This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

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.

Asynchronous Events

       Default.

Consequences Of Errors

       The invoking shell may terminate.

       Thefollowingsectionsareinformative.

Copyright

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 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 .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                        NEWGRP(1POSIX)

Description

       The  newgrp  utility  shall  create a new shell execution environment with a new real and effective group
       identification. Of the attributes listed in Section2.12, ShellExecutionEnvironment,  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 POSIX.1‐2017 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 POSIX.1‐2017 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 the first argument is '-', the results are unspecified.

       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.

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 POSIX.1‐2017, Section8.2, InternationalizationVariables 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.

Examples

       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.

Extended Description

       None.

Future Directions

       None.

Input Files

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

Name

       newgrp — change to a new group

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.

Options

       The newgrp utility shall conform to the Base Definitions volume of POSIX.1‐2017,  Section12.2,  UtilitySyntaxGuidelines, except for the unspecified usage of '-'.

       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.

Output Files

       None.

Prolog

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

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 POSIX.1‐2017, even given the existence of the  multiple
       group permissions feature in the System Interfaces volume of POSIX.1‐2017, 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  POSIX.1‐2017;  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.

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.

Stdin

       Not used.

Stdout

       Not used.

Synopsis

       newgrp [-l][group]