Provided by: schroot_1.4.25-1_amd64 bug

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 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  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/lib/schroot/mount/my-session↵

       ·      Unmount any mounted filesystems

       ·      Remove /var/lib/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 © 2005-2011  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).