Bash Scripting Basics

# CHAPTER 14

Bash Scripting Basics #

1. Introduction #

2. Learning Objectives #

• Understand the purpose of a shell script and the necessity of the Shebang (#!/bin/bash).

• Grant execution permissions to a script file using chmod +x.

• Declare and utilize variables within a Bash script.

• Prompt a user for input using the read command.

• Implement basic logical flow using if/then/else statements.

• Automate repetitive actions using for loops.

3. Anatomy of a Bash Script #

1. The Shebang (#!): The very first line of every script must tell the operating system *which* interpreter to use to read the code. For Bash, it is always:

#!/bin/bash

2. Execution Permissions: By default, Linux text files are not allowed to run as programs (for security). You must manually grant the Execute (x) permission.

# Create the file
touch backup.sh

# Make it executable!
chmod +x backup.sh

# Run the script (The ./ tells Linux to look in the current folder)
./backup.sh

4. Variables and User Input #

#!/bin/bash

# Define a variable
SERVER_NAME="Database_01"

# Print the variable using a Dollar Sign ($)
echo "Starting backup for $SERVER_NAME"

# Ask the user a question
echo "What is your name?"
read USERNAME
echo "Welcome to the system, $USERNAME"

5. Conditional Logic (If / Else) #

#!/bin/bash

echo "Enter your age:"
read AGE

if [ $AGE -ge 18 ]; then
    echo "You are an adult. Access granted."
else
    echo "You are a minor. Access denied."
fi

*Common comparison operators:* -eq (equal), -ne (not equal), -gt (greater than), -lt (less than).

6. Automation (For Loops) #

#!/bin/bash

# Loop through a list of words
for FILE in report1 report2 report3
do
    echo "Creating file: $FILE.txt"
    touch $FILE.txt
done

7. Diagrams/Visual Suggestions #

8. Best Practices #

• Use Exit Codes: When a Linux command succeeds, it silently returns a 0 to the operating system. If it fails, it returns a 1 (or higher). A professional script always checks if the previous command succeeded before continuing. You can check the success of the last command using the special variable $?.

  ping -c 1 google.com
  if [ $? -eq 0 ]; then
      echo "Internet is working!"
  fi
  ```

### 9. Common Mistakes
- **Spaces around the equals sign:** In Python or JavaScript, typing `name = "Alex"` is perfect. In Bash, typing `name = "Alex"` will cause a catastrophic crash. Bash interprets the first word (`name`) as a command to run, and fails. You MUST type `name="Alex"` with zero spaces.

### 10. Mini Project: Build a Backup Automation Script
Let's build a real-world DevOps tool!
1. `nano auto_backup.sh`
2. Write the following code:

bash #!/bin/bash

# Variables TARGETDIR="/var/log" BACKUPFILE="logbackup$(date +%F).tar.gz" DESTINATION="/tmp"

echo "Starting backup of $TARGETDIR..."

# Create the compressed tarball tar -czvf $DESTINATION/$BACKUPFILE $TARGETDIR 2>/dev/null

if [ $? -eq 0 ]; then echo "SUCCESS! Backup saved to $DESTINATION/$BACKUPFILE" else echo "ERROR! Backup failed." fi ``

1. 3. Save and Exit.

1. 4. Make it executable: chmod +x autobackup.sh

1. 5. Run it: ./autobackup.sh

1. 1. Explain the operational purpose of the Shebang (#!/bin/bash) at the top of a shell script. What happens if you run a script without one?

1. 2. Identify the syntax error in the following Bash variable declaration: IP_ADDRESS = "192.168.1.5".

• Q: A junior engineer writes a script containing cd /var/logs followed by rm -rf *. Explain the catastrophic risk of this script if the /var/logs directory is accidentally deleted before the script runs, and how you would utilize an exit code check ($?) to secure it.

• Q: Explain the mechanical difference between running a script by typing ./script.sh versus placing the script in the /usr/local/bin directory and simply typing script.

• Q: Walk me through the fundamental syntax differences in assigning a variable in Bash versus evaluating a mathematical comparison inside a Bash if statement (specifically regarding spaces).

16. Next Chapter Recommendation #