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"BASH inbuilt VARIABLES for easy scripting" as the title says this small tutorial will make bash scripting  easier. Do you know its possible to control "for" loop if you set a inbuilt variable  . . .  I won't say that variable now …. Please continue reading to know that ….

First we start with $0 $1 .. $9, $*, $@ and $#. To explain these variables lets write a simple script called "test-param.sh". 

#!/bin/bash

echo "The script file is $0"
echo "The first parameter passed to $0 is $1 and the second one is $2"
echo "$@ are the total parameters passed to $0"
echo "The total number of parameters passed is $#"

Now execute the script "test-param.sh"

$ chmod u+x test-param.sh

$ ./test-param.sh p1 p2 p3 p4 p5 p6 p7 p8 p9

The script file is ./test-param.sh
The first parameter passed to ./test-param.sh is p1 and the second parameter is p2
p1 p2 p3 p4 p5 p6 p7 p8 p9 are the parameters passed to ./test-param.sh
The total number of parameters passed is 9

I hope you have understand what those variables are used for.

$0 - To get the base name (script file name).
$1 to $9 - Will give the parameters (or arguments) passed to a script during execution.
$@ - Will display all parameters passed to a script.
$* - Will also display all parameters passed to a script. But they are not separated by space.  (p1p2p3  etc)
@# - Will give the total number of parameters passed to a script.

TIP

What if there are more that 9 parameters. How to display a 10th parameter. Answer is use, ${10}

Now $?, called as "exit status". In linux when a command is executed without any errors, it produce an exit status, 0. ie, $? will be 0. A value other than 0 is considered as an error. Technically, $? is exit status of the most recently executed foreground pipeline.

Lets illustrate this,

$ ls anil.txt

If ls can list anil.txt, the exit status ($?) will be "0". If ls can't list anil.txt exit status will be a value not equal to 0.

Another example.

$ echo "Linux rocks" | grep -q windows (-q will suppress grep output)
$ echo $?
1

Why? Because grep can't find windows in"Linux rocks". In general, if the last command doesn't produce the desired output (an error) the exit status ($?) will be a value greater than "0". Zero means success.

Some more inbuilt variables

$$ - Priocess ID of the shell. If used inside a script $$ will get process id of the script.
$! - Process ID of last executed background process.

When using for loop you may sometimes want to loop through a ":" separated or "-" separated fields. How will you do that. By setting inbuilt variable IFS (Internal field separator). See the following example to understand this.

$ IFS="-"
$ for i in `echo that-is-cool`; do  echo $i; done

that
is
cool

If IFS is not set to "-" the output of for loop will be. Because the default value of IFS is space " "

that-is-cool

Always remember to unset IFS, after its use.

unset IFS 

More to come . . .

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Please post your comments on this. If possible your on suggestions and tips.
Thanks.