MESG(1)                    POSIX Programmer's Manual                   MESG(1)



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

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(1)
