schroot allows the user to run a command or a login shell in a chroot
environment. If no command is specified, a login shell will be started in the
user's current working directory inside the chroot.
The command is a program, plus as many optional arguments as required. Each
argument may be separately quoted.
The directory the command or login shell is run in depends upon the context.
See --directory option below for a complete description.
All chroot usage will be logged in the system logs. Under some circumstances,
the user may be required to authenticate themselves; see the section
If no chroot is specified, the chroot name or alias 'default' will be
used as a fallback. This is equivalent to "--chroot=default".
There is often a need to run programs in a virtualised environment rather than
on the host system directly. Unlike other virtualisation systems such as
kvm or Xen, schroot does not virtualise the entire system; it only
virtualises the filesystem, and some parts of the filesystem may still be
shared with the host. It is therefore fast, lightweight and flexible.
However, it does not virtualise other aspects of the system, such as shared
memory, networking, devices etc., and so may be less secure than other systems,
depending upon its intended use. Some examples of existing uses for schroot
Running an untrusted program in a sandbox, so that it can't interfere with
files on the host system; this may also be used to limit the damage a
compromised service can inflict upon the host
Using a defined or clean environment, to guarantee the
reproducibility and integrity of a given task
Using different versions of an operating system, or even different operating
systems altogether, e.g. different GNU/Linux distributions
Running 32-bit programs using a 32-bit chroot on a 64-bit host system
Automatic building of Debian packages using
which builds each package in a pristine chroot snapshot when using LVM
snapshots or unions
Supporting multiple system images in a cluster setup, where modifying the base
image is time-consuming and/or supporting all the required configurations
needed by users is difficult: different chroots can support all the different
configurations required, and cluster users may be given access to the chroots
they need (which can include root access for trusted users to maintain their
A chroot may be used directly as root by running
but normal users are not able to use this command. schroot allows access
to chroots for normal users using the same mechanism, but with permissions
checking and allowing additional automated setup of the chroot environment,
such as mounting additional filesystems and other configuration tasks. This
automated setup is done through the action of setup scripts which may be
customised and extended to perform any actions required.
schroot accepts the following options:
Show help summary.
Print version information.
List all available chroots.
Print detailed information about the specified chroots.
Print configuration of the specified chroots. This is useful for testing that
the configuration in use is the same as the configuration file. Any comments
in the original file will be missing.
Print location (path) of the specified chroots. Note that chroot types which
can only be used within a session will not have a location until they are
Print only essential messages.
Print all messages.
Specify a chroot or active session to use. This option may be used multiple
times to specify more than one chroot, in which case its effect is similar to
--all. The chroot name may be prefixed with a namespace; see the
section "Chroot Namespaces", below.
Select all chroots, source chroots and active sessions. When a command has
been specified, the command will be run in all chroots, source chroots and
active sessions. If --info has been used, display information about
all chroots. This option does not make sense to use with a login shell (when
no command has been specified). This option is equivalent to
"--all-chroots --all-source-chroots --all-sessions".
Select all chroots. Identical to --all, except that source chroots and
active sessions are not considered.
Select all active sessions. Identical to --all, except that chroots
and source chroots are not considered.
Select all source chroots. Identical to --all, except that chroots
and sessions are not considered.
Change to directory inside the chroot before running the command or login
shell. If directory is not available, schroot will exit with an error
The default behaviour is as follows (all directory paths are inside the
chroot). A login shell is run in the current working directory. If this is
not available, it will try $HOME (when --preserve-environment is
used), then the user's home directory, and / inside the chroot in turn.
A command is always run in the current working directory inside the chroot. If
none of the directories are available, schroot will exit with an error status.
Run as a different user. The default is to run as the current user. If
required, the user may be required to authenticate themselves with a password.
For further information, see the section "Authentication", below.
Preserve the user's environment inside the chroot environment. The default is
to use a clean environment; this option copies the entire user environment and
sets it in the session. The environment variables allowed are subject to
certain restrictions; see the section "Environment", below.
Begin, run and end a session automatically. This is the default action, so
does not require specifying in normal operation.
Begin a session. A unique session identifier (session ID) is returned on
standard output. The session ID is required to use the other session options.
Note that the session identifier may be specified with the
Recover an existing session. If an existing session has become unavailable,
for example becoming unmounted due to a reboot, this option will make the
session available for use again, for example by remounting it. The session ID
is specified with the --chroot option.
Run an existing session. The session ID is specified with the --chroot
End an existing session. The session ID is specified with the --chroot
Name a session. The specified session-name replaces the default session
name containing an automatically-generated session ID. The session name must
not contain a namespace qualifier, since sessions are always created within the
'session:' namespace. The session name is also subject to the chroot
naming restrictions documented in
Force a session operation, even if it would otherwise fail. This may be used
to forcibly end a session, even if it has active users. This does not
guarantee that the session will be ended cleanly; filesystems may not be
unmounted, for example.
End of options. Used to indicate the end of the schroot options; any following
options will be passed to the command being run, rather than to schroot.
If the user is not an allowed user, or a member of the allowed groups (or if
changing to root, the allowed root users or allowed root groups) for the
specified chroot(s), permission will be immediately denied. If switching
users, and the user running the command has access, the user will be required
to authenticate themselves using the credentials of the user being switched to.
