chroot.sh, for a typical modern GNU/Linux system

username=$1
newroot=$2
if [ -z "$newroot" ]; then
 echo Usage: $0 USERNAME NEW_ROOT >&2
 exit 1
fi
echo Will chroot to $newroot then become user $username >&2
if [ "$newroot" != "/" -a -x $newroot/bin/bash ]; then
 cd $newroot
 sudo mount -t proc proc proc/
 sudo mount -t sysfs sys sys/
 sudo mount -o bind /dev dev/
 sudo mount -t devpts devpts dev/pts
 sudo mount -o bind /media/sda3 media/sda3
 sudo chroot . /bin/bash -c "su - $username"
 # upon exit from chroot, undo the mounts
 sudo umount media/sda3
 sudo umount dev/pts
 sudo umount dev
 sudo umount sys
 sudo umount proc
else
 echo Unexpected root directory $newroot, nothing done >&2
fi