Explain this shell script to me.

[virus_killer]

Guys,

I am new to shell script, and want to learn more about it. i have got one script to report on status of ADSL connection. i can understand half of it (or less then half). can you guys please help me out in this.

Here is the script,
-------------------------------------------------------------------------------------
CONFIG="$1"
if [ -z "$CONFIG" ] ; then
get_device
[ -z "$CONFIG" ] && CONFIG=/etc/ppp/pppoe.conf
fi
if [ ! -f "$CONFIG" -o ! -r "$CONFIG" ] ; then
echo "$0: Cannot read configuration file '$CONFIG'" >& 2
exit 1
fi
. $CONFIG
PPPOE_PIDFILE="$PIDFILE.pppoe"
PPPD_PIDFILE="$PIDFILE.pppd"
if [ "$DEMAND" != "no" ] ; then
echo "Note: You have enabled demand-connection; adsl-status may be inaccurate."
fi
# If no PPPOE_PIDFILE, connection is down, unless we're using the Linux plugin
if [ "$LINUX_PLUGIN" = "" ] ; then
if [ ! -r "$PPPOE_PIDFILE" ] ; then
echo "adsl-status: Link is down (can't read pppoe PID file $PPPOE_PIDFILE)"
exit 1
fi
fi
# If no PPPD_PIDFILE, something fishy!
if [ ! -r "$PPPD_PIDFILE" ] ; then
echo "adsl-status: Link is down (can't read pppd PID file $PPPD_PIDFILE)"
exit 1
fi
PPPD_PID=`cat "$PPPD_PIDFILE"`
# Sigh. Some versions of pppd put PID files in /var/run; others put them
# in /etc/ppp. Since it's too messy to figure out what pppd does, we
# try both locations.
for i in /etc/ppp/ppp*.pid /var/run/ppp*.pid ; do
if [ -r $i ] ; then
PID=`cat $i`
if [ "$PID" = "$PPPD_PID" ] ; then
IF=`basename $i .pid`
netstat -rn | grep " ${IF}\$" > /dev/null
# /sbin/ifconfig $IF | grep "UP.*POINTOPOINT" > /dev/null
if [ "$?" != "0" ] ; then
echo "adsl-status: Link is attached to $IF, but $IF is down"
exit 1
fi
echo "adsl-status: Link is up and running on interface $IF"
/sbin/ifconfig $IF
exit 0
fi
fi
done
echo "adsl-status: Link is down -- could not find interface corresponding to"
echo "pppd pid $PPPD_PID"
exit 1
-------------------------------------------------------------------------------------

If possible then please explain each line of the script. that what it does.

Any reply would be highly appreciated.

 

[Faun]

#$1 is the command line argument that is passed to shell script
CONFIG="$1"

#checking if string is empty or not
if [ -z "$CONFIG" ] ; then
get_device
#again checking if the string is empy, if yes then setting value of CONFIG to /etc/ppp....
[ -z "$CONFIG" ] && CONFIG=/etc/ppp/pppoe.conf
fi

#checking if either file pppoe.conf doesn't exists or is not readable
# -f checks for file existence, -o is or, -r is checks if file is readabel, ! negates the output
if [ ! -f "$CONFIG" -o ! -r "$CONFIG" ] ; then
echo "$0: Cannot read configuration file '$CONFIG'" >& 2
exit 1
fi


. $CONFIG
PPPOE_PIDFILE="$PIDFILE.pppoe"
PPPD_PIDFILE="$PIDFILE.pppd"
if [ "$DEMAND" != "no" ] ; then
echo "Note: You have enabled demand-connection; adsl-status may be inaccurate."
fi
# If no PPPOE_PIDFILE, connection is down, unless we're using the Linux plugin
if [ "$LINUX_PLUGIN" = "" ] ; then
if [ ! -r "$PPPOE_PIDFILE" ] ; then
echo "adsl-status: Link is down (can't read pppoe PID file $PPPOE_PIDFILE)"
exit 1
fi
fi
# If no PPPD_PIDFILE, something fishy!
if [ ! -r "$PPPD_PIDFILE" ] ; then
echo "adsl-status: Link is down (can't read pppd PID file $PPPD_PIDFILE)"
exit 1
fi
PPPD_PID=`cat "$PPPD_PIDFILE"`
# Sigh. Some versions of pppd put PID files in /var/run; others put them
# in /etc/ppp. Since it's too messy to figure out what pppd does, we
# try both locations.
for i in /etc/ppp/ppp*.pid /var/run/ppp*.pid ; do
if [ -r $i ] ; then
PID=`cat $i`
if [ "$PID" = "$PPPD_PID" ] ; then
IF=`basename $i .pid`
netstat -rn | grep " ${IF}\$" > /dev/null
# /sbin/ifconfig $IF | grep "UP.*POINTOPOINT" > /dev/null
if [ "$?" != "0" ] ; then
echo "adsl-status: Link is attached to $IF, but $IF is down"
exit 1
fi
echo "adsl-status: Link is up and running on interface $IF"
/sbin/ifconfig $IF
exit 0
fi
fi
done
echo "adsl-status: Link is down -- could not find interface corresponding to"
echo "pppd pid $PPPD_PID"
exit 1
-------------------------------------------------------------------------------------

If possible then please explain each line of the script. that what it does.

Any reply would be highly appreciated.

its easy, what part you didnt understand ? Am feeling a bit lazy typing.

I dont use dialer to connect to internet.

 

[virus_killer]

Thanks for your reply. i really appreciate. i am trying to learn shell scripts.so you can say i am a newbie to this shell script. i am referring books as well. but i have found this way very easy to learn. like comments added to the each lines makes it easy for me to understand.

I would really appreciate your effort if you will explain me rest of the script.