This manual page covers various frequently asked questions about configuration
and usage of schroot.
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 by removing or
commenting out the databases in the NSSDATABASES file in the script-config file
for the chroot. These are /etc/schroot/default/nssdatabases and
/etc/schroot/default/config 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
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.
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
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
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.
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
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
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/schroot/schroot-listmounts -m /var/lib/schroot/mount/my-session␍
Unmount any mounted filesystems
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!
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␍
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 $SESSION␍
Now we have created the session and got the session ID, we can run commands in
it using the session ID:
% schroot --run-session -c squeeze-57a69547-e014-4f5d-a98b-f4f35a005307 -- command1␍
% schroot --run-session -c "$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␍
When we are done with the session, we can remove it with --end-session:
% schroot --end-session -c squeeze-57a69547-e014-4f5d-a98b-f4f35a005307␍
% 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 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