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 correspond-
       ing 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. Pro-
       cesses 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  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  use-
       ful 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 Technol-
       ogy -- 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 original Standard can  be  obtained
       online at http://www.opengroup.org/unix/online.html .

IEEE
/The Open Group						       2003								  MESG(1P)