Shell Scripting Basics Explained

Key Concepts

• Shell Script
• Shebang Line
• Variables
• Input and Output
• Conditionals
• Loops
• Functions
• Command Substitution
• Error Handling
• Comments
• Script Execution

Shell Script

A shell script is a text file containing a series of commands that the shell interpreter can execute. It allows automation of tasks and simplifies complex operations.

Example: A shell script to greet the user and display the current date.

#!/bin/bash
echo "Hello, $USER!"
date
    

Shebang Line

The shebang line, also known as a hashbang, is the first line in a shell script that specifies the interpreter to be used. It typically starts with #! followed by the path to the interpreter.

Example: #!/bin/bash tells the system to use the Bash shell to execute the script.

Variables

Variables in shell scripting are used to store data that can be referenced and manipulated. They are defined by assigning a value to a name.

Example: Assigning a value to a variable and displaying it.

name="Alice"
echo "Hello, $name!"
    

Input and Output

Input and output in shell scripting involve reading data from the user or a file and displaying data to the terminal or a file. Commands like read and echo are commonly used.

Example: Reading user input and displaying it.

echo "Enter your name:"
read name
echo "Hello, $name!"
    

Conditionals

Conditionals allow scripts to make decisions based on certain conditions. The if, elif, and else statements are used for this purpose.

Example: Checking if a number is positive, negative, or zero.

echo "Enter a number:"
read num
if [ $num -gt 0 ]; then
    echo "Positive"
elif [ $num -lt 0 ]; then
    echo "Negative"
else
    echo "Zero"
fi
    

Loops

Loops are used to execute a set of commands repeatedly. The for, while, and until loops are commonly used in shell scripting.

Example: Using a for loop to print numbers from 1 to 5.

for i in {1..5}; do
    echo $i
done
    

Functions

Functions in shell scripting allow you to group commands into reusable blocks. They can be called multiple times within a script.

Example: Defining and calling a function to greet the user.

greet() {
    echo "Hello, $1!"
}
greet "Alice"
    

Command Substitution

Command substitution allows you to use the output of a command as an argument to another command. It is done using $(command) or backticks command.

Example: Using command substitution to get the current date.

current_date=$(date)
echo "Today's date is $current_date"
    

Error Handling

Error handling in shell scripting involves checking the success or failure of commands and taking appropriate actions. The exit command and $? variable are commonly used.

Example: Checking if a command succeeded and exiting if it failed.

ls /nonexistent_directory
if [ $? -ne 0 ]; then
    echo "Directory not found"
    exit 1
fi
    

Comments

Comments are used to add explanatory notes to the script that are ignored by the interpreter. They start with the # symbol.

Example: Adding a comment to explain a section of the script.

# This script greets the user
echo "Hello, $USER!"
    

Script Execution

To execute a shell script, you need to make it executable using the chmod command and then run it using the shell interpreter.

Example: Making a script executable and running it.

chmod +x myscript.sh
./myscript.sh