Shell Scripting, The Easy way

Shell scripting is nothing but a group of commands put together and executed one after another in a sequential way. It is fairly useful if you are using any of the UNIX variants. Without beating about the bush let us get into shell scripting.

Let’s start by mentioning the steps to write and execute a shell script

Step 1: touch a file

e.g.: touch Firstshellscript.sh

Step 2: Open the file using the command vi or pico

e.g.: vi Firstshellscript.sh

Step 3: All bash shell scripts should begin with "#!/bin/bash ". This line is called the Shebang and this line looks like a comment but its not. It’s a message, which talks about the interpreter to be used for this script.

Normally the format is #! <path of the bash installation if its a bash script >

Step 4: Write the code / script which you want to develop in the file named Firstshellscript.sh. Here let’s write our first shell script and let it be the normal hello world display script.

Step 5: For hello world to be displayed in the shell script. Put this content in the file Firstshellscript.sh.

echo “Hello World"

Step 6: Next step is to make the script executable by using the command chmod

e.g.: chmod 744 Firstshellscript.sh

or

chmod u + x Firstshellscript.sh

Step 7: Execute the script using the command sh

i.e. bash> sh Firstshellscript.sh

If you want to see the entire execution then use the command

sh -xv < script name>

Step 8: The above command would display the contents like what is shown below

bash> sh Firstshellscript.sh

bash > Hello World

Success, we have written our first shellscript and executed it.

A hello world script will look like what is shown below, if you cat or open the script named Firstshellscript.sh

cat Firstshellscript.sh

#!/bin/bash

echo Hello World

Comments in a Shell

All lines beginning with # is a comment in shell scripting. You can have multiple comments by using colon and single quotes.

e.g.:

:' This is a comment line

Again this is a comment line

My God again this is a comment line’

Notes: This will not work if there is a single quote in between the contents.

Variables

As you all know, variables are the most significant part of any software language, be it Perl, C or anything. Similarly, in shell scripting as well variables are very significant and is classified mainly into two. They are System Variables and User Defined Variables.

·         System Variables

The System Variables are variables which are already defined and kept in the OS and they are also called Environment Variables. These variables are all named in capital letters. One can see these variables and their values by executing the command set. Examples of System variables are PWD, HOME, USER etc. The values of these system variables can be displayed individually by echoing the System variables i.e. echo $HOME, will display the value stored in the system variable HOME.

To set a System Variable use "set" command

e.g.: bash > set $PATH=/home/blessen/shellscript

·         User Defined Variables

These kinds of variables are commonly used in scripting. They are normal variables but their variable name should not be in capital letters, should not start with a number etc. An ideal naming of variable will be like _define_tempval.

To set or define a user defined variable, please read below.

When we assign or create a variable we just write the variable name, equal to its value i.e. _define_tempval = blessen. Now to use / display the value in the variable _define_tempval we have to use echo command i.e.

echo _define_tempval. The out put of which will be blessen

Please find an example script which will set a variable named username and displays its content on the screen when it is executed.

#!/bin/bash

username=blessen

echo " The username is $username"

·         Command Line Arguments

These are variables which pass values or argument to a script to process it. These variables which are passed into the script are accessed using $1,$2...$n where $1 is the first command line argument and $2 the next etc. The delimiter is space here. $0 is the name of the script. The variable $# will display the number of command line argument supplied.

Let me explain. Consider a script which will take in 2 command line arguments and displays it. The name of the script is commandline.sh and the script will look like the one below

#!/bin/bash

echo "The first variable is $1"

echo "The second variable is $2"

When I execute commandline.sh with command line argument like blessen and lijoe then the output of the script will be like the one shown below

bash>sh commandline.sh blessen lijoe

The first variable is blessen

The second variable is lijoe

·         Exit status variable

This variable tells us if the command executed just above this was successful or not. The variable is represented using $?. If the value is 0, it means that the command, which was executed just above this, was successful. But if the value is any other number it means that the above command was unsuccessful. Thus it is very useful in scripting.

For testing, create a file named test by using the command touch test. Then try cating the file

bash > cat test

Then check the value of $?.

bash> echo $?

0

The value is zero because the command was successful. Now try catting a file, which is not there. Let it be xyz1.

bash> cat xyz1

bash> echo $?

1

The value 1 for the exit status shows that the above command was unsuccessful.

·         Scope of a Variable

I am sure most of the programmers have learned and most probably worked with variables and its scope. In shell also we use the scope of a variable for various programming activities. In shell there are two types of scope. One is global and other is local scope. From the name itself you can understand that scope of global variable is throughout the program i.e. any other shell program can use these variables for its functioning and its set using the export command.

Syntax is:

variable1=<its value>

export variable1

In shell program the local variables are defined using a local tag before the variables, while it is defined.

Syntax is:

local variable=<some value>

The below script will demonstrate the scope of a local and global variable.

#!/bin/bash

function display()

{

local local_var=100

global_var=blessen

echo " local variable is $local_var\n";

echo "global variable is $global_var\n";

}

echo " ======================"

display

echo "=======outside ========"

echo "local variable outside function is $local_var\n";

echo "global variable outside function is $global_var\n";

Input and Output in shell scripting

For taking inputs from keyboard we will have to use a tool provided by shell which is called read. The read command will read the values, which are typed from keyboard, and assigns it to the variable mentioned along with it.

