Shell Scripting

Shell Scripting Basics

📜 1. Introduction to Shell Scripting

A shell script is a text file containing a series of commands that are executed by the shell, which is a command-line interface (CLI) in Linux. Shell scripts allow you to automate repetitive tasks, manage system operations, and enhance the power of the shell environment.

🧑‍💻 What Is a Shell?

The shell is a program that takes commands from the user and executes them. Common shells include:

• Bash (Bourne Again Shell): The most common and widely used shell in Linux.
• Zsh: A feature-rich shell with additional capabilities.
• Fish: A user-friendly shell with intuitive features.

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✍️ 2. Writing Your First Shell Script

A basic shell script can be as simple as running a few commands, and it can be made executable to automate tasks.

📝 Step-by-Step Guide:

1. Create a new file: Use your favorite text editor (e.g., nano, vim) to create a new script file. For example, let's create a file named hello_world.sh.

   nano hello_world.sh

2. Add the shebang line: The first line of your script should be a special comment called a "shebang" (#!/). This line tells the system which shell to use to execute the script.

   #!/bin/bash

3. Add your script content: After the shebang line, add commands you want to execute. For example:

   #!/bin/bash
   echo "Hello, World!"

4. Save and exit: Save the file and exit the editor. If using nano, press CTRL+O, then ENTER, and CTRL+X to exit.

5. Make the script executable: You need to give the script executable permissions.

   chmod +x hello_world.sh

6. Run the script: Finally, you can execute your script by running it with ./.

   ./hello_world.sh

   Output:

   Hello, World!

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🔄 3. Variables in Shell Scripting

In shell scripts, you can store values in variables and use them throughout the script.

🧑‍💻 Defining Variables:

To define a variable, just type the variable name followed by = and the value (no spaces around the = sign).

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

This will output:

Hello, Lucy!

🧑‍💻 Common Variables:

• $USER: The username of the currently logged-in user.
• $HOME: The home directory of the current user.
• $PWD: The current working directory.

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🎯 4. Control Structures in Shell Scripting

Control structures such as if statements, loops, and case statements help you control the flow of execution in your shell script.

🧑‍💻 If Statements:

If statements allow you to run code based on conditions.

Example:

#!/bin/bash
if [ $1 -gt 10 ]; then
  echo "The number is greater than 10."
else
  echo "The number is less than or equal to 10."
fi

Explanation:

• $1 refers to the first command-line argument.
• -gt checks if the number is greater than 10.

To run the script with a number argument:

./number_check.sh 15

Output:

The number is greater than 10.

🧑‍💻 For Loops:

For loops are used for repetitive tasks.

Example:

#!/bin/bash
for i in {1..5}
do
  echo "Loop iteration $i"
done

Output:

Loop iteration 1
Loop iteration 2
Loop iteration 3
Loop iteration 4
Loop iteration 5

🧑‍💻 While Loops:

While loops continue as long as a condition is true.

Example:

#!/bin/bash
count=1
while [ $count -le 5 ]
do
  echo "Count is $count"
  ((count++))
done

Output:

Count is 1
Count is 2
Count is 3
Count is 4
Count is 5

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🧩 5. Functions in Shell Scripting

Functions allow you to organize your code into reusable blocks. You can define functions and call them when needed.

🧑‍💻 Defining Functions:

#!/bin/bash
function greet {
  echo "Hello, $1!"
}

greet "Lucy"

Output:

Hello, Lucy!

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⏳ 6. Command-Line Arguments

Shell scripts can accept arguments that are passed when the script is executed. These arguments are stored in special variables like $1, $2, etc.

🧑‍💻 Using Command-Line Arguments:

#!/bin/bash
echo "The first argument is: $1"
echo "The second argument is: $2"

Run the script like this:

./args_script.sh Hello World

Output:

The first argument is: Hello
The second argument is: World

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🧹 7. Redirecting Output

You can redirect the output of your commands to files or other locations using > and >>.

🧑‍💻 Redirecting Output:

• > overwrites the file.
• >> appends to the file.

Example:

#!/bin/bash
echo "This is a test" > output.txt

This will overwrite the contents of output.txt with "This is a test".

To append:

echo "Appending text" >> output.txt

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🛠️ 8. Debugging Shell Scripts

Debugging shell scripts is crucial when things don’t go as expected. You can use the -x option to print each command and its arguments as they are executed.

🧑‍💻 Enable Debugging:

#!/bin/bash
set -x
echo "This will print each command."

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🧰 9. Working with Files and Directories

Shell scripts can interact with the file system, such as creating files, directories, or checking if they exist.

🧑‍💻 File Operations:

• Create a file: touch filename

• Create a directory: mkdir directory_name

• Check if a file exists:

  if [ -f "myfile.txt" ]; then
    echo "File exists"
  else
    echo "File does not exist"
  fi

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⚠️ 10. Error Handling in Shell Scripting

It's important to handle errors in your shell scripts to avoid unexpected behavior. You can use exit codes to indicate success or failure.

🧑‍💻 Exit Status:

• 0 means success.
• Non-zero values indicate errors.

Example:

#!/bin/bash
if [ ! -f "myfile.txt" ]; then
  echo "Error: File not found!"
  exit 1
else
  echo "File found!"
fi