Provided by: schroot_1.6.10-4ubuntu0.1_amd64 
NAME
schroot - frequently asked questions
DESCRIPTION
This manual page covers various frequently asked questions about configuration and usage
of schroot.
CONFIGURATION
Why is schroot overwriting configuration files in the chroot?
By default, schroot copies over the system NSS databases (‘passwd’, ‘shadow’, ‘group’,
‘gshadow’, ‘services’, ‘protocols’, ‘networks’, and ‘hosts’, etc.) into the chroot. The
reason for this is that the chroot environment is not a completely separate system, and it
copying them over keeps them synchronised. However, this is not always desirable,
particularly if installing a package in the chroot creates system users and groups which
are not present on the host, since these will disappear next time the databases are copied
over.
The suggested workaround here is to disable the copying. This may be achieved by setting
the setup.nssdatabases key to be empty in schroot.conf. In prior schroot releases, this
was done by commenting out the NSSDATABASES file for the chroot
(/etc/schroot/default/config by default). The database list may also be customised by
editing the file containing the database list (/etc/schroot/default/nssdatabases by
default).
In the future, we will be working on a better scheme for keeping the host and chroot
databases in sync which can merge entries rather than overwriting the entire database,
which would preserve chroot-specific changes.
Should I use the plain or directory chroot type?
These two chroot types are basically equivalent, since they are both just directories in
the filesystem. plain is very simple and does not perform any setup tasks; the only
reason you would want to use it is if you're upgrading from a program such as dchroot(1)
or chroot(8) which don't do anything other than running a command or shell in a directory.
On the other hand, directory chroots do run setup scripts, which can mount additional
filesystems and do other setup tasks.
ADVANCED CONFIGURATION
What are snapshots and unions?
Some chroot types support cloning. This means when you start a session, you get a copy of
the chroot which lasts just for the lifetime of the session. This is useful when you want
a temporary clean copy of a system for a single task, which is then automatically deleted
when you're done with it. For example, the Debian package build dæmons run sbuild(1) to
build Debian packages, and this program uses schroot to create a clean build environment
for each package. Without snapshotting, the chroot would need to be reset to its initial
state at the end of each build to make it ready for the next one, and any debris left over
from package removals or earlier builds could interfere with the next build.
The most commonly-used snapshotting method is to use LVM snapshots (chroot type
‘lvm-snapshot’). In this case the chroot must exist on an LVM logical volume (LV);
snapshots of an LV may then be made with lvcreate(8) during chroot session setup.
However, these use up a lot of disk space. A newer method is to use Btrfs snapshots which
use up much less disk space (chroot type ‘btrfs-snapshot’), and may be more reliable than
LVM snapshots. Btrfs is however still experimental, but it is hoped that it will become
the recommended method as it matures.
Unions are an alternative to snapshots. In this situation, instead of creating a copy of
the chroot filesystem, we overlay a read-write temporary filesystem on top of the chroot
filesystem so that any modifications are stored in the overlay, leaving the original
chroot filesystem untouched. The Linux kernel has yet to integrate support for union
filesystems such as aufs and unionfs, so LVM snapshots are still the recommended method at
present.
USAGE
Can I run a dæmons in a chroot?
A common problem is trying to run a dæmon in a chroot, and finding that this doesn't work.
Typically, the dæmon is killed shortly after it starts up.
When schroot runs, it begins a session, runs the specified command or shell, waits for the
command or shell to exit, and then it ends the session. For a normal command or shell,
this works just fine. However, dæmons normally start up by running in the background and
detaching from the controlling terminal. They do this by forking twice and letting the
parent processes exit. Unfortunately, this means schroot detects that the program exited
(the dæmon is a orphaned grandchild of this process) and it then ends the session. Part
of ending the session is killing all processes running inside the chroot, which means the
dæmon is killed as the session ends.
In consequence, it's not possible to run a dæmon directly with schroot. You can however
do it if you create a session with --begin-session and then run the dæmon with
--run-session. It's your responsibility to end the session with --end-session when the
daemon has terminated or you no longer need it.
How do I manually cleaning up a broken session?
Occasionally, it may be necessary to manually clean up sessions. If something changes on
your system which causes the setup scripts to fail when ending a session, for example
removal of a needed file or directory, it may not be possible for schroot to clean
everything up automatically. For each of the session directories listed in the “Session
directories” section in schroot(1), any files with the name of the session ID need
deleting, and any directories with the name of the session ID need umounting (if there are
any filesystems mounted under it), and then also removing.
