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 .