Linux Shell Scripting Beginner Tutorials – Part II

In this second series, you will learn about script debugging, conditional statements and case statements.

Debugging shell script

Everything goes wrong at some point. You can debug the shell script for error like any other programming languages using two switches “-v” and “-x”. Just append the switched before the script name to debug the script.

./ -x script.sh
./ -v script.sh

Command line arguments:

Arguments are something which will follows the command. For example , echo $0 , here echo is the command and $0 Is an argument passed to the command. $0 , by default will return the name of the shell script. Create a script file with .sh extension and try out the following example.

#! /bin/sh
echo $0 $1 $2

save the file and pass any two strings with the script file.

./scriptname.sh demo string

Once you run your script with two arguments, you will get the names of two arguments and shell script name as output because $1 and $2 gets the arguments passed with the command.

Conditional Statements

You can use conditional statements like if, else in the script to make decisions. Let’s look at a simple example which uses conditional statement.

if [condition];
Then
“code goes here”
Elif[condition[;
Then
“code goes here”
Else
“code goes here”
fi

#! /bin/sh
echo "Guess this websites name"
read guess
if [ "$guess" = "scriptcrunch" ]; then
    echo "You got it right!! Bingo"
else
    echo "Your are wrong :( try again"
fi;

In the above code snippet, we get an user input from keyboard. The if statement checks if the input matches the “scriptcrunch” string. If the string is matched it will display the message from the “ then block” or else the “else” statement will be invoked and the message inside the else block will be displayed. “fi” closes the if code block. Note: When we deal with mathematical expressions, we use “–eq” instead of “=” in the if condition.

Working with loops

Shell script supports for and while loops.

Syntax:
For ((initial-condition;condition;iterator))
Do
  command1
  command2
Done

while[condition]
Do
Command1
Command2
Done

echo "enter you name"
read name
echo "how many times you want  to print your name"
read times
echo -e "here you go n n ****************************n"
for((i=1;i<=times;i++))
do
echo "$name"
done

In the above script, name stores the name you enter in the keyboard using read function. Times store the input value given by the user. In for loop we use times variable to print the name “n”times. ie the value given by the user.

echo -e  "How many numbers you want to print???n"
read number
echo -e "n________________n"
x=1
while [ $x -le $number ]
do
  echo " $x"
  x=$(( $x + 1 ))
done
echo -e "n________________n"

case statements

You can use case statements in shell script. The case statement will execute the commands which match the value mentioned in the case statement. Save the following script in case.sh file and change the permissions to 755 to execute the script.

echo "enter any number between 1 and 6"
read number
case $number in
   "1") echo "you have won a BMW X6";;
   "2") echo "you have won a BENZ";;
   "3") echo "you have won a PRIUS";;
   "4") echo "You have won 1 million dollors";;
   *) echo "Sorry!! Better luck next time  ";;
esac