mesg(1p) — Linux manual page

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
                 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(1p), write(1p)

       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(1P)

Pages that refer to this page: talk(1p), who(1p), write(1p)

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