mesg - permit or deny messages
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 operands shall be supported in the POSIX locale:
- Grant permission to other users to send messages to the terminal device.
- Deny permission to other users to send messages to the terminal device.
The following environment variables shall affect the execution of mesg:
- Provide a default value for the internationalization variables that are unset or null. (See XBD 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 standard error.
- [XSI] Determine the location of message catalogs for the processing of LC_MESSAGES.
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.
The following exit values shall be returned:
- Receiving messages is allowed.
- Receiving messages is not allowed.
- An error occurred.
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 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.
XBD Environment Variables
First released in Issue 2.
This utility is marked as part of the User Portability Utilities option.
The mesg utility is moved from the User Portability Utilities option to the Base. User Portability Utilities is now an option for interactive utilities.
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