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 .