Syntax is: read <variable name>

For outputting, we use echo command and we have already dealt with it in our above explanation.

Syntax is: echo "statement to be displayed"

Arithmetic Operations in shell scripting

Like all other scripting languages shell script also allows us to play with numerical and functions associated with it like addition, subtraction, multiplication and division. To do these arithmetic operations a function called expr is used, which tells the shell script interpreter that these are numerical on which the specified function is to be performed, i.e. expr a + b means add a and b .

Syntax: expr <expression>

e.g.: sum=`expr 12 + 20`

Similarly syntax can be used for Subtraction, Division and Multiplication. There is another way to handle Arithmetic operations; include the variables and function inside a square bracket which starts with a $sign. The syntax is

Syntax is: $[expression / statement]

e.g.: echo $[12 + 10]

Conditional Loops

Lets have some fun with a conditional statements like "if condition “. Most of the time, we shell programmers have situations where we have to compare two variables and execute certain statement depending on true or false of the condition. So in such cases, where we have to execute certain set of commands, depending on certain conditions then we have to use if condition. The syntax of if is show below

Syntax:

if [Conditional statement]

then

..Any commands/statements ..

fi

For conditional check we can also use a different form of if condition using the test condition. The syntax of test is test <condition>. The new if condition will look like the one below

if test Conditional statement

then

..Any commands/statements ..

fi

The below script will prompt for entering the username and if the user name is blessen then it will display the message showing that I have successfully logged in. If it’s not blessen then it will display the message “wrong username”.

#!/bin/bash

echo " Enter your username:"

read username

if test $username=blessen

then

echo " Success!!!, you are now logged in"

else

echo “Sorry, wrong username"

fi

Test the above for yourself.

Variable Comparison

In shell script while have a different way to compare variables . If the value of a variables to be compared are numerical then you have to use options like

-eq For equal to

-ne Not Equal to

-lt Less than

-le Less than or equal to

-gt Greater than

-ge Greater then or equal to

If the value of variables to be compared are strings then you have to use options like

= Equal too

!= Not Equal too

> Greater than

= Greater than or equal to

< Less than

<= Less than or equal to

Loops

In this section we will deal with the loop statements like for and while loop.

·         For Loop

Most commonly used loop is for loop and in shell script there are 2 types of for loop. One type of for loop is similar to the C programs for loop and the other type of for loop will be new to you all.

Syntax for first type of for loop:

for ((initialization;condition;increment/decrement))

do

....statements....

done

Syntax for second type of for loop:

for <variable> in <list of option >

do

....statements ....

done

The below script will read all the contents of /etc/passwd file and displays each line one by one.

e.g.:

#!/bin/bash

count=0

for i in `cat /etc/passwd'

do

count=`expr $count + 1`

echo "Line $count is being displayed"

echo $i

done

echo "End of file"

Another example on forloop, where we use ‘seq’ generator to generate the sequence.

e.g.:

#!/bin/bash

for i in `seq 1 5`

do

echo $i

done

·         While Loop

While loop is another useful loop used in all programming languages. This loop will continue executing till the condition specified is met.

Syntax is: while [condition]

do

...statement ....

done

The below example script is a simple script which assigns value one to the num variable and adds one to the value of num each time it goes round the loop until the value of num is less than 5.

e.g.:

#!/bin/bash

num=1

while [$num -lt 5]; do

num=$[$num + 1]

echo $num

done

Select and Case Statement

We have learned the switch case in C programming. The combination of select and case provides us with the same feature. Select statement is not at all a part of case statement but I just put it together for you to understand how both of it can be used in programming

Syntax of Select:

select <variable> in <list>

do

..statements...

done

Syntax of Case:

case $<variable> in

<option1>) statements;;

<option2>) statements;;

*) echo "Sorry wrong option"

esac

The example below will explain the usage of select and case together. Example below will display the options of service's which needs to be restarted in a machine. When a particular option is selected then the corresponding service will be restarted.

e.g.:

#!/bin/bash

select options in "apache" "named" "sendmail"

do

echo "***********************"

case $options 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 "***********************"

break

# If this break is not there then we wont get a shell prompt

done

Functions

In the modern world where all programmers use OOPs model for programming. Even we the shell programmers are also not far behind. We too break our codes into small chunks called functions and call them by name in the main program. This approach helps in debugging, code re-usability etc.

Syntax for function is:

function <name of function ()>

{# start of function

statements

} #end of function

The function is invoked by just writing their function names in the main program.

Please find an example below which uses function for code re-usability.

e.g.:

#!/bin/bash

function sumcalc

{

echo "Enter the first number:"

read num1

echo "Enter the second number:"

read num2

sum=$[$num1 + $num2]

}

sumcalc

echo "$sum -- O/P from function sumcalc"

Debugging Shell Script

Now and then, I have found people debugging their C program and other programs. Hence I wish to include the option to debug shell script for our shell programmers. We shell programmer are not far behind any other programmers. To debug a shell program use the option -x and -v along with sh where using -x option will expand each simple command, for command, case command, select command, or arithmetic for command, display the expanded value of PS4, followed by the command and its expanded arguments or associated word list . -v option is for verbose.

Syntax and Usage of debug options

e.g. sh -xv Firstshellscript.sh