Case, Loop & Command line args un Unix

Bash Case Statement
The Bash case statement is the simplest form of IF-THEN-ELSE with many ELIF
elements.
Using the case statement makes our bash script more readable and easier to maintain.
These are generally applied to simplify the complex conditions having multiple
different choices.
The Bash case statement follows a similar logic as the Javascript or C switch
statement. There is a slight difference, as follows:
The Bash case statement takes a value once and tests that value multiple times. It
stops searching for a pattern once it has found it and executed the statement linked with
it, which is almost opposite in case of the C switch statement.

Bash Case Statement Conti…
Case Statement Syntax
Patterns
A pattern and its commands make a clause, which
ends with ;;.
Patterns support special characters.
The ) terminates a pattern list.
The | operator separates multiple patterns.
case expression in
pattern_1)
statements
;;
pattern_2)
statements
;;
pattern_3|pattern_4|pattern_5)
statements
;;
pattern-n)
statements
;;
*)
statements
;;
esac

There are some key points of bash case statements:
Each case statement in bash starts with the 'case' keyword, followed by the case
expression and 'in' keyword.
 The case statement is closed by 'esac' keyword.
We can apply multiple patterns separated by | operator. The ) operator indicates the
termination of a pattern list.
A pattern containing the statements is referred to as a clause, and it must be
terminated by double semicolon (;;).
An asterisk symbol (*) is used as a final pattern to define the default case.
 It is used as a default case when used as the last case.

Example: #!/bin/bash
echo "Do you know Unix Operating System?"
read -p "Yes/No? :" Answer
case $Answer in
Yes|yes|y|Y)
echo "That's amazing."
echo
;;
No|no|N|n)
echo "It's easy. Let's start learning from Online Platform."
;;
esac
#!/bin/bash
echo "enter a and b"
read a

Example:To Perform Arithmetic Operation
#!/bin/sh
echo "enter a and b"
read a
read b
echo "enter operation to be performed"
read op
case $op in
+) c=èxpr $a + $b` ;;
-) c=èxpr $a - $b` ;;
/) c=èxpr $a / $b` ;;
*) c=èxpr $a * $b` ;;
*) echo "no valid operation specified" ;;
esac
echo Result after performing operation on a and b is
echo $c

Loop
#1) Unix For loop statement
Sytax:
For varname in list
do
Echo “Statement”
done
Keywords are for, in, do, done
List is a list of variables which are separated by spaces. If list is not mentioned in the
for statement, then it takes the positional parameter value that were passed into the
shell.
Varname is any variable assumed by the user.
Example: Print table of 2. using for loop
#!/bin/sh
for num in {2..20.2}
do
Echo “Table of 2 : $num”
done

Loop
Syntax of for like C programming language.
Syntax: for (( cond1; cond2; cond3 ))
do
echo “Statement”
done
Look at the above snapshot, condition1 indicates initialization, cond2
indicates condition and cond3 indicates updation.
#!/bash/sh
For (( num=1; num<=10; num++ ))
do
echo “$num”
done

Loop
#2) while loop:
Linux scripting while loop is similar to C language while loop. There is a condition in
while. And commands are executed till the condition is valid. Once condition becomes
false, loop terminates.
Syntax: while [condition]
do
commands
done
Example: printing number in reverse order. [10 to 0]
#!/bin/sh
i=10;
while [ $i -le 0 ]
do
echo “$i”
let i-- ;
done
while infinite loop:

Loop
while infinite loop:
Infinite loop is also called endless loop. It is made with while true (it means condition
will always be true) or while : (it means an empty expression), where colon (:) is
equivalent to no operation.
Example:
#!/bash/sh
while true or while :
do
Echo “This is infinite loop. Please ctrl + c to Exit.”
done

Loop
#3) Until loop:
It is similar to while loop. The only difference is that until statement executes its code
block while its conditional expression is false, and while statement executes its code
block while its conditional expression is true.
Syntax: until [ condition ];
do
commands
done
Example: example to display number from 5 to 15.
#!/bin/sh
i=5;
Until [ $i –gt 15 ];
do
Echo “$i”
i=$(( i+1 ))
done

Command line argument
The Unix shell is used to run commands, and it allows users to pass run time
arguments to these commands.
These arguments, also known as command line parameters, that allows the users to
either control the flow of the command or to specify the input data for the command.
While running a command, the user can pass a variable number of parameters in the
command line.
Within the command script, the passed parameters are accessible using ‘positional
parameters’. These range from $0 to $9, where $0 refers to the name of the
command itself, and $1 to $9 are the first through to the ninth parameter, depending
on how many parameters were actually passed.
Example:
$ Hello student this is command line argement
Here $0 would be assigned sh
$1 would be assigned Hello
$2 would be assigned student
And so on …
We will now look at some additional commands to process these parameters.
#1) set

We will now look at some additional commands to process these parameters.
#1) set
This command can be used to set the values of the positional parameters on the
command line.
Example:
$ set Welcome to Unix World
$ echo $1 $2
Welcome to
Here, “Welcome” was assigned to $1 and “to” was assigned to $2 and so on.
#2) shift
This command is used to shift the position of the positional parameters. i.e. $2 will be
shifted to $1 all the way to the tenth parameter being shifted to $9. Note that if in case
there are more than 9 parameters, this mechanism can be used to read beyond the 9th.
Example:
$ set hello good morning how do you do welcome to Unix tutorial.
Here, ‘hello’ is assigned to $1, ‘good’ to $2 and so on to ‘to’ being assigned to $9. Now
the shift command can be used to shift the parameters ‘N’ places.

Example:
$ shift 2 $ echo $1Now $1 will be ‘morning’ and so on to $8 being ‘unix’ and $9 being
‘tutorial’.

Case, Loop & Command line args un Unix

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