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.TH MESG "1P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.

.SH NAME
mesg
\(em permit or deny messages
.SH SYNOPSIS
.LP
.nf
mesg \fB[\fRy|n\fB]\fR
.fi
.SH DESCRIPTION
The
.IR mesg
utility shall control whether other users are allowed to send messages
via
.IR write ,
.IR 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,
.IR 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.
.SH OPTIONS
None.
.SH OPERANDS
The following operands shall be supported in the POSIX locale:
.IP "\fIy\fR" 10
Grant permission to other users to send messages to the terminal
device.
.IP "\fIn\fR" 10
Deny permission to other users to send messages to the terminal
device.
.SH STDIN
Not used.
.SH "INPUT FILES"
None.
.SH "ENVIRONMENT VARIABLES"
The following environment variables shall affect the execution of
.IR mesg :
.IP "\fILANG\fP" 10
Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of POSIX.1\(hy2008,
.IR "Section 8.2" ", " "Internationalization Variables"
for the precedence of internationalization variables used to determine
the values of locale categories.)
.IP "\fILC_ALL\fP" 10
If set to a non-empty string value, override the values of all the
other internationalization variables.
.IP "\fILC_CTYPE\fP" 10
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).
.IP "\fILC_MESSAGES\fP" 10
.br
Determine the locale that should be used to affect the format and
contents of diagnostic messages written (by
.IR mesg )
to standard error.
.IP "\fINLSPATH\fP" 10
Determine the location of message catalogs for the processing of
.IR LC_MESSAGES .
.SH "ASYNCHRONOUS EVENTS"
Default.
.SH STDOUT
If no operand is specified,
.IR mesg
shall display the current terminal state in an unspecified format.
.SH STDERR
The standard error shall be used only for diagnostic messages.
.SH "OUTPUT FILES"
None.
.SH "EXTENDED DESCRIPTION"
None.
.SH "EXIT STATUS"
The following exit values shall be returned:
.IP "\00" 6
Receiving messages is allowed.
.IP "\01" 6
Receiving messages is not allowed.
.IP >1 6
An error occurred.
.SH "CONSEQUENCES OF ERRORS"
Default.
.LP
.IR "The following sections are informative."
.SH "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
.IR mesg
has successfully completed. These actions may include, but are not
limited to: another invocation of the
.IR mesg
utility, login procedures; invocation of the
.IR stty
utility, invocation of the
.IR chmod
utility or
\fIchmod\fR()
function, and so on.
.SH EXAMPLES
None.
.SH RATIONALE
The terminal changed by
.IR 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.
.P
The method of checking each of the first three file descriptors in
sequence until a terminal is found was adopted from System V.
.P
The file
.BR /dev/tty
is not specified for the terminal device because it was thought to be
too restrictive. Typical environment changes for the
.IR n
operand are that write permissions are removed for
.IR others
and
.IR 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.
.P
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.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fItalk\fR\^",
.IR "\fIwrite\fR\^"
.P
The Base Definitions volume of POSIX.1\(hy2008,
.IR "Chapter 8" ", " "Environment Variables"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

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