1MESG(1P) POSIX Programmer's Manual MESG(1P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 mesg — permit or deny messages
13
15 mesg [y|n]
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18 The mesg utility shall control whether other users are allowed to send
19 messages via write, talk, or other utilities to a terminal device. The
20 terminal device affected shall be determined by searching for the first
21 terminal in the sequence of devices associated with standard input,
22 standard output, and standard error, respectively. With no arguments,
23 mesg shall report the current state without changing it. Processes with
24 appropriate privileges may be able to send messages to the terminal
25 independent of the current state.
26
28 None.
29
31 The following operands shall be supported in the POSIX locale:
32 y Grant permission to other users to send messages to the ter‐
34 minal device.
35 n Deny permission to other users to send messages to the termi‐
37 nal device.
38
40 Not used.
41
43 None.
44
46 The following environment variables shall affect the execution of mesg:
47 LANG Provide a default value for the internationalization vari‐
49 ables that are unset or null. (See the Base Definitions vol‐
50 ume of POSIX.1‐2017, Section 8.2, Internationalization Vari‐
51 ables for the precedence of internationalization variables
52 used to determine the values of locale categories.)
53 LC_ALL If set to a non-empty string value, override the values of
55 all the other internationalization variables.
56 LC_CTYPE Determine the locale for the interpretation of sequences of
58 bytes of text data as characters (for example, single-byte as
59 opposed to multi-byte characters in arguments).
60 LC_MESSAGES
62 Determine the locale that should be used to affect the format
63 and contents of diagnostic messages written (by mesg) to
64 standard error.
65 NLSPATH Determine the location of message catalogs for the processing
67 of LC_MESSAGES.
68
70 Default.
71
73 If no operand is specified, mesg shall display the current terminal
74 state in an unspecified format.
75
77 The standard error shall be used only for diagnostic messages.
78
80 None.
81
83 None.
84
86 The following exit values shall be returned:
87 0 Receiving messages is allowed.
89 1 Receiving messages is not allowed.
91 >1 An error occurred.
93
95 Default.
96 The following sections are informative.
98
100 The mechanism by which the message status of the terminal is changed is
101 unspecified. Therefore, unspecified actions may cause the status of the
102 terminal to change after mesg has successfully completed. These actions
103 may include, but are not limited to: another invocation of the mesg
104 utility, login procedures; invocation of the stty utility, invocation
105 of the chmod utility or chmod() function, and so on.
106
108 None.
109
111 The terminal changed by mesg is that associated with the standard
112 input, output, or error, rather than the controlling terminal for the
113 session. This is because users logged in more than once should be able
114 to change any of their login terminals without having to stop the job
115 running in those sessions. This is not a security problem involving
116 the terminals of other users because appropriate privileges would be
117 required to affect the terminal of another user.
118 The method of checking each of the first three file descriptors in
120 sequence until a terminal is found was adopted from System V.
121 The file /dev/tty is not specified for the terminal device because it
123 was thought to be too restrictive. Typical environment changes for the
124 n operand are that write permissions are removed for others and group
125 from the appropriate device. It was decided to leave the actual
126 description of what is done as unspecified because of potential differ‐
127 ences between implementations.
128 The format for standard output is unspecified because of differences
130 between historical implementations. This output is generally not useful
131 to shell scripts (they can use the exit status), so exact parsing of
132 the output is unnecessary.
133
135 None.
136
138 talk, write
139 The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment
141 Variables
142
144 Portions of this text are reprinted and reproduced in electronic form
145 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
146 table Operating System Interface (POSIX), The Open Group Base Specifi‐
147 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
148 Electrical and Electronics Engineers, Inc and The Open Group. In the
149 event of any discrepancy between this version and the original IEEE and
150 The Open Group Standard, the original IEEE and The Open Group Standard
151 is the referee document. The original Standard can be obtained online
152 at http://www.opengroup.org/unix/online.html .
153 Any typographical or formatting errors that appear in this page are
155 most likely to have been introduced during the conversion of the source
156 files to man page format. To report such errors, see https://www.ker‐
157 nel.org/doc/man-pages/reporting_bugs.html .
158IEEE/The Open Group 2017 MESG(1P)