Shell Scripting

What is shell script?

A shell script is a file containing a series of commands.The shell reads this file and carried out the commands as thought they have been carried out directly on the command line.

Following steps are required to write shell script:

Step 1: Use any editor like vi or gedit to write shell script.

Syntax: vi <<Script file name>>

Example: vi hello.sh

Step 2: After writing shell script change permission of shell script as follows

Syntax: chmod <permission> <filename>

Example: chmod 777 hello.sh

Note:For file permissions checkout my linux file permissions blog.

Step 3: Execute your script as

Syntax: bash <file name>

Example: bash hello.sh or ./hello.sh

First Shell Script

#!/bin/bash

echo "Hello World"

The first line called "shebang" or "bang" line. It is nothing but the absolute path to the bash interpreter.

Shell Variable

A variable is nothing more than a pointer to the actual data. The shell enables you to create, assign, and delete variables.

Variable Names

The name of a variable can contain only letters (a to z or A to Z), numbers ( 0 to 9) or the underscore character ( _).

By convention, Unix shell variables will have their names in UPPERCASE.

Example: VAR_1, TOKEN_A, _NAME

Defining Variables

Variables are defined as follows −

variable_name=variable_value

For example −

NAME= "Khushbu"

The above example defines the variable NAME and assigns the value "Khushbu" to it. Variables of this type are called scalar variables. A scalar variable can hold only one value at a time.

Accessing Values

To access the value stored in a variable, prefix its name with the dollar sign ($) −

For example, the following script will access the value of defined variable NAME and print it on STDOUT −

#!/bin/sh

NAME="Khushbu"

echo $NAME

Read-Only Variables

Shell provides a way to mark variables as read-only by using the read-only command. After a variable is marked read-only, its value cannot be changed.

#!/bin/sh

NAME="Khushbu"

readonly NAME

NAME="Tiwari"

The above script will generate the following result −

/bin/sh: NAME: This variable is read only.

Unsetting Variables

Unsetting or deleting a variable directs the shell to remove the variable from the list of variables that it tracks. Once you unset a variable, you cannot access the stored value in the variable.

Following is the syntax to unset a defined variable using the unset command −

unset variable_name

Example:

#!/bin/sh

NAME="Khushbu"

unset NAME

echo $NAME

The above example does not print anything. You cannot use the unset command to unset variables that are marked readonly.

Variable Types

• Local Variables − A local variable is a variable that is present within the current instance of the shell. It is not available to programs that are started by the shell. They are set at the command prompt.

• Environment Variables − An environment variable is available to any child process of the shell. Some programs need environment variables in order to function correctly. Usually, a shell script defines only those environment variables that are needed by the programs that it runs.

• Shell Variables − A shell variable is a special variable that is set by the shell and is required by the shell in order to function correctly. Some of these variables are environment variables whereas others are local variables.

Shell Basic Operators

We will discuss the following operators −

• Arithmetic Operators

• Relational Operators

• Boolean Operators

• String Operators

• File Test Operators

The following example shows how to add two numbers −

The above script will generate the following result −

#!/bin/sh

val=`expr 2 + 2`

echo "Total value : $val"

Output: Total value : 4

The following points need to be considered while adding −

• There must be spaces between operators and expressions. For example, 2+2 is not correct; it should be written as 2 + 2.

• The complete expression should be enclosed between ‘ ‘, called the backtick.

Arithmetic Operators

The following arithmetic operators are supported by Bourne Shell.

Assume variable a holds 10 and variable b holds 20 then −

Show Examples

It is very important to understand that all the conditional expressions should be inside square braces with spaces around them, for example [ $a == $b ] is correct whereas, [$a==$b] is incorrect.

All the arithmetical calculations are done using long integers.

Relational Operators

Bourne Shell supports the following relational operators that are specific to numeric values. These operators do not work for string values unless their value is numeric.

For example, following operators will work to check a relation between 10 and 20 as well as in between "10" and "20" but not in between "ten" and "twenty".

Assume variable a holds 10 and variable b holds 20 then −

It is very important to understand that all the conditional expressions should be placed inside square braces with spaces around them. For example, [ $a <= $b ] is correct whereas, [$a <= $b] is incorrect.

Boolean Operators

The following Boolean operators are supported by the Bourne Shell.

Assume variable a holds 10 and variable b holds 20 then −

String Operators

The following string operators are supported by Bourne Shell.

Assume variable a holds "abc" and variable b holds "efg" then −

File Test Operators

We have a few operators that can be used to test various properties associated with a Unix file.

Assume a variable file holds an existing file name "test" the size of which is 100 bytes and has read, write and execute permission on −

Shell Decision Making

Unix Shell supports conditional statements which are used to perform different actions based on different conditions. We will now understand two decision-making statements here −

• The if...else statement

• The case...esac statement

The if...else statements

If else statements are useful decision-making statements which can be used to select an option from a given set of options.

Unix Shell supports following forms of if…else statement −

• if...fi statement

• if...else...fi statement

• if...elif...else...fi statement

Most of the if statements check relations using relational operators discussed in the previous chapter.

