mmdebstrap/README.md

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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
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For the full documentation use:
pod2man ./mmdebstrap | man -l -
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The sales pitch in comparison to debootstrap
--------------------------------------------
Summary:
- more than one mirror possible
- security and updates mirror included for Debian stable chroots
- 3-6 times faster
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- 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
- variant installing only Essential:yes packages and dependencies
- temporary chroots by redirecting to /dev/null
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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 **3-6 times faster** than debootstrap. The
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timings were carried out on a laptop with an Intel Core i5-5200U.
| variant | mmdebstrap | debootstrap |
| ------- | ---------- | ------------ |
| minbase | 14.18 s | 51.47 s |
| buildd | 20.55 s | 59.38 s |
| - | 18.98 s | 127.18 s |
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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
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privileges to create a file (the chroot tarball) in one's home directory.
Thus, mmdebstrap provides multiple options to create a chroot tarball with the
right permissions **without superuser privileges**. Depending on what is
available, it uses either Linux user namespaces, fakechroot or proot.
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).
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When creating a chroot tarball with debootstrap, the temporary chroot directory
cannot be on a filesystem that has been mounted with nodev. In unprivileged
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mode, **mknod is never used**, which means that /tmp can be used as a temporary
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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
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is used to allow one to create a **foreign architecture chroot**.
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Limitations in comparison to debootstrap
----------------------------------------
Debootstrap supports creating a Debian chroot on non-Debian systems but
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mmdebstrap requires apt and is thus limited to Debian and derivatives.
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There is no `SCRIPT` argument.
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The following options, don't exist: `--second-stage`, `--exclude`,
`--resolve-deps`, `--force-check-gpg`, `--merged-usr` and `--no-merged-usr`.
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Tests
=====
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The script `coverage.sh` runs mmdebstrap in all kind of scenarios to execute
all code paths of the script. It verifies its output in each scenario and
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displays the results gathered with Devel::Cover. It also compares the output of
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mmdebstrap with debootstrap in several scenarios. To run the testsuite, run:
./make_mirror.sh
CMD=./mmdebstrap ./coverage.sh
To also generate perl Devel::Cover data, omit the `CMD` environment variable.
But that will also take a lot longer.
The `make_mirror.sh` script will be a no-op if nothing changed in Debian
unstable. You don't need to run `make_mirror.sh` before every invocation of
`coverage.sh`. When you make changes to `make_mirror.sh` and want to regenerate
the cache, run:
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touch -d yesterday shared/cache/debian/dists/unstable/Release
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The script `coverage.sh` does not need an active internet connection by
default. An online connection is only needed by the `make_mirror.sh` script
which fills a local cache with a few minimal Debian mirror copies.
By default, `coverage.sh` will skip running a single test which tries creating
a Ubuntu Focal chroot. To not skip that test, run `coverage.sh` with the
environment variable `ONLINE=yes`.
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Bugs
====
mmdebstrap has bugs. Report them here:
https://gitlab.mister-muffin.de/josch/mmdebstrap/issues
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Contributors
============
- Johannes Schauer (main author)
- Helmut Grohne
- Benjamin Drung
- Steve Dodd