To write your first shell script, you will need to use a text editor to create a new file. You can use any text editor that you like, such as vi, nano, or gedit.

Here is an example of a simple shell script that displays the message “Hello, World!” on the screen:

#!/bin/bash

echo "Hello, World!"

To make the script executable, you will need to give it execute permissions using the chmod command:

chmod +x myscript.sh

You can then run the script by typing its name:

./myscript.sh

This will execute the commands in the script and display the message “Hello, World!” on the screen.

Here is a brief explanation of the lines in the script:

• The first line, #!/bin/bash, is called the shebang line. It tells the system which interpreter to use to execute the script. In this case, it is telling the system to use the bash shell to execute the script.
• The second line, echo "Hello, World!", is a command that displays the message “Hello, World!” on the screen. The echo command is a built-in command in the bash shell that prints its arguments to the standard output.

I hope this helps! Let me know if you have any questions.

There are total 3 types of looping statements that can be used in bash programming  

1. while statement
2. for statement
3. until statement

To control the flow of the loop, two control statements are used they are, 
 

1. break
2. continue

Their descriptions and syntax are as follows: 

while statement 

Here the command is evaluated and based on the resulting loop will be executed, if the command raises to false then the loop will be terminated 

for statement 

The for loop operates on lists of items. It repeats a set of commands for every item in a list. Here var is the name of a variable and word1 to wordN are sequences of characters separated by spaces (words). Each time the for loop executes, the value of the variable var is set to the next word in the list of words, word1 to wordN.

until statement 

 The until loop is executed as many times as the condition/command evaluates to false. The loop terminates when the condition/command becomes true. 

In this post, we will learn how to use inbuild special symbols in the shell to make our code simple.
Let’s assume we have a file in the temp directory and we will use the inbuilt keyword to test the functionality.

We have 2 files in the Desktop

The ls command shows all the files in the current directory.

root@vps43086554:~/Desktop# ls
sample.sh sample.txt
root@vps43086554:~/Desktop#

Now will make a simple shell script sample.sh where we will write an if condition where the condition will check with the help of existing keywords.

if [ -e sample.txt ];
then echo “Inside if ture part”
fi

In the above the if condition we have written an if the condition where we have written -e and a file name.

File Operators

-e

Ture if file exists.
Menas it will check if the file exists in the directory then it will return true.

Prerequisite -Create 2 files sample.sh sample.txt

if [ -e sample.txt ];
then echo “Inside if ture part”
fi

Output-
root@vps43086554:~/Desktop# sh sample.sh
Inside if ture part
root@vps43086554:~/Desktop#

-d

True if the file is a directory.

Prerequisite- Create a folder named a folder.

Code to create a folder named as folder.

root@vps43086554:~/Desktop# mkdir folder
root@vps43086554:~/Desktop# ls
folder sample.sh sample.txt

Code to check the folder is exists or not.

if [ -e folder ];
then echo “Inside if ture part”
fi

Output

root@vps43086554:~/Desktop# sh sample.sh
Inside if ture part

-f

True if the file exists and the file is a regular file.

Prerequisite- Create an invalid file named non_normarl.gfd.

Code-

if [ -f non_normal.gfd ];
then echo “Inside if ture part”
fi
if [ -f sample.txt ];
then echo “Inside if ture part of valid file”
fi

root@vps43086554:~/Desktop# sh sample.sh
Inside if ture part of valid file

-r

True if the file is readable by you

-s

True if the file exists and is not empty.

-w

True if the file is writable by you.

-x

True if the file is executable by you.

String Operators

-z “String”

True if the string is empty

-n “String”

True if the file is not empty.

STRING1=STRING2

True id the strings are equal.

STRING1!=STRING2

True if the strings are not equal.

agr1 -eq arg2

True if arg1 is equal to arg2

agr1 -nq arg2

True if arg1 is not equal to arg2

arg1 -lt arg2

True if the arg1 is less than arg2

arg1 -le arg2

True if arg1 is less than or equal to arg2

arg1 -gt arg2

True if the arg1 is greater than arg2

arg1 -ge arg2

True if the arg1 is greater than or equal to arg2

Conditional Statements: There are total 5 conditional statements available in the bash programming,Which can be implemented in the different situation in the bash/shell programming.

