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 terminal device.

       n         Deny permission to other users to send messages to the terminal 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 variables that are unset or null. (See the
                 Base  Definitions  volume  of POSIX.1‐2017, 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 (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 differences 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.1‐2017, Chapter 8, Environment Variables

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                                          MESG(1POSIX)