Shell Scripting

Your first Shell script!!

• Open the terminal
• type the below command: this is to create a file
• vi helloworld.sh
• hit escape button (to enter the insert mode in vi editor)
• type the below
• echo "helloworld..!!"
• to save the script hit escape and colon (:)type wq! (like this :wq!)and hit enter
• now to execute the program use the below command
• . ./helloworld.sh
• hit enter
• output is as below
• helloworld..!!

Printing on screen

Example:1

This statement prints the statement as is on screen

echo Hello, how are you doing?

output:

Hello, how are you doing?

Example:2

Printing the value of any variable

a=10

echo $a

output:

10

Reading the user input

the below shell script reads 2 inputs from the user and prints it on the screen.

echo "Enter the name:"

read name

echo"Enter last name:"

read lname

echo "hello $name $lname, how are you?? "

Operators in Shell

Mathematical

[x -eq y] => x and y values are equal??

[x -ne y] => x and y are not equal ??

[x -lt y] => x is less than y??

[x -le y] => x is less than or equal to y??

[x -gt y] => x is greater than y??

[x -ge y] => x is greater than or equal to y??

String

string1 =string2 => string1 and string2 are equal??

string1 != string2 => string1 and string2 are not equal??

string1 => string1 is not NULL

-n string1 => string1 is not NULL and does exists

-z string1  => string1 is NULL and exists

Logical

AND (&&)

OR (||)

NOT (!)

AND (&&)

Syntax:

command1 || command2

Here command2 is only when command 1 is successful.

Example: 

Make a directory and change the directory to newly created one.

mkdir new_dir && cd new_dir

OR (||)

Syntax:

command1 || command2

Here command2 is only when command 1 fails.

Example: 

Make a directory if it doesn't exists.

[-d new_dir] || mkdir new_dir