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 (con-
       sult 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 IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence  of  international-
              ization 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 ter-
       minals 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 environ-
       ment  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()

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003  Edition,  Standard  for
       Information  Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
       right (C) 2001-2003 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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                      MESG(1P)