Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
while [ $TRUE ] #Endless loop.
do
tail −$CHECK_LINES $LOGFILE> $TEMPFILE
# Saves last 100 lines of system log file as temp file.
# Necessary, since newer kernels generate many log messages at log on.
search=`grep $KEYWORD $TEMPFILE`
# Checks for presence of the "IP address" phrase,
# indicating a successful logon.
if [ ! −z "$search" ] # Quotes necessary because of possible spaces.
then
echo "On−line"
rm −f $TEMPFILE
exit $ONLINE
else
echo −n "."
fi
sleep 1
done
# Clean up temp file.
# −n option to echo suppresses newline,
# so you get continuous rows of dots.
# Note: if you change the KEYWORD variable to "Exit",
# this script can be used while on−line to check for an unexpected logoff.
# Exercise: Change the script, as per the above note,
# and prettify it.
exit 0
# Nick Drage suggests an alternate method:
while true
do ifconfig ppp0 | grep UP 1> /dev/null && echo "connected" && exit 0
echo −n "." # Prints dots (.....) until connected.
sleep 2
done
# Problem: Hitting Control−C to terminate this process may be insufficient.
# (Dots may keep on echoing.)
# Exercise: Fix this.
# Stephane Chazelas has yet another alternative:
CHECK_INTERVAL=1
while ! tail −1 "$LOGFILE" | grep −q "$KEYWORD"
do echo −n .
sleep $CHECK_INTERVAL
done
echo "On−line"
# Exercise: Discuss the strengths and weaknesses
# of each of these various approaches.
Chapter 30. Debugging 323