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

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 privi-
       leges 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, Inter-
	      nationalization 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 interna-
	      tionalization 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 argu-
	      ments).

       LC_MESSAGES
	      Determine the locale that should be used to affect the format and contents of diag-
	      nostic 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 unspeci-
       fied 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, invoca-
       tion 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 termi-
       nal 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()

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, 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
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					  MESG(P)
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