mmdebstrap
==========

An alternative to debootstrap which uses apt internally and is thus able to use
more than one mirror and resolve more complex dependencies.

Usage
-----

Use like debootstrap:

    sudo mmdebstrap unstable ./unstable-chroot

Without superuser privileges:

    mmdebstrap unstable unstable-chroot.tar

With complex apt options:

    cat /etc/apt/sources.list | mmdebstrap > unstable-chroot.tar

The sales pitch in comparison to debootstrap
--------------------------------------------

Summary:

 - more than one mirror possible
 - security and updates mirror included for Debian stable chroots
 - 2-3 times faster
 - chroot with apt in 11 seconds
 - gzipped tarball with apt is 27M small
 - bit-by-bit reproducible output
 - unprivileged operation using Linux user namespaces, fakechroot or proot
 - can operate on filesystems mounted with nodev
 - foreign architecture chroots with qemu-user

The author believes that a chroot of a Debian stable release should include the
latest packages including security fixes by default. This has been a wontfix
with debootstrap since 2009 (See #543819 and #762222). Since mmdebstrap uses
apt internally, support for multiple mirrors comes for free and stable or
oldstable **chroots will include security and updates mirrors**.

A side-effect of using apt is being **2-3 times faster** than debootstrap. The
timings were carried out on a laptop with an Intel Core i5-5200U.

| variant | mmdebstrap | debootstrap  |
| ------- | ---------- | ------------ |
| minbase | 25.25 s    | 51.47 s      |
| buildd  | 30.99 s    | 59.38 s      |
| -       | 29.85 s    | 127.18 s     |

Apt considers itself an `Essential: yes` package. This feature allows one to
create a chroot containing just the `Essential: yes` packages and apt (and
their hard dependencies) in **just 11 seconds**.

If desired, a most minimal chroot with just the `Essential: yes` packages and
their hard dependencies can be created with a gzipped tarball size of just 34M.
By using dpkg's `--path-exclude` option to exclude documentation, even smaller
gzipped tarballs of 21M in size are possible. If apt is included, the result is
a **gzipped tarball of only 27M**.

These small sizes are also achieved because apt caches and other cruft is
stripped from the chroot. This also makes the result **bit-by-bit
reproducible** if the `$SOURCE_DATE_EPOCH` environment variable is set.

The author believes, that it should not be necessary to have superuser
privileges to create a file (the chroot tarball) in one's home directory. If
mmdebstrap is run by an unprivileged user, either Linux user namespaces,
fakechroot or proot are used to create a chroot tarball. Debootstrap supports
fakechroot but will not create a tarball with the right permissions by itself.
Support for Linux user namespaces and proot is missing (see bugs #829134 and
#698347, respectively).

When creating a chroot tarball with debootstrap, the temporary chroot directory
cannot be on a filesystem that has been mounted with nodev. In unprivileged
mode, mknod is never used, which means that /tmp can be used as a temporary
directory location even if if it's mounted with nodev as a security measure.

If the chroot architecture cannot be executed by the current machine, qemu-user
is used to allow one to create a foreign architecture chroot.

Limitations in comparison to debootstrap
----------------------------------------

Debootstrap supports creating a Debian chroot on non-Debian systems but
mmdebstrap requires apt.

There is no `SCRIPT` argument.

There is no `--second-stage` option.

Tests
=====

The script `test.sh` compares the output of mmdebstrap with debootstrap in
several scenarios. Since debootstrap needs superuser privileges, `test.sh`
needs `sudo` to run.

Bugs
====

mmdebstrap has bugs. Report them here:
https://gitlab.mister-muffin.de/josch/mmdebstrap/issues