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MESG(1P)                   POSIX Programmer's Manual                  MESG(1P)
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.
NAME
       mesg -- permit or deny messages
SYNOPSIS
       mesg [y|n]
DESCRIPTION
       The mesg utility shall control whether other users are allowed to  send
       messages  via write, talk, or other utilities to a terminal device. The
       terminal device affected shall be determined by searching for the first
       terminal  in  the  sequence  of devices associated with standard input,
       standard output, and standard error, respectively. With  no  arguments,
       mesg shall report the current state without changing it. Processes with
       appropriate privileges may be able to send  messages  to  the  terminal
       independent of the current state.
OPTIONS
       None.
OPERANDS
       The following operands shall be supported in the POSIX locale:
       y         Grant  permission to other users to send messages to the ter-
                 minal device.
       n         Deny permission to other users to send messages to the termi-
                 nal device.
STDIN
       Not used.
INPUT FILES
       None.
ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of mesg:
       LANG      Provide  a  default  value for the internationalization vari-
                 ables that are unset or null. (See the Base Definitions  vol-
                 ume  of POSIX.1-2008, Section 8.2, Internationalization Vari-
                 ables 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 (by mesg) to
                 standard error.
       NLSPATH   Determine the location of message catalogs for the processing
                 of LC_MESSAGES.
ASYNCHRONOUS EVENTS
       Default.
STDOUT
       If  no  operand  is  specified, mesg shall display the current terminal
       state in an unspecified format.
STDERR
       The standard error shall be used only for diagnostic messages.
OUTPUT FILES
       None.
EXTENDED DESCRIPTION
       None.
EXIT STATUS
       The following exit values shall be returned:
        0    Receiving messages is allowed.
        1    Receiving messages is not allowed.
       >1    An error occurred.
CONSEQUENCES OF ERRORS
       Default.
       The following sections are informative.
APPLICATION USAGE
       The mechanism by which the message status of the terminal is changed is
       unspecified. Therefore, unspecified actions may cause the status of the
       terminal to change after mesg has successfully completed. These actions
       may  include,  but  are  not limited to: another invocation of the mesg
       utility, login procedures; invocation of the stty  utility,  invocation
       of the chmod utility or chmod() function, and so on.
EXAMPLES
       None.
RATIONALE
       The  terminal  changed  by  mesg  is  that associated with the standard
       input, output, or error, rather than the controlling terminal  for  the
       session.  This is because users logged in more than once should be able
       to change any of their login terminals without having to stop  the  job
       running  in  those  sessions.  This is not a security problem involving
       the terminals of other users because appropriate  privileges  would  be
       required to affect the terminal of another user.
       The  method  of  checking  each  of the first three file descriptors in
       sequence until a terminal is found was adopted from System V.
       The file /dev/tty is not specified for the terminal device  because  it
       was  thought to be too restrictive. Typical environment changes for the
       n operand are that write permissions are removed for others  and  group
       from  the  appropriate  device.  It  was  decided  to  leave the actual
       description of what is done as unspecified because of potential differ-
       ences between implementations.
       The  format  for  standard output is unspecified because of differences
       between historical implementations. This output is generally not useful
       to  shell  scripts  (they can use the exit status), so exact parsing of
       the output is unnecessary.
FUTURE DIRECTIONS
       None.
SEE ALSO
       talk, write
       The Base Definitions volume of  POSIX.1-2008,  Chapter  8,  Environment
       Variables
COPYRIGHT
       Portions  of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal  and  Electronics  Engineers,  Inc  and  The  Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum  1  applied.)  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.unix.org/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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                             MESG(1P)