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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 dif-
       fer (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 util-
       ities to a terminal device. The terminal device affected shall be determined by searching for the first  termi-
       nal  in  the  sequence  of devices associated with standard input, standard output, and standard error, respec-
       tively.  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  prece-
              dence 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  exam-
              ple, 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 writ-
              ten (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 con-
       trolling 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 prob-
       lem involving the terminals of other users because appropriate privileges would be required to affect the  ter-
       minal 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.  Typi-
       cal  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()

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  Issue  6,  Copyright (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 Stan-
       dard,  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 .



IEEE/The Open Group                  2003                             MESG(1P)