For example, to remove a session named my-session by hand:
• Remove the session configuration file
% rm /var/lib/schroot/session/my-session↵
• Check for mounted filesystems
% /usr/lib/x86_64-linux-gnu/schroot/schroot-listmounts -m \
/var/run/schroot/mount/my-session↵
• Unmount any mounted filesystems
• Remove /var/run/schroot/mount/my-session
• Repeat for the other directories such as /var/lib/schroot/union/underlay,
/var/lib/schroot/union/overlay and /var/lib/schroot/unpack
NOTE: Do not remove any directories without checking if there are any filesystems mounted
below them, since filesystems such as /home could still be bind mounted. Doing so could
cause irretrievable data loss!
ADVANCED USAGE
How do I use sessions?
In normal use, running a command might look like this:
% schroot -c squeeze -- command↵
which would run the command command in the squeeze chroot. While it's not apparent that a
session is being used here, schroot is actually doing the following steps:
• Creating a session using the squeeze chroot. This will be automatically given a
unique name, such as squeeze-57a69547-e014-4f5d-a98b-f4f35a005307, though you don't
usually need to know about this
• Setup scripts are run to create the session chroot and configure it for you
• The command command is run inside the session chroot
• Setup scripts are run to clean up the session chroot
• The session is deleted
Now, if you wanted to run more than one command, you could run a shell and run them
interactively, or you could put them into shell script and run that instead. But you
might want to do something in between, such as running arbitrary commands from a program
or script where you don't know which commands to run in advance. You might also want to
preseve the chroot state in between commands, where the normal automatic session creation
would reset the state in between each command. This is what sessions are for: once
created, the session is persistent and won't be automatically removed. With a session,
you can run as many commands as you like, but you need to create and delete the session by
hand since schroot can't know by itself when you're done with it unlike in the single
command case above. This is quite easy:
% schroot --begin-session -c squeeze↵
squeeze-57a69547-e014-4f5d-a98b-f4f35a005307
This created a new session based upon the squeeze chroot. The unique name for the
session, the session ID, was printed to standard output, so we could also save it as a
shell variable at the same time like so:
% SESSION=$(schroot --begin-session -c squeeze)↵
% echo $SESSION↵
squeeze-57a69547-e014-4f5d-a98b-f4f35a005307
Now we have created the session and got the session ID, we can run commands in it using
the session ID:
% schroot --run-session -c squeeze-57a69547-e014-4f5d-a98b-f4f35a005307 \
-- command1↵
or
% schroot --run-session -c "$SESSION" -- command1↵
and then as many more commands as we like
% schroot --run-session -c "$SESSION" -- command2↵
% schroot --run-session -c "$SESSION" -- command3↵
% schroot --run-session -c "$SESSION" -- command4↵
etc.
When we are done with the session, we can remove it with --end-session:
% schroot --end-session -c squeeze-57a69547-e014-4f5d-a98b-f4f35a005307↵
or
% schroot --end-session -c "$SESSION"↵
Since the automatically generated session names can be long and unwieldy, the
--session-name option allows you to provide you own name:
% schroot --begin-session -c squeeze --session-name my-name↵
my-name
CONTRIBUTING
Getting help and getting involved
The mailing list <buildd-tools-devel@lists.alioth.debian.org> is used for both user
support and development discussion. The list may be subscribed to from the project
website at https://alioth.debian.org/projects/buildd-tools/ or the Mailman list interface
at http://lists.alioth.debian.org/mailman/listinfo/buildd-tools-devel.
Reporting bugs
On Debian systems, bugs may be reported using the reportbug(1) tool, or alternatively by
mailing <submit@bugs.debian.org> (see http://bugs.debian.org for details on how to do
that).
Getting the latest sources
schroot is maintained in the git version control system. You can get the latest sources
from git://git.debian.org/git/buildd-tools/schroot.
% git clone git://git.debian.org/git/buildd-tools/schroot↵
The master branch containes the current development release. Stable releases are found on
branches, for example the 1.4 series of releases are on the schroot-1.4 branch.
AUTHORS
Roger Leigh.
COPYRIGHT
Copyright © 2005-2012 Roger Leigh <rleigh@debian.org>
schroot is free software: you can redistribute it and/or modify it under the terms of the
GNU General Public License as published by the Free Software Foundation, either version 3
of the License, or (at your option) any later version.
SEE ALSO
dchroot(1), schroot(1), sbuild(1), schroot-setup(5), schroot.conf(5).