Easy Shell Scripting

**Step 1:** Open the file using an editor (e.g., "vi" or "pico".)

vi Firstshellscript.sh

#!/bin/sh
echo "Hello World"

chmod 744 Firstshellscript.sh

chmod +x Firstshellscript.sh

bash$ ./Firstshellscript.sh
Hello World

sh -x Firstshellscript.sh
+ echo 'Hello World'
Hello World

bash$ cat Firstshellscript.sh
#!/bin/sh
echo Hello World

# This is a comment line.
# This is another comment line.

: 'This is a comment line.

Again, this is a comment line.

My God, this is yet another comment line.'

bash$ SCRIPT_PATH=/home/blessen/shellscript
bash$ export SCRIPT_PATH

bash$ export SCRIPT_PATH=/home/blessen/shellscript

bash$ echo $define_tempval 
blessen

#!/bin/sh

username=blessen
echo "The username is $username"

#!/bin/sh

echo "The first variable is $1"
echo "The second variable is $2"

bash$ ./commandline.sh blessen lijoe
The first variable is blessen
The second variable is lijoe

bash$ cat test

bash$ echo $?
0

bash$ cat xyz1
bash$ echo $?
1

#!/bin/sh

display()
{
    local local_var=100
    global_var=blessen
    echo "local variable is $local_var"
    echo "global variable is $global_var"
}

echo "======================"
display
echo "=======outside ========"
echo "local variable outside function is $local_var"
echo "global variable outside function is $global_var"

======================
local variable is 100
global variable is blessen
=======outside ========
local variable outside function is 
global variable outside function is blessen

read <variable_name>

echo "statement to be displayed"

sum=`expr 12 + 20`

$[expression operation statement]

echo $[12 + 10]

if [ conditional statement ]
then
	... Any commands/statements ...
fi

#!/bin/sh

echo "Enter your username:"
read username

if [ "$username" = "blessen" ]
then
	echo 'Success!!! You are now logged in.'
else
	echo 'Sorry, wrong username.'
fi

for ((initialization; condition; increment/decrement))
do
	...statements...
done

#!/bin/sh

for (( i=1; $i <= 10; i++ ))
do
	echo $i
done

for <variable> in <list>
do
	...statements...
done

#!/bin/sh

count=0
for i in `cat /etc/group`
do
	count=`expr "$count" + 1`
	echo "Line $count is being displayed"
	echo $i
done

echo "End of file"

#!/bin/sh

for i in `seq 1 5`
do
	echo $i
done

while [ condition ]
do
	...statement...
done

#!/bin/sh

num=1

while [$num -lt 5]; do num=$[$num + 1]; echo $num; done

select <variable> in <list>
do
	...statements...
done

case $<variable> in
	<option1>) statements ;;
	<option2>) statements ;;
	*) echo "Sorry, wrong option" ;;
esac

#!/bin/bash

echo "***********************"
select opt in apache named sendmail
	do
	case $opt in
		apache) 	/etc/rc.d/init.d/httpd restart;;
		named) 		/etc/rc.d/init.d/named restart;;
		sendmail)	/etc/rc.d/init.d/sendmail restart;;
		*) 			echo "Nothing will be restarted"
	esac
	echo "***********************"

	# If this break is not here, then we won't get a shell prompt.
	break

done

<name of function> ()
{	# start of function
	statements
} 	# end of function

#!/bin/sh

sumcalc ()
{
	sum=$[$1 + $2]
}

echo "Enter the first number:"
read num1
echo "Enter the second number:"
read num2

sumcalc $num1 $num2

echo "Output from function sumcalc: $sum"

Easy Shell Scripting

Introduction

Comments in a Shell

Variables

System Variables

User-Defined Variables

Commandline Arguments

Exit status variable

Scope of a Variable

Input and Output in Shell Scripting

Arithmetic Operations in Shell Scripting

Conditional Statements

Variable Comparison

Loops

The "for" Loop

While Loop

Select and Case Statement

Functions

Debugging Shell Scripts