#!/bin/sh
# The first thing I usually do in a script is define "functions"
# A function (like the one below) takes the form of
# function_name () {
# The Commands You Want Inside
# }
# Then you can just call the function_name to execute it!
# I like functions, even short ones like this, because
# it makes my "if" statements small and easy to read...
use_msg () {
echo "Usage:  `basename $0` [name]"
}
# Below is some very basic sanity checking.
# Basically, what it says, is "if there isn't exactly one argument,
# print the use_msg, then exit with errors"
# the "$#" operator counts the number of arguments. "-ne"
# Stands for "not equal".
if [ "$#" -ne "1" ] ; then
        use_msg
        exit 2
fi
# Okay, so we made it this far, lets try to extract the real name
# from the passwd file. So we're searching passwd for a
# line beginning with (^) the user id (${1}). I put curly
# braces around some of my variables to help myself keep
# it seperate from the rest. You probably don't even really need
# it here. Also, make sure you are using backticks instead of
# single quotes! The backtick key is usually found near the top
# left of your keyboard, under the Esc key
name_t=`grep ^${1} /etc/passwd | cut -d: -f5 | cut -d, -f1`
#OK, if we can't extract the real name of the user, the variable
# $name_t will be empty. If that's the case, lets use the UserID
# instead. Otherwise, use the name. It'll make the script look
# nicer later on
if [ "$name_t" = "" ] ; then
        name="$1"
else
        name="$name_t"
fi
# Here's your trick to see if the user exists:
who | grep $1 >/dev/null 2>&1
# Now watch what we're doing with the "logg" variable. If the
# user above command is successful ( exited with 0 status),
# then make the "logg" var empty. If the command did not 
# succeed (e.g. the user isn't logged in), then make logg equal
# to "not ". The space on the end is important, once again to
# make the output readable. Try it without the space, and
# see what happens.
if [ "$?" = "0" ] ; then
        logg=""
else
        logg="not "
fi
# Alrighty! This is the fruits of your labor! You echo out the
# name, which we defined above (it'll either be the full name,
# or the usedid you gave it on the command line, remember?),
# then you'll see what happens with the logg var. Remember,
# since it's empty when the user is logged it, the string will say
# "is logged in". If the user is not logged in, it will insert
# "not", then a space to say "is not logged in". The curly braces
# are needed right here to keep $logg from looking like
# $logglogged, which the shell will not understand.
echo "${name} is ${logg}logged in"