switch_root vs pivot_root vs chroot【转】

1. pivot_root can/should be used together with chroot

       pivot_root new_root put_old

       pivot_root moves the root file system of the current process to the
       directory put_old and makes new_root the new root file system.

       cd new_root
       pivot_root . old-root
       exec chroot . command

       umount /old-root

        Note that chroot must be available under the old root and under the new root, because pivot_root may or may not have implicitly changed the root directory of the shell.

2. switch_root newroot init [arg...]

       switch_root moves already mounted /proc, /dev and /sys to newroot and
       makes newroot the new root filesystem and starts init process. switch_root is typically used with initramfs.

       WARNING: switch_root removes recursively all files and directories on
       the current root filesystem.

   The following shell script fragment demonstrates how to use switch_root:

  # First, find and mount the new filesystem.
  mkdir /newroot
  mount /dev/whatever /newroot

  # Unmount everything else you've attached to rootfs.  (Moving the filesystems
  # into newroot is something useful to do with them.)

  mount --move /sys /newroot/sys
  mount --move /proc /newroot/proc
  mount --move /dev /newroot/dev

  # Now switch to the new filesystem, and run /sbin/init out of it.  Don't
  # forget the "exec" here, because you want the new init program to inherit
  # PID 1.

  exec switch_root /newroot /sbin/init