[code lang=”bash”]expr length “`users | sed -e “s/\($USER\|\[ \]*\)//ig”`”[/code]

The first thing executed by the shell will be the thing under french apostrophe ( ` ). Theses are for evaluation a command, and replace it by what it outputs (normally print on the screen). The command users print the list of connected users into a pipe, to sed which evaluate its command as a regular expression (-e parameter). The command is “s” for substitute, and the rest tells him to find “$USER” (replaced by the currently connected user, you) and spaces and replace them by … nothing. So this part will be an empty string if there is no other users connected than “$USER” and something not empty if there is.

The “expr length” return the length of a string. So this commands print 0 if there is no other user connected, and >0 if there is some !

In a shell script to do something if yes or no…
[code lang=”bash”]#!/bin/sh
usersstripped=`users | sed -e “s/\(tom\|\[ \]*\)//ig”`
connected=`expr length “$usersstripped”`
if [ “$connected” -eq “0” ] ; then
echo “No other user is connected”
else
echo “Other user connected !”
fi[/code]

It is essentially the same command but done in two times, as in a shell script this command would not be evaluated correctly.

And if you want to put that in a cron to eg. send you a mail, you just have to type “crontab -e” and put a line like :

[code]* * * * * /home/tom/connected[/code]

To launch it every minutes. But if you do that you’d better do something like detecting a connection…

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