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MESG(1P) POSIX Programmer's Manual MESG(1P)
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.
mesg — permit or deny messages
mesg [y|n]
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.
None.
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.
Not used.
None.
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 POSIX.1‐2008, 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.
Default.
If no operand is specified, mesg shall display the current terminal
state in an unspecified format.
The standard error shall be used only for diagnostic messages.
None.
None.
The following exit values shall be returned:
0 Receiving messages is allowed.
1 Receiving messages is not allowed.
>1 An error occurred.
Default.
The following sections are informative.
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.
None.
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.
None.
talk(1p), write(1p)
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables
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 .
IEEE/The Open Group 2013 MESG(1P)
Pages that refer to this page: talk(1p), who(1p), write(1p)
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