [ "x$found" = "x1" ]; do
    found=0
    for pid in $(ls -1 /proc); do
      link=$(readlink /proc/$pid/root || true)
      if [ "x$link" != "x" ]; then
        if [ "x${link:0:${#prefix}}" == "x$prefix" ]; then
          kill -9 $pid && echo "Killed process $pid" || true
          found=1
        fi
      fi
    done
  done

  umount -v -l "$prefix"/tmp/.X11-unix || true
  umount -v -l "$prefix"/dev || true
  umount -v -l "$prefix"/proc || true
  umount -v -l "$prefix"/sys || true
  rm -v "$lockfile"
}

lock() {
  local count
  count="$(cat "$lockfile" 2>/dev/null || echo 0)"
  echo $(expr "$count" + 1) > "$lockfile"
}

unlock() {
  local count
  count="$(cat "$lockfile")"
  echo $(expr "$count" - 1) | tee "$lockfile"
}

prefix="$1"
lockfile="$1/.chroot_lock"
shift
if [ "x" = "x$prefix" ] || ! [ -d "$prefix" ]; then
  printf "error: invalid chroot directory\n" >&2
  exit 1
fi

if [ $# = 0 ]; then
  printf "error: at least two arguments required" >&2
fi

trap finish EXIT
lock
mkdir -p "$prefix/tmp/.X11-unix"
ln -sfv /tmp/.X11-unix "$prefix"/tmp/.X11-unix || echo "warning: X11 will not be available" >&2
mount -v -t devtmpfs devtmpfs "$prefix"/dev || true
mount -v --rbind /dev/pts "$prefix"/dev/pts || true
mount -v -t proc proc "$prefix"/proc || true
mount -v -t sysfs sysfs "$prefix"/sys || true
chroot "$prefix" "$@" || exit 0