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

Feel free to modify this script according to your needs: :

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

Do not forget to make script executable before execution:

$ chmod +x


./ -d 22:34


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:


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

Page 64 of 73

Free Linux eBooks

Do you have the right skills?

Our IT Skills Watch page reflects an up to date IT skills demand leaning towards the Linux and Unix environment. We have considered a number of skills and operating systems.

See the result...

Linux Online Training

Learn to run Linux servers and prepare for LPI certification with Linux Academy. 104 available video lessons with PDF course notes with your own server!

Go to top