Linux System Administration

Today I have tried to install RHEL6 i386 as a virtual machine using virtualbox. Right after startup I got a error message saying:

This kernel requires the following features not present: pae
Unable to boot - please use a kernel appropriate for your CPU

If you get this error enable PAE/NX on your virtual machine by navigating to:

Settings->System->Processor->Extended Features and tick: PAE/NX

This is a simple skeleton of a bash countdown script. The script takes two arguments . Here are some examples of its usage:

  • countdown time to 90 minutes from now:
./bash-countdown.sh -m 90
  • countdown time to 23.3.2036 from now:
./bash-countdown.sh -d "Mar 23 2036"
  • countdown time to 21:06 from now:
./bash-countdown.sh -d 21:06
  • countdown time to 21:06:45 from now:
./bash-countdown.sh -d 21:06:45

Feel free to modify this script according to your needs:

bash-countdown.sh :

#!/bin/bash 
 
if ["$#" -lt "2" ] ; then 
echo"Incorrect usage ! Example:" 
echo'./countdown.sh -d  "Jun 10 2011 16:06"' 
echo'or' 
echo'./countdown.sh -m  90' 
exit 1 
fi 
 
now=`date +%s` 
 
if ["$1" = "-d" ] ; then 
until=`date -d "$2" +%s` 
sec_rem=`expr$until - $now` 
echo"-d" 
if [$sec_rem -lt 1 ]; then 
echo"$2 is already history !" 
fi 
fi 
 
if ["$1" = "-m" ] ; then 
until=`expr 60 \*$2` 
until=`expr$until + $now` 
sec_rem=`expr$until - $now` 
echo"-m" 
if [$sec_rem -lt 1 ]; then 
echo"$2 is already history !" 
fi 
fi 
 
 
while [$sec_rem -gt 0 ]; do 
clear 
date 
letsec_rem=$sec_rem-1 
interval=$sec_rem 
seconds=`expr$interval % 60` 
interval=`expr$interval - $seconds` 
minutes=`expr$interval % 3600 / 60` 
interval=`expr$interval - $minutes` 
hours=`expr$interval % 86400 / 3600` 
interval=`expr$interval - $hours` 
days=`expr$interval % 604800 / 86400` 
interval=`expr$interval - $hours` 
weeks=`expr$interval / 604800` 
echo"----------------------------" 
echo"Seconds: "$seconds 
echo"Minutes: "$minutes 
echo"Hours:   "$hours 
echo"Days:    "$days 
echo"Weeks:   "$weeks 
sleep 1 
done 

Do not forget to make bash-countdown.sh script executable before execution:

$ chmod +x bash-countdown.sh

Exectute:

./bash-countdown.sh -d 22:34

Output:

Fri Jun 11 20:55:19 EST 2010
----------------------------
Seconds:  40
Minutes:  38
Hours:    1
Days:     0
Weeks:    0

Not everyone has unlimited Internet download. Sometimes Internet provides provide peak and off-peak hours and hardly is someone going to stay up at night to trigger their downloads.

There are many tools in Linux which allows user to set a timer to automatically start download without user intervention. Here is a simplest form of automatic off-peak download involving at, wget and shutdown command. First create a file which will contain wget and shutdown commands:

mydownloads.txt:

wget ftp://ftp.redhat.com/pub/redhat/rhel/beta/6/i386/iso/RHEL6.0-20100414.0-AP-i386-DVD1.iso
wget http://ftp.monash.edu.au/pub/linux/CentOS/5.5/isos/i386/CentOS-5.5-i386-bin-DVD.iso
shutdown -h now

Now use at command to execute all command within mydownloads.txt file:

at -m 02:00 < mydownloads.txt

At command will automatically start download at 2AM and when downloads are finished it will shutdown your PC.

NOTE: For shutdown you need sudo or run command as root



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