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DESCRIPTION
The external wall command performs a write to all users currently logged on to the system. wall differs from a write in two ways. First, it reads all of its input from the standard input. Redirection or a pipe may be used. After receiving the entire input (an EOF is encountered) wall sends the message to all users. Second, it is a unidirectional utility. It does not imply a response from the recipients. It functions as a broadcasting tool instead of a bidirectional communication tool.
NOTE: The wall command is supported on System V as an administrative command.
COMMAND FORMAT
Following is the general format of the wall command.
wall
FURTHER DISCUSSION
The wall message precedes its message text with the following line:
Broadcast Message from username
It is usually used by the super-user to warn all users of immediate problems. The most common warning is the system is being shutdown within 60 seconds.
The super-user can write to any user's terminal regardless of the permissions set by the mesg command.
Some common formats include:
wall < mesgfile
- or -
cat mesgfile | wall
DIAGNOSTICS AND BUGS
The following messages may be returned by the wall command if it encounters problems.
"Cannot send to /dev/ttyXX" | |
Wall could not open the user's tty for writing. Possible problems exist with the tty driver or hardware. | |
"Permission denied" | |
The person you are trying to wall has denied other users from writing to his/her terminal. This means the recipient has done a mesg -n. | |
"Can no longer write to user" | |
The recipient performed a mesg -n after you started your wall command. This denies you permission to write to the recipient's tty. |
RELATED COMMANDS
Refer to the mesg command described in Module 88 and the write command described in Module 161.
RELATED FILES
The following files are used by wall:
/etc/utmp | Used to locate the destination users. The ttys are also retrieved from this file. |
/etc/ttyXX | A tty device file is used to write to the other users. |
standard input | The input is read from your terminal unless redirected from a file or piped from a previous command. |
The wall command is commonly used by the system administrator (super-user) to broadcast warnings to all users currently logged on to the system. It can be used by a user like yourself. This is not done often since not very many users have a need to send message to everyone on the system.
TYPICAL OPERATION
In this activity you use the wall command to broadcast a message to everyone logged onto the system. Note you're performing this at your own risk. If you have a group of hard-core, leave-me- alone type users, you may want to skip this exercise. But if you want to display a message on every user's screen, proceed. Begin at the shell prompt.
cj> wall This is a broadcast message of the UNIX Beginners Broadcasting Foundation. Please disregard this message and get back to your work immediately. Ctrl-D
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