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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 .

Any typographical or formatting errors that appear

in this page are most likely

to have been introduced during the conversion of the source files to

man page format. To report such errors, see

https://www.kernel.org/doc/man-pages/reporting_bugs.html .