debootstrap is a tool which will install a Debian base system into a subdirectory of another, already installed system. It doesn't require an installation CD, just access to a Debian repository. It can also be installed and run from another operating system, so, for instance, you can use debootstrap to install Debian onto an unused partition from a running Gentoo system. It can also be used to create a rootfs for a machine of a different architecture. This is "cross-debootstrapping". There is also a largely equivalent version written in C: cdebootstrap, which is smaller, and thus used in debian-installer.
Debootstrap can only use one repository for its packages. If you need to merge packages from different repositories (the way apt does) to make a rootfs, or you need to automatically customise the rootfs, then use Multistrap.
Since DebianTesting and DebianUnstable change rapidly, if you are going to attempt to install DebianTesting or DebianUnstable, it is recommended that you download the latest version of debootstrap from the package page and use that. Normally it would not be wise to install a package from unstable onto stable, but in this case it's safe, since debootstrap is really just a shell script and its dependencies are minimal.
To locate a mirror near you, see the list of Debian worldwide mirror sites.
To setup a Jessie system:
main # cd / main # mkdir /jessie-chroot main # debootstrap jessie ./jessie-chroot http://httpredir.debian.org/debian/
Full process to create a complete Debian installation of sid (unstable):
main # export MY_CHROOT=/sid-root main # cd / main # mkdir $MY_CHROOT main # debootstrap --arch i386 sid $MY_CHROOT http://httpredir.debian.org/debian/ [ ... watch it download the whole system ] main # echo "proc $MY_CHROOT/proc proc defaults 0 0" >> /etc/fstab main # mount proc $MY_CHROOT/proc -t proc main # echo "sysfs $MY_CHROOT/sys sysfs defaults 0 0" >> /etc/fstab main # mount sysfs $MY_CHROOT/sys -t sysfs main # cp /etc/hosts $MY_CHROOT/etc/hosts main # cp /proc/mounts $MY_CHROOT/etc/mtab main # chroot $MY_CHROOT /bin/bash chroot # dselect [ you may use aptitude, install mc and vim ... ] main # echo "8:23:respawn:/usr/sbin/chroot $MY_CHROOT " \ "/sbin/getty 38400 tty8" >> /etc/inittab [ define a login tty that will use this system ] [ i.e. create tty8 with `mknod tty8 c 4 8' and run `passwd' ] main # init q [ reload init ]
C implementation of Debootstrap
cdebootstrap is used to create a Debian base system from scratch.