Important Concepts

 File Names and Permissions

In the above example, we have saved the file using gfg.sh name and also provided execute permission using chmod command. Now, let’s understand why we have done that. 

While writing bash scripts we should save our file with the .sh extension, so that the Linux system can execute it. When we first create a file with the .sh extension, it doesn’t have any execute permission and without the execute permission the script will not work. So, we should provide execute permission to it using the chmod command. 

The filename of a bash script can be anything but by convention, it is recommended to use snake case ( my_bash_script.sh ) or hyphens ( my-bash-script.sh ) for naming a script file.

Variables

We can use variables in bash scripting. Below is a sample program to understand the usage of variables in Bash scripts.

Example Script:

Name="SATYAJIT GHOSH"
Age=20

echo "The name is $Name and Age is $Age"

Output of Variables:

The name is SATYAJIT GHOSH and Age is 20

So, here is have declared two variables Name and another one is Age. These variables are accessible using $Name and $Age. That means, we can declare a variable in a bash script using VariableName=Value and can access it using $VariableName. Here is the terminal shell pictorial depiction after executing the above script as follows:  

 

There are two types of variables present within Bash Scripting. Conventionally, If a variable, is declared inside a function then it is generally a local variable and if it is declared outside then it is a global variable. In the case of a bash script, this concept is a little bit different, here any variable whether it is written inside a function or outside a function by default is a global variable. If we want to make a local variable then we need to use the keyword “local”.

Note: It is best practice to always use a local variable inside a function to avoid any unnecessary confusion.  

An example of the same is given below –

Example Script:

#!/bin/bash

var1="Apple" #global variable
myfun(){
    local var2="Banana" #local variable
    var3="Cherry" #global variable
    echo "The name of first fruit is $var1"
    echo "The name of second fruit is $var2"
}
myfun #calling function

echo "The name of first fruit is $var1"
#trying to access local variable
echo "The name of second fruit is $var2"
echo "The name of third fruit is $var3"

Output of local and global variables:

The name of first fruit is Apple
The name of second fruit is Banana
The name of first fruit is Apple
The name of second fruit is 
The name of third fruit is Cherry

Here in this above example, var2 is a local variable, so when we are accessing it from the function it is doing fine but when we are trying to access it outside the function, it is giving us an empty result in the output.

On the other hand, unlike programming languages, even though var3 is defined inside a function still it is acting as a global variable and it can be accessed outside the function. Below is the terminal shell depiction after executing the script –

 

Input and Output

Input & output are fundamental concepts for shell scripting. A script can take one or more inputs and can also produce zero or many outputs. It may even produce some errors.  Let’s understand this with an example –

Example Script:

echo "Enter filename"
read filename

if [ -e $filename ]
then
echo "$filename is exits on the directory"
cat $filename

else
    cat > $filename
    echo "File created"
fi

Output of Input & Output:

First time:

Enter filename
geeks.txt
Hello Geek
File created

Second time:

Enter filename
geeks.txt
geeks.txt is exits on the directory
Hello Geek

So, in this above example the first time, the script could not find any file with that file name, and the else block gets executed. It created the file and put some data into that file. When we run it a second time with the same file name, then it finds the file. So, is the if block gets executed and that displays the contents of the file.  Reading the file contents is input and on the first time putting data into the file is considered to be output. Here we have used > for storing the content in a file.  The > notation is used to redirect stdout to a file. On the other hand, we can use  2> notation to redirect stderr, and &>  to redirect both stdout and stderr.

Below is the terminal shell pictorial depiction after executing the following script – 

 

Functions

In programming, A function is a block of code that performs some tasks and it can be called multiple times for performing tasks. The simplest example of the use of function in Bash scripting can be given as –

Example Script:

#!/bin/bash
#It is a function
myFunction () {
echo Hello World from GeeksforGeeks
}

#function call
myFunction 

Output of Functions:

Hello World from GeeksforGeeks

The above example shows a function that prints something when called.

So, the basic syntax for writing functions within a Bash Script will be –

Syntax of Functions:

#for defining
function_name(){
    commands
    .....
}

function_name # for calling

Besides this, we can also have functions with passing arguments and with return values.