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L i n u x H e l p
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Shell scripting quick start
There are a few basic things that one must understand about how a shell works
in order to understand how to write scripts. I will try to be as brief and to-the-point as I can possibly be throughout this tutorial.
Shell redirection
There are three fundamental ways to redirect input and output in your shell.
A "pipe" |
Overwrite > or <
Append >> or <<
Starting to write the script:
She-Bang (Hash (#) Bang (!))
The She-Bang is the first line in any shell script and is the actual execute line for the script. In other words, the script executes using the program pointed to by the she-bang. Normally, any line in a shell script that begins with a "#" is a comment. The she-bang is an exception.
#!/bin/bash
Variables
Run the command 'env' on your shell and you will see all of the variables that your shell currently has set. You can set variables like these for use in your shell scripts. *NOTE* From this point on in this tutorial, each of the scriptlets can be copied to your favorite editor and will run as displayed here. Copy one and play around with it to see how it ticks.
#!/bin/bash
VAR="0" # Set the variable
echo $VAR # echo it to the shell
if-then construct
Now that you know how to assign a variable, we're going to learn how to test for that variable and execute differently based on the variable's value.
#!/bin/bash
VAR="0"
if [ $VAR -eq "0" ]; then # if $VAR is equal to 0
echo "The variable \$VAR is set to 0"
else
echo "The variable \$VAR is not set to 0"
fi
for loop
#!/bin/bash
VAR="foo bar baz"
for member in $VAR # for each member of $VAR
do
echo $member # echo $member to the shell
done
while loop
#!/bin/bash
i="0";
while [ $i -le 10 ]; # while $i is less than or equal to 10
do
echo $i; # echo $i to the shell
let i=$i+1; # increment $i by 1
done
Reading input from the shell
#!/bin/bash
echo "Is this fun, or WHAT?!? [y/n]"
read ANSWER
if [ $ANSWER == "n" -o $ANSWER == "N" ]; then
echo "Oh, you're not having fun? Well, I am!"
elif [ $ANSWER == "y" -o $ANSWER == "Y" ]; then
echo "I'm glad you're enjoying yourself ;-)"
else
echo "You must answer the question with \"y\" or \"n\""
echo
$0
fi
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L i n u x W o r l d N e w s
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What in the hell is a KrnlPanic? Well, a KrnlPanic is me! Actually, let's start with "What is a kernel?". The
kernel is the core of your operating system (OS), whether your OS is Linux, Unix or windows. The kernel takes
care of all process management (what program runs and when), memory management (which parts of memory get used for what) and
also, the kernel takes care of interfacing the OS with your computer's hardware (disk drives, sound card, modem, network card, etc).
Now...since the kernel is doing all of these extremely important jobs, it stands to reason
that if it has an error, it will be a Bad Thing(tm).
If you use windows, you know a kernel panic as a "BSOD" or Blue Screen of Death. Or how about "Invalid Page
Fault in KERNEL32.DLL". I'm sure you've probably seen both of those. In Linux, a Kernel Panic is normally plainly stated
so. At boot time is when you will see most linux panics. I have yet to see a linux kernel panic while the system is running.
You may have seen "Kernel Panic: init not found" or "Kernel Panic: VFS unable to mount root fs on 2:00". All of these previously
listed errors are because of something that happened to the kernel that it couldn't handle, whether it was an access to an
invalid memory location or the inability to find the initialization files it requires.
I hope that sufficiently explains things. Oh yeah...KrnlPanic is also my name ;-)
- Rick
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S l a s h d o t / F r e s h m e a t
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Slashdot and Freshmeat Headlines at 1328383374
(Unixtime)
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