Does anyone know how to write a script to find out if a particular user is logged in to unix and send a greeting if there logged in; otherwise, if they're not return a message they're not logged on. I'm still a beginner at this and I know you can use the 'who' command to find who's logged in but how do send a greeting to them by using the 'write' or 'echo' command. I tried using the 'write' command but it kept saying the write permission is off.

The environment variable mesg is set to either 'y' or 'n'. If it is set to 'n' for a user the refuse write messages and talk messages. A wall message gets sent to everyone that is logged on to the current system. A wall message is terminated with control-d. The root user ignores to value of mesg and can write to a user's console regardless.