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.
The following environment variables shall affect the execution of mesg:
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.)
If set to a non-empty string value, override the values of
all the other internationalization variables.
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).
Determine the locale that should be used to affect the
format and contents of diagnostic messages written (by mesg) to
Determine the location of message catalogs for the
processing of LC_MESSAGES.
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.
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
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
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