#!/bin/sh -u
# wait until userid log in, then print a message and exit.
# Syntax:  $0 userid
# Purpose:
#  Wait until the given userid logs in, then print a message and exit.
#  The script also exits if $USER (the person running the script)
#  is no longer logged in.  (The script exits when $USER logs off.)
#  This could be run in the background.
# Some comments in this script are "teacher" comments that would not
# normally appear in your own shell scripts.
# -IAN! idallen@idallen.ca

# Use standard search path, friendly umask, ASCII collating and sorting.
# Set the language and character set to be ASCII/C standard.
#
PATH=/bin:/usr/bin ; export PATH
LC_COLLATE=C ; export LC_COLLATE
LANG=C ; export LANG
umask 022

# validate the number of inputs and make sure input isn't empty
#
if [ $# -ne 1 ] ; then
	echo 1>&2 "$0: Expecting 1 userid arg; found $# ($*)"
	exit 1
fi

# Transfer $1 to a shell variable to make the script more readable.
# (Note that you must do this *after* you check $#, not before!)
#
userid="$1"

if [ -z "$userid" ] ; then
	echo 1>&2 "$0: Expecting 1 userid; found empty string; nothing done"
	exit 1
fi

echo "Now waiting for $userid to log in ..."

# Keep waiting for $userid to log in as long as $USER is still logged in.
# (If the person running the script logs off, this script exits too.)
# Note the use of the shell "!" operator to reverse the return status.
# The return status is set only by the *last* command in a pipeline.
#
while ! who | grep "$userid" ; do
    	echo "DEBUG still waiting for $userid ..."
	if ! who | grep "$USER" >/dev/null ; then
		echo 1>&2 "$0: Userid $USER is no longer logged in."
		exit 1
	fi
	sleep 30
done

echo "$0: Userid $userid is now logged in."

exit 0