📜 Lesson 8.1: Your First Bash Script

What Is a Shell Script?

A shell script is simply a text file containing a sequence of commands. Instead of typing each command one at a time in the terminal, you write them all in a file and run them together.

Shell scripts are perfect for:

• Automation — backups, log rotation, deployments
• Repetitive tasks — setting up new machines, batch file renaming
• Complex pipelines — chaining commands with logic and error handling
• Quick tools — system health checks, report generators

Writing Your First Script

Let's create the classic "Hello, World!" script.

Step 1: Create the File

nano hello.sh

Step 2: Write the Script

#!/bin/bash
# My first Bash script!
# This prints a friendly greeting.

echo "Hello, World!"
echo "I am a Bash script running on $(hostname)"
echo "Today is $(date)"

Step 3: Make It Executable

chmod +x hello.sh

Step 4: Run It

./hello.sh

Hello, World!
I am a Bash script running on ray-desktop
Today is Mon Apr 14 10:30:00 PDT 2026

That's it! You just wrote and ran a shell script. Let's break down what each piece does.

The Shebang Line

The very first line of a script should be the shebang (also called hashbang):

#!/bin/bash

This tells the operating system which interpreter to use when executing the script. Without it, the system might try to use a different shell (like sh or dash), which could behave differently.

Running Scripts

There are three ways to run a script, and they behave differently:

Method 1: ./script.sh (Execute)

# Requires the file to be executable (chmod +x)
# Uses the shebang line to pick the interpreter
# Runs in a NEW subprocess
./hello.sh

Method 2: bash script.sh (Explicit Interpreter)

# Does NOT require the file to be executable
# Ignores the shebang — uses whichever shell you specify
# Runs in a NEW subprocess
bash hello.sh

Method 3: source script.sh (Source in Current Shell)

# Runs in the CURRENT shell (not a subprocess)
# Variables and changes persist after the script ends
# Useful for loading config or environment variables
source hello.sh
# Shorthand:
. hello.sh

Output with echo and printf

#!/bin/bash

# Basic output
echo "Hello, World!"

# Variables are expanded in double quotes
NAME="Ray"
echo "Hello, $NAME!"

# Single quotes are LITERAL — no expansion
echo 'Hello, $NAME!'   # Prints: Hello, $NAME!

# Escape characters with -e
echo -e "Line one\nLine two\tTabbed"

# Print without a trailing newline
echo -n "Loading..."

# printf for formatted output (more control)
printf "%-15s %5d\n" "Apples" 42
printf "%-15s %5d\n" "Bananas" 7
printf "%-15s %5d\n" "Cherries" 156

Apples               42
Bananas                7
Cherries             156

Comments

Everything after # on a line is a comment — ignored by Bash. Use them generously.

#!/bin/bash

# ============================================
# backup.sh — Back up the projects directory
# Author: Ray De La Paz
# Date: 2026-04-14
# ============================================

# Define the source and destination
SOURCE="/home/ray/projects"
DEST="/mnt/backup"

# Create a timestamped archive
tar -czf "$DEST/projects-$(date +%Y%m%d).tar.gz" "$SOURCE"  # Uses gz compression

echo "Backup complete!"  # Let the user know

Good commenting practices:

• Add a header block at the top with the script's purpose, author, and date
• Comment the why, not the what — # Compress using gzip isn't helpful when the command clearly uses -z, but # gzip chosen for speed over bzip2 is
• Use blank lines and comment dividers to organize sections

Exit Codes

Every command (and script) returns an exit code — a number between 0 and 255 that indicates success or failure.

# Check the exit code of the last command
echo "Hello"
echo $?     # Prints: 0 (success)

ls /nonexistent
echo $?     # Prints: 2 (error)

# Use exit codes in your own scripts
#!/bin/bash
if [ -d "/home/ray/projects" ]; then
    echo "Projects directory exists"
    exit 0
else
    echo "ERROR: Projects directory not found!" >&2
    exit 1
fi

# && runs the next command ONLY if the first succeeds (exit 0)
mkdir /tmp/test && echo "Created!"

# || runs the next command ONLY if the first fails (exit non-zero)
mkdir /tmp/test || echo "Failed to create!"

# Common pattern: try something, handle failure
cd /some/dir || { echo "Can't cd!"; exit 1; }

A Useful Example

Here's a practical script that checks basic system health:

#!/bin/bash
# ============================================
# syscheck.sh — Quick system health report
# ============================================

echo "=============================="
echo "  System Health Report"
echo "  $(date)"
echo "=============================="
echo ""

echo "🖥️  Hostname: $(hostname)"
echo "🐧 OS: $(lsb_release -ds 2>/dev/null || cat /etc/os-release | grep PRETTY_NAME | cut -d= -f2)"
echo "⏱️  Uptime:$(uptime -p)"
echo ""

echo "💾 Disk Usage (/):"
df -h / | tail -1 | awk '{printf "   Used: %s of %s (%s)\n", $3, $2, $5}'
echo ""

echo "🧠 Memory:"
free -h | awk '/Mem:/ {printf "   Used: %s of %s\n", $3, $2}'
echo ""

echo "⚙️  CPU Load (1m / 5m / 15m):"
echo "   $(cat /proc/loadavg | awk '{print $1, $2, $3}')"
echo ""

echo "🔥 Top 5 Processes by Memory:"
ps aux --sort=-%mem | head -6 | tail -5 | awk '{printf "   %-8s %5s%%  %s\n", $1, $4, $11}'
echo ""

echo "=============================="
echo "  Report complete!"
echo "=============================="

# Save it, make it executable, run it
chmod +x syscheck.sh
./syscheck.sh

Exercises

#!/bin/bash
MY_VAR="Hello from the script!"
echo "Variable set: $MY_VAR"

Knowledge Check

Summary

🚀 What's Next?

Your scripts can do more than just run commands in order. The next lesson covers Variables, Input, and Conditionals — making your scripts smart and interactive.