On systems supporting Pluggable Authentication Modules (PAM), schroot will use
PAM for authentication and authorisation of users. If and when required,
schroot will prompt for a password. If PAM is not available, all
authentication will automatically fail (user switching is not supported
Note that when PAM is in use, the root user is not granted any special
privileges by default in the program. However, the default PAM configuration
permits root to log in without a password (pam_rootok.so), but this may
be disabled to prevent root from accessing any chroots except if specifically
permitted. In such a situation, root must be added to the allowed users or
groups as for any other user or group. If PAM is not available, the root user
will be permitted to access all chroots, even when not explicitly granted
There are three different types of chroot: regular chroots, source chroots and
session chroots. These different types of chroot are separated into different
namespaces. A namespace is a prefix to a chroot name. Currently there
are three namespaces: 'chroot:', 'source:' and
'session:'. Use --list --all to list all available chroots
in all namespaces. Because ':' is used as the separator between
namespace and chroot names, it is not permitted to use this character in chroot
Depending upon the action you request schroot to take, it may look for the
chroot in one of the three namespaces, or a particular namespace may be
specified. For example, a chroot named "sid" is actually named
"chroot:sid" if the namespace is included, but the namespace may be
omitted for most actions.
Some chroot types, for example LVM snapshots and Btrfs snapshots, provide
session-managed copy-on-write snapshots of the chroot. These also provide a
source chroot to allow easy access to the filesystem used as a source for
snapshotting. These are regular chroots as well, just with the snapshotting
disabled. For a chroot named "sid-snapshot" (i.e. with a fully
qualified name of "chroot:sid-snapshot"), there will also be a
corresponding source chroot named "source:sid-snapshot". Earlier
versions of schroot provided source chroots with a '-source' suffix.
These are also provided for compatibility. In this example, this would be
called "chroot:sid-snapshot-source". These compatibility names will
be dropped in a future version, so programs and scripts should switch to using
the namespace-qualified names rather than the old suffix.
All sessions created with --begin-session are placed within the
'session:' namespace. A session named with --session-name may
have any name, even the same name as the chroot it was created from, providing
that it is unique within this namespace. This was not permitted in previous
versions of schroot which did not have namespaces.
Actions and default namespaces
All actions use 'chroot:' as the default namespace, with some session
actions being the exception. --run-session,
--recover-session and --end-session use 'session:' as
the default namespace instead, since these actions work on session chroots.
The upshot is that the namespace is usually never required except when you need
to work with a chroot in a namespace other than the default, such as when using
a source chroot. To make chroot selection unambiguous, it is always possible
to use the full name including the namespace, even when not strictly required.
Use -- to allow options beginning with '-' or '--'
in the command to run in the chroot. This prevents them being interpreted as
options for schroot itself. Note that the top line was echoed to standard
error, and the remaining lines to standard output. This is intentional, so
that program output from commands run in the chroot may be piped and redirected
as required; the data will be the same as if the command was run directly on
the host system.
If the user 'rleigh' was in root-users in
/etc/schroot/schroot.conf, or one of the groups he belonged to was in
root-groups, they would be granted root access without
authentication, but the PAM authorisation step is still applied.
A chroot may be needed to run more than one command. In particular, where the
chroot is created on the fly from an LVM LV or a file on disc, there is a need
to make the chroot persistent while a given task (or set of tasks) is
performed. Sessions exist for this purpose. For simple chroot types such as
'plain' and 'directory', sessions may be created but are not
Let's start by looking at a session-capable chroot:
% schroot -i -c sid-snap␍
--------- Chroot ---------
Description Debian sid snapshot
Users maks rleigh
Root Groups root sbuild
Environment Filter ^(BASH_ENV|CDPATH|ENV|HOSTALIASES|I
Run Setup Scripts true
Script Configuration script-defaults
Session Managed true
Mount Options -o atime,async,user_xattr
Source Groups root rleigh
Source Root Users
Source Root Groups root rleigh
LVM Snapshot Options --size 2G -c 128
Note that the Session Managed option is set to 'true'. This is a
requirement in order to use session management, and is supported by most chroot
types. Next, we will create a new session:
If something is not working, and it's not clear from the error messages what is
wrong, try using the --debug=level option to turn on debugging
messages. This gives a great deal more information. Valid debug levels are
'none', and 'notice', 'info', 'warning' and
'critical' in order of increasing severity. The lower the severity
level, the more output.
On the mips and mipsel architectures, Linux kernels up to and
including at least version 2.6.17 have broken
support, which results in a failure to set the personality. This will be seen
as an "Operation not permitted" (EPERM) error. To work around this
problem, set personality to 'undefined', or upgrade to a more
By default, the environment is not preserved, and the following environment
variables are defined: HOME, LOGNAME, PATH, SHELL, TERM (preserved if already
defined), and USER. The environment variables SCHROOT_COMMAND, SCHROOT_USER,
SCHROOT_GROUP, SCHROOT_UID and SCHROOT_GID are set inside the chroot specifying
the command being run, the user name, group name, user ID and group ID,
The following, potentially dangerous, environment variables are removed for
safety by default: BASH_ENV, CDPATH, ENV, HOSTALIASES, IFS, KRB5_CONFIG,
KRBCONFDIR, KRBTKFILE, KRB_CONF, LD_.*, LOCALDOMAIN, NLSPATH, PATH_LOCALE,
RES_OPTIONS, TERMINFO, TERMINFO_DIRS, and TERMPATH. If desired, the
environment-filter configuration key will allow the exclusion list to
the modified; see
for further details.
The system-wide chroot definition file. This file must be owned by the root
user, and not be writable by other.
Additional chroot definitions may be placed in files under this directory.
They are treated in exactly that same manner as /etc/schroot/schroot.conf. Each
file may contain one or more chroot definitions. Note that the files in this
directory follow the same naming rules as
when run with the --lsbsysinit option.
The system-wide chroot setup script directories. See
Directory containing helper programs used by setup scripts.
Each directory contains a directory or file with the name of each session. Not
all chroot types make use of all the following directories.
Directory containing the session configuration for each active session.
Directory used to mount the filesystems used by each active session.
Directory used for filesystem union source (underlay).
Directory used for filesystem union writeable overlay.
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