Ubuntu Manpage: really - gain privilege or run commands a different user

name
synopsis
description
options
security considerations
environment
author
availability
see also

NAME

       really - gain privilege or run commands a different user

SYNOPSIS

       really [options] [command args... ]

DESCRIPTION

       really  checks whether the caller is allowed, and if it is it changes its uids and gids (and perhaps root
       directory) according to the command line options and executes the specified command.

       If no options are specified, the uid will be set to 0 and the  gids  and  root  directory  will  be  left
       unchanged.

       If no command is specified, really will run $SHELL -i.

       A caller is allowed if it has write access to /etc/inittab and is also member of the group root.  This is
       most easily achieved by making  inittab  group-writeable  by  some  suitable  group  containing  all  the
       appropriate users, and making /etc/inittab group-owned by that group and group-writeable.  The root group
       is perhaps a good choice if it isn't being used for anything else.

OPTIONS

-u username | --user username
Sets the uid, gid, and supplementary group list, according to username's entry in the password and
group databases.

-i username | --useronly username
Sets only the uid according to username's entry in the password database.

-I uid | --uidonly uid
Sets the uid to the numeric value uid (which need not correspond to any existing user in the
password database).

-g groupname | --group groupname
groupname is looked up in the group database and its gid is appended to the process's
supplementary groups list. If this is the first gid specified it will also be set as the primary
gid.

-G gid | --gid gid
gid is appended to the process's supplementary groups list. (gid need not correspond to any
existing group in the group database.) If this is the first gid specified it will also be set as
the primary gid.

-z | --groupsclear
Clears the process's supplementary groups list. When using this option you must also specify -g
or -G. The process's groups will then be exactly those specified. The relative position of -z in
the argument list is not relevant.

-R root-dir | --chroot root-dir
The program will have its root directory set to root-dir.

Do not use this option unless you know what you are doing: Unlike chroot(8), the current working
directory will remain unchanged. This means that if the current directory isn't underneath the
specified new root, the program will still be able to access files outside the new root by using
relative pathnames. If this isn't what you want, please use the chroot utility instead.

-- Indicates the end of the options. The next argument (if present) will be interpreted as the
command name, even if it starts with a hyphen.

SECURITY CONSIDERATIONS

       really is designed so that installing it setuid root is extremely unlikely to compromise the security  of
       any  system.  It will check using access(2) whether the real user is allowed to write to /etc/inittab and
       if this check fails really will exit without even attempting to parse its command line.

       really is not designed to be resistant to malicious command  line  arguments.   Do  not  allow  untrusted
       processes  to  pass  options to really, or to specify the command to be run.  Whether it is safe to allow
       relatively untrusted processes to pass options to the command which is to be run depends on the behaviour
       of that command and its security status.

       Attempting  to  use  really  to  drop  privilege is dangerous unless the calling environment is very well
       understood.  There are many inherited process properties and resources which might be used by the  callee
       to  escalate  its  privilege  to  that of the (root-equivalent) caller.  For this function, it is usually
       better to use userv if possible.

ENVIRONMENT

       really does not manipulate the environment at all.  The calling  program  is  run  in  exactly  the  same
       environment  as the caller passes to really.  In particular, really will not add sbin directories to PATH
       so really-enabled accounts will usually need to have these directories on their configured PATH to  start
       with.

       SHELL is used to find the default shell to use in interactive mode (ie, when no command is specified).

AUTHOR

       This version of really was written by Ian Jackson <ian@chiark.greenend.org.uk>.

       It and this manpage are Copyright (C) 1992-5,2004,2013 Ian Jackson <ian@chiark.greenend.org.uk>.

       really  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, or (at  your  option)  any
       later version.

       really  is  distributed  in  the  hope that it will be useful, but WITHOUT ANY WARRANTY; without even the
       implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.   See  the  GNU  General  Public
       License for more details.

       You  should  have received a copy of the GNU General Public License along with this file; if not, consult
       the Free Software Foundation's website at www.fsf.org, or the GNU Project website at www.gnu.org.

AVAILABILITY

       really is currently part of chiark-utils and is available for download from ftp.chiark.greenend.org.uk in
       /users/ian/chiark-utils/, in source and pre-compiled binary form, and also from Ian Jackson's cvsweb.