The case...esac Statement

You can use multiple if...elif statements to perform a multiway branch. However, this is not always the best solution, especially when all of the branches depend on the value of a single variable.

Unix Shell supports case...esac statement which handles exactly this situation, and it does so more efficiently than repeated if...elif statements.

There is only one form of case...esac statement which has been described in detail here −

• case...esac statement

The case...esac statement in the Unix shell is very similar to the switch...case statement we have in other programming languages like C or C++ and PERL, etc.

Shell Loops Type

we will examine the following types of loops available to shell programmers −

• The while loop

• The for loop

• The until loop

• The select loop

You will use different loops based on the situation. For example, the while loop executes the given commands until the given condition remains true; the until loop executes until a given condition becomes true.

Once you have good programming practice you will gain the expertise and thereby, start using appropriate loop based on the situation. Here, while and for loops are available in most of the other programming languages like C, C++ and PERL, etc.

Nesting Loops

All the loops support nesting concept which means you can put one loop inside another similar one or different loops. This nesting can go up to unlimited number of times based on your requirement.

Here is an example of nesting while loop. The other loops can be nested based on the programming requirement in a similar way −

Nesting while Loops

It is possible to use a while loop as part of the body of another while loop.

Syntax

while command1 ; # this is loop1, the outer loop
do
   Statement(s) to be executed if command1 is true

   while command2 ; # this is loop2, the inner loop
   do
      Statement(s) to be executed if command2 is true
   done

   Statement(s) to be executed if command1 is true
done

Example

Here is a simple example of loop nesting. Let's add another countdown loop inside the loop that you used to count to nine −

#!/bin/sh

a=0
while [ "$a" -lt 10 ]    # this is loop1
do
   b="$a"
   while [ "$b" -ge 0 ]  # this is loop2
   do
      echo -n "$b "
      b=`expr $b - 1`
   done
   echo
   a=`expr $a + 1`
done

This will produce the following result. It is important to note how echo -n works here. Here -n option lets echo avoid printing a new line character.

0
1 0
2 1 0
3 2 1 0
4 3 2 1 0
5 4 3 2 1 0
6 5 4 3 2 1 0
7 6 5 4 3 2 1 0
8 7 6 5 4 3 2 1 0
9 8 7 6 5 4 3 2 1 0

Shell Loop Control

So far you have looked at creating loops and working with loops to accomplish different tasks. Sometimes you need to stop a loop or skip iterations of the loop.

In this chapter, we will learn following two statements that are used to control shell loops−

• The break statement

• The continue statement

The infinite Loop

All the loops have a limited life and they come out once the condition is false or true depending on the loop.

A loop may continue forever if the required condition is not met. A loop that executes forever without terminating executes for an infinite number of times. For this reason, such loops are called infinite loops.

Example

Here is a simple example that uses the while loop to display the numbers zero to nine −

#!/bin/sh

a=10

until [ $a -lt 10 ]
do
   echo $a
   a=`expr $a + 1`
done

This loop continues forever because a is always greater than or equal to 10 and it is never less than 10.

The break Statement

The break statement is used to terminate the execution of the entire loop, after completing the execution of all of the lines of code up to the break statement. It then steps down to the code following the end of the loop.

Syntax

The following break statement is used to come out of a loop −

break

The break command can also be used to exit from a nested loop using this format −

break n

Here n specifies the n^th enclosing loop to the exit from.

Example

Here is a simple example which shows that loop terminates as soon as a becomes 5 −

#!/bin/sh

a=0

while [ $a -lt 10 ]
do
   echo $a
   if [ $a -eq 5 ]
   then
      break
   fi
   a=`expr $a + 1`
done

Upon execution, you will receive the following result −

0
1
2
3
4
5

Here is a simple example of nested for loop. This script breaks out of both loops if var1 equals 2 and var2 equals 0 −

Live Demo

#!/bin/sh

for var1 in 1 2 3
do
   for var2 in 0 5
   do
      if [ $var1 -eq 2 -a $var2 -eq 0 ]
      then
         break 2
      else
         echo "$var1 $var2"
      fi
   done
done

Upon execution, you will receive the following result. In the inner loop, you have a break command with the argument 2. This indicates that if a condition is met you should break out of outer loop and ultimately from the inner loop as well.

1 0
1 5

The continue statement

The continue statement is similar to the break command, except that it causes the current iteration of the loop to exit, rather than the entire loop.

This statement is useful when an error has occurred but you want to try to execute the next iteration of the loop.

Syntax

continue

Like with the break statement, an integer argument can be given to the continue command to skip commands from nested loops.

continue n

Here n specifies the n^th enclosing loop to continue from.

Example

The following loop makes use of the continue statement which returns from the continue statement and starts processing the next statement −

Live Demo

#!/bin/sh

NUMS="1 2 3 4 5 6 7"

for NUM in $NUMS
do
   Q=`expr $NUM % 2`
   if [ $Q -eq 0 ]
   then
      echo "Number is an even number!!"
      continue
   fi
   echo "Found odd number"
done

Upon execution, you will receive the following result −

Found odd number
Number is an even number!!
Found odd number
Number is an even number!!
Found odd number
Number is an even number!!
Found odd number

Happy Learning,

Khushbu Tiwari