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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017, 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.1‐2017, Chapter 8,
Environment Variables
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 2017 MESG(1P)
Pages that refer to this page: talk(1p), who(1p), write(1p)
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