Part – 2:

Q) How we get the PID of current shell ?

Ans - $ echo “$$”<Enter – Key>

Q) How we get the exit status of last command ?

Ans - $ echo “$?”<Enter – Key>

Q) How we get the process number of last back ground process ?

Ans - $ echo “$!”<Enter – Key>

Q) What are different types of Unix Special Variable ?

Ans –

$0 – Return Current Script File Name

$1 – Return 1^st Command line argument

$2 – Return 2^nd Command line argument

$n – Return nth Command line argument

$# - Return total number of Command line arguments

$@ or $* - Return all command line arguments within double quotes.

Example –

vi script6.sh

#! /bin/sh

echo “The PID of current Shell: $$”

echo “The total number of command line arguments: $#”

echo “The 1^st command line argument: $1“

echo “The 2^nd command line argument: $2”

echo “The name of your script: $0”

ESC + :wq<Enter – Key>

$ sh script6.sh 23 12<Enter – Key>

Output –

The PID of current shell: 2354

The total number of command line arguments: 2

The 1^st command line argument: 23

The 2^nd command line argument: 12

The name of your script: script6.sh

Q) What are the operators in Shell Script ?

Ans – There are 5 operators present in shell script. Such as –

1. Arithmetic Operator

2. Relational Operator

3. Boolean Operator

4. String Operator

5. File Test Operator

Q) What are arithmetic operators ?

Ans –

+ -> `expr $a + $b` Addition Operator

- -> `expr $a - $b` Subtraction Operator

* -> `expr $a \* $b` Multiplication Operator

/ -> `expr $a / $b` Division Operator

% -> `expr $a % $b` Modulus Operator

= -> c=`expr $a + $b` Assignment Operator

== -> if [ $a == $b ] Comparison Operator – if both string or character are equal return true otherwise false

!= -> if [ $a != $b ] Comparison Operator - if both string or character are not equal return true otherwise false

Where a and b are operands.

Example –

#! /bin/sh

a=45

b=23

s=`expr $a + $b`

sub=`expr $a - $b`

mul=`expr $a \* $b`

d=`expr $a / $b`

mod=`expr $a % $b`

echo “Sum = $s”

echo “Subtract = $sub”

echo “Multiplication = $mul”

echo “Division = $d”

echo “Modulus = $mod”

Q) What are Relational operators ?

Ans –

For numbers, it operates.

-eq -> equal operator

-ne -> Not equal operator

-gt -> greater than operator

-ge -> greater than equal to

-lt -> less than

-le -> less than equal to

Example 

a=34

b=21

if [ $a –eq $b ]

Q) What are Boolean operators ?

Ans –

-o (logical OR) – It takes multiple conditions, any one condition is true, then total expression is true.

-a (logical AND) – It takes multiple conditions and returns true if all the conditions are true.

Q) What are String Operators ?

Ans –

= -> check if the value of two string operands are equal or not

!= -> check if the value of string operands are not equal if yes return true else false

-z -> Check if the size of operand is zero if yes return true else false.

-n -> check if the size of operand is non – zero if yes returns true else false.

Q) What are File Test Operators ?

Ans –

If [ -r $FileName ] -> checks if the file is readable or not

If [ -w $FileName ] -> checks if the file is writable or not

If [ -x $FileName ] -> checks if the file is executable or not

If [ -f $FileName ] -> checks if the file is ordinary file or not

If [ -d $FileName ] -> checks if the file is directory file or not

If [ -e $FileName ] -> checks if the file name any other files or directories exists or not

If [ -s $FileName ] -> checks if the size of file is non – zero or not