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.
Since pivot_root(8) simply calls pivot_root(2), we refer to
the man page of the latter for further details.
Note that, depending on the implementation of pivot_root, root and
cwd of the caller may or may not change. The following is a sequence for
invoking pivot_root that works in either case, assuming that
pivot_root and chroot are in the current PATH:
pivot_root . put_old
exec chroot . command
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.
Note that exec chroot changes the running executable, which is
necessary if the old root directory should be unmounted afterwards.
Also note that standard input, output, and error may still point to a
device on the old root file system, keeping it busy. They can easily be
changed when invoking chroot (see below; note the absence of
leading slashes to make it work whether pivot_root has changed the
shells root or not).
Change the root file system to /dev/hda1 from an interactive shell:
mount /dev/hda1 /new-root
pivot_root . old-root
exec chroot . sh <dev/console >dev/console 2>&1
Mount the new root file system over NFS from 10.0.0.1:/my_root and run
ifconfig lo 127.0.0.1 up # for portmap
# configure Ethernet or such
portmap # for lockd (implicitly started by mount)
mount -o ro 10.0.0.1:/my_root /mnt
killall portmap # portmap keeps old root busy
pivot_root . old_root
exec chroot . sh -c umount /old_root; exec /sbin/init \
<dev/console >dev/console 2>&1