1. if statement
2. if-else statement
3. if..elif..else..fi statement (Else If ladder)
4. if..then..else..if..then..fi..fi..(Nested if)
5. switch statement

if statement

If statement is the simplest statement in the series.Where it is used to compare the two variable. This block will process if specified condition is true.
Syntax:

if [ expression ]
then
   statement
fi

if-else statement

If specified condition is not true in if part then else part will be execute.This can be helpful when we have 3 variables and need to compare between
Syntax

if [ expression ]
then
   statement1
else
   statement2
fi

if..elif..else..fi statement (Else If ladder)

We can use to compare more than 3 variables by using this block.To use multiple conditions in one if-else block, then elif keyword is used in shell. If expression1 is true then it executes statement 1 and 2, and this process continues. If none of the condition is true then it processes else part.
Syntax

if [ expression1 ]
then
   statement1
   statement2
   .
elif [ expression2 ]
then
   statement3
   statement4
   .
else
   statement5
fi

if..then..else..if..then..fi..fi..(Nested if)

Generally Nested block is used for comparison of multiple nested conditions. if-else block can be used when, one condition is satisfies then it again checks another condition. In the syntax, if expression1 is false then it processes else part, and again expression2 will be check.
Syntax:

if [ expression1 ]
then
   statement1
   statement2
   .
else
   if [ expression2 ]
   then
      statement3
      .
   fi
fi

switch statement

case statement works as a switch statement if specified value match with the pattern then it will execute a block of that particular pattern
When a match is found all of the associated statements until the double semicolon (;;) is executed.
A case will be terminated when the last command is executed.
If there is no match, the exit status of the case is zero.

Syntax:

case  in
   Pattern 1) Statement 1;;
   Pattern n) Statement n;;
esac

In the last post we learned how to declare variables and how to naming conventions.Where we can only use the “_” underscore symbol in the variable name.Where excluding “_” symbol we can not able to use other symbol. This is because those characters are used in the names of special Unix variables. These variables are reserved for specific functions.

For example, the $ character represents the process ID number, or PID, of the current shell −

$echo $$

The above command writes the PID of the current shell −

29949

The following table shows a number of special variables that you can use in your shell scripts −

Command-Line Arguments

The command-line arguments $1, $2, $3, …$9 are positional parameters, with $0 pointing to the actual command, program, shell script, or function and $1, $2, $3, …$9 as the arguments to the command.

Following script uses various special variables related to the command line −

#!/bin/sh

echo "File Name: $0"
echo "First Parameter : $1"
echo "Second Parameter : $2"
echo "Quoted Values: $@"
echo "Quoted Values: $*"
echo "Total Number of Parameters : $#"

Here is a sample run for the above script −

$./test.sh Zara Ali
File Name : ./test.sh
First Parameter : Zara
Second Parameter : Ali
Quoted Values: Zara Ali
Quoted Values: Zara Ali
Total Number of Parameters : 2

Special Parameters $* and $@

There are special parameters that allow accessing all the command-line arguments at once. $* and $@ both will act the same unless they are enclosed in double quotes, “”.

Both the parameters specify the command-line arguments. However, the “$*” special parameter takes the entire list as one argument with spaces between and the “$@” special parameter takes the entire list and separates it into separate arguments.

We can write the shell script as shown below to process an unknown number of commandline arguments with either the $* or $@ special parameters −

#!/bin/sh

for TOKEN in $*
do
   echo $TOKEN
done

Here is a sample run for the above script −

$./test.sh Zara Ali 10 Years Old
Zara
Ali
10
Years
Old

Note − Here do…done is a kind of loop that will be covered in a subsequent tutorial.

Exit Status

The $? variable represents the exit status of the previous command.

Exit status is a numerical value returned by every command upon its completion. As a rule, most commands return an exit status of 0 if they were successful, and 1 if they were unsuccessful.

Some commands return additional exit statuses for particular reasons. For example, some commands differentiate between kinds of errors and will return various exit values depending on the specific type of failure.

Following is the example of successful command −

$./test.sh Zara Ali
File Name : ./test.sh
First Parameter : Zara
Second Parameter : Ali
Quoted Values: Zara Ali
Quoted Values: Zara Ali
Total Number of Parameters : 2
$echo $?
0
$