Ubuntu Manpage: newgrp - change to a new group

NAME

       newgrp - change to a new group

SYNOPSIS

       newgrp [-l][group]

DESCRIPTION

       The  newgrp  utility  shall  create  a new shell execution environment with a new real and
       effective group identification. Of the attributes listed in Shell Execution Environment  ,
       the  new  shell  execution  environment  shall retain the working directory, file creation
       mask, and exported variables from the previous environment (that is,  open  files,  traps,
       unexported  variables,  alias  definitions, shell functions, and set options may be lost).
       All other aspects of the process environment that are preserved  by  the  exec  family  of
       functions  defined  in  the System Interfaces volume of IEEE Std 1003.1-2001 shall also be
       preserved by newgrp; whether other aspects are preserved is unspecified.

       A failure to assign the new group identifications (for example, for security or  password-
       related reasons) shall not prevent the new shell execution environment from being created.

       The newgrp utility shall affect the supplemental groups for the process as follows:

        * On systems where the effective group ID is normally in the supplementary group list (or
          whenever the old effective group ID actually is in the supplementary group list):

           * If the new effective group ID is also in the supplementary group list, newgrp  shall
             change the effective group ID.

           * If  the  new effective group ID is not in the supplementary group list, newgrp shall
             add the new effective group ID to the list, if there is room to add it.

        * On systems where the effective group ID is not normally in the supplementary group list
          (or whenever the old effective group ID is not in the supplementary group list):

           * If  the  new  effective  group  ID  is in the supplementary group list, newgrp shall
             delete it.

           * If the old effective group ID is not in the supplementary list, newgrp shall add  it
             if there is room.

       Note:  The  System  Interfaces volume of IEEE Std 1003.1-2001 does not specify whether the
              effective group ID of a process is included in its supplementary group list.

       With no operands, newgrp shall change the effective group back to the groups identified in
       the  user's  user entry, and shall set the list of supplementary groups to that set in the
       user's group database entries.

       If a password is required for the specified group, and the user is not listed as a  member
       of  that  group  in  the  group  database, the user shall be prompted to enter the correct
       password for that group. If the user is listed as a member  of  that  group,  no  password
       shall  be  requested.  If  no  password  is  required  for  the  specified  group,  it  is
       implementation-defined whether users not listed as members of that  group  can  change  to
       that  group.  Whether  or  not  a  password  is  required,  implementation-defined  system
       accounting or security mechanisms may impose additional  authorization  restrictions  that
       may  cause  newgrp  to  write  a diagnostic message and suppress the changing of the group
       identification.

OPTIONS

       The newgrp utility shall conform to the Base Definitions volume  of  IEEE Std 1003.1-2001,
       Section 12.2, Utility Syntax Guidelines.

       The following option shall be supported:

       -l     (The  letter  ell.)  Change  the  environment to what would be expected if the user
              actually logged in again.

OPERANDS

       The following operand shall be supported:

       group  A group name from the group database or a non-negative numeric group ID.  Specifies
              the  group ID to which the real and effective group IDs shall be set. If group is a
              non-negative numeric string and exists in the group database as a group  name  (see
              getgrnam()),  the numeric group ID associated with that group name shall be used as
              the group ID.

STDIN

       Not used.

INPUT FILES

       The file /dev/tty shall be used to read a single line of text for password checking,  when
       one is required.

ENVIRONMENT VARIABLES

       The following environment variables shall affect the execution of newgrp:

       LANG   Provide  a  default  value for the internationalization variables that are unset or
              null. (See the  Base  Definitions  volume  of  IEEE Std 1003.1-2001,  Section  8.2,
              Internationalization Variables for the precedence of internationalization variables
              used to determine the values of locale categories.)

       LC_ALL If set to  a  non-empty  string  value,  override  the  values  of  all  the  other
              internationalization variables.

       LC_CTYPE
              Determine  the  locale for the interpretation of sequences of bytes of text data as
              characters (for  example,  single-byte  as  opposed  to  multi-byte  characters  in
              arguments).

       LC_MESSAGES
              Determine  the  locale  that  should  be  used to affect the format and contents of
              diagnostic messages written to standard error.

       NLSPATH
              Determine the location of message catalogs for the processing of LC_MESSAGES .

ASYNCHRONOUS EVENTS

       Default.

STDOUT

       Not used.

STDERR

       The standard error shall be used for  diagnostic  messages  and  a  prompt  string  for  a
       password,  if  one is required. Diagnostic messages may be written in cases where the exit
       status is not available.  See the EXIT STATUS section.

OUTPUT FILES

       None.

EXTENDED DESCRIPTION

       None.

EXIT STATUS

       If newgrp succeeds in creating a new shell execution environment, whether or not the group
       identification  was  changed successfully, the exit status shall be the exit status of the
       shell.  Otherwise, the following exit value shall be returned:

       >0     An error occurred.

CONSEQUENCES OF ERRORS

       The invoking shell may terminate.

       The following sections are informative.

APPLICATION USAGE

There is no convenient way to enter a password into the group database. Use of group
passwords is not encouraged, because by their very nature they encourage poor security
practices. Group passwords may disappear in the future.

A common implementation of newgrp is that the current shell uses exec to overlay itself
with newgrp, which in turn overlays itself with a new shell after changing group. On some
implementations, however, this may not occur and newgrp may be invoked as a subprocess.

The newgrp command is intended only for use from an interactive terminal. It does not
offer a useful interface for the support of applications.

The exit status of newgrp is generally inapplicable. If newgrp is used in a script, in
most cases it successfully invokes a new shell and the rest of the original shell script
is bypassed when the new shell exits. Used interactively, newgrp displays diagnostic
messages to indicate problems. But usage such as:

newgrp foo
echo $?

is not useful because the new shell might not have access to any status newgrp may have
generated (and most historical systems do not provide this status). A zero status echoed
here does not necessarily indicate that the user has changed to the new group
successfully. Following newgrp with the id command provides a portable means of
determining whether the group change was successful or not.

EXAMPLES

       None.

RATIONALE

Most historical implementations use one of the exec functions to implement the behavior of
newgrp. Errors detected before the exec leave the environment unchanged, while errors
detected after the exec leave the user in a changed environment. While it would be useful
to have newgrp issue a diagnostic message to tell the user that the environment changed,
it would be inappropriate to require this change to some historical implementations.

The password mechanism is allowed in the group database, but how this would be implemented
is not specified.

The newgrp utility was retained in this volume of IEEE Std 1003.1-2001, even given the
existence of the multiple group permissions feature in the System Interfaces volume of
IEEE Std 1003.1-2001, for several reasons. First, in some implementations, the group
ownership of a newly created file is determined by the group of the directory in which the
file is created, as allowed by the System Interfaces volume of IEEE Std 1003.1-2001; on
other implementations, the group ownership of a newly created file is determined by the
effective group ID. On implementations of the latter type, newgrp allows files to be
created with a specific group ownership. Finally, many implementations use the real group
ID in accounting, and on such systems, newgrp allows the accounting identity of the user
to be changed.

FUTURE DIRECTIONS

       None.

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .