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  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 , 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
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                                   2003                                               MESG(P)