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PROGRAM:

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 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.

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.

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