How to Bash Shell Script Functions and Loops

1. 
   1
   Go to Linux and open a terminal window. Type the following command at the command prompt: vi FunLoop.sh. This will create the file you are going to use as a script file and open it for editing.
   Ad
2. 
   2
   Enter the shell statement on the first line of the file. Every shell script needs this to tell it what shell to run in. Type #!/bin/bash.
3. 
   3
   Create the variables you will need for this script. You will need one to capture the choice entered by the user and one to set the directory based on the user's choice. Type directory="" and choice=0.
4. 
   4
   Create your first function. It is a straightforward code and it has a name that is case sensitive. Name it menuList. Everything between the opening and closing brackets is any code you wish it to be.
5. 5
   Type the name and parentheses on the first line after the variables. It should look like: menuList () .
6. 6
   Type an opening bracket on another line by itself {.
7. 7
   Type the command to clear the screen on a line by itself. It should look like this: clear.
8. 8
   Type the echo statements that make up the menu.

   echo "1. Run ls on my home directory.".

   echo "2. Run ls on the root directory.".

   echo "3. Run ls on the bin directory.".

   echo "4. Exit program".
9. 9
   Type a closing bracket on another line by itself }.
10. 
    10
    Create a function to run the ls command.
11. 11
    Type the name and parentheses on the first line after the variables. You should add runLS ().
12. 12
    Type a opening bracket on another line by itself {.
13. 13
    Type the command to run the ls. ls -la $directory where $directory is the one specified by the menu choice.
14. 14
    Type a closing bracket on another line by itself }..
15. 
    15
    Create the loop and if statements to run your functions.
16. 16
    Create the while statement with the word while followed by a conditional statement. while [ $choice -ne 4]. Where $choice is the input from the user and 4 is the menu item to tell the program to close, -ne is a conditional operator used in bash shell scripting for number evaluation.
17. 17
    Continue the while statement with the do keyword. Type in do.
18. 18
    Next comes the code you wish to run every time the while loop runs. You have four menu items so you will need four if statements and a catch all for values that don't meet the menu items' requirements. Write the if statement as follows:

    if [ $choice -eq 1 ]

    then

    $directory="~/"

    runLS

    elif [ $choice -eq 2 ]

    then

    $directory="/"

    runLS

    elif [ $choice -eq 3 ]

    then

    $directory="/bin"

    runLS

    elif [ $choice -eq 4 ]

    then

    clear

    echo "Thank you for using list files"

    sleep 3

    exit

    else

    echo "Invalid choice, please try again"

    fi

    done.
19. 19
    Save and exit the file by typing: Esc, :x.
20. 20
    Run chmod to give permissions to the file to be executable. Type in: chmod u+x FunLoop.sh.
21. 21
    This is a simple program to show how to create a function and some loops to reduce repeating code.
    • run the program by typing ./FunLoop.sh

Sample code that you can use:

!/bin/bash.
 
directory=""
choice=0
 
menuList () {
clear
echo "1. Run ls on my home directory."
echo "2. Run ls on the root directory."
echo "3. Run ls on the bin directory."
echo "4. Exit program"
}
 
runLS ()
{
ls -la $directory
sleep 2
}
 
while [ $choice -ne 4 ]
do
menuList
read -p "Enter your choice: " choice
if [ $choice -eq 1 ]
then
$directory="~/"
runLS
elif [ $choice -eq 2 ]
then
$directory="/"
runLS
elif [ $choice -eq 3 ]
then
$directory="/bin"
runLS
elif [ $choice -eq 4 ]
then
clear
echo "Thank you for using list files"
sleep 3
exit
else
echo "Invalid choice, please try again"
fi
done