Creating Your First Bash Script

When to Create a Bash Script

• When you find yourself repeating the same sequence of commands
• When you need to automate system tasks that run regularly
• When you want to create a tool that others can easily use
• When you need to process multiple files in the same way
• When you want to combine multiple commands into a single operation

Common Script Creation Steps

Practical Examples

# Creating your first script
vim hello_world.sh  # Open vim editor to create a new file

# Inside the editor, type:
#!/bin/bash
# My first bash script
echo "Hello, World!"

# Save the file, exit vim, then make it executable
chmod 700 hello_world.sh

# Run your script
./hello_world.sh

Understanding and Using Comments

Comments in Bash scripts are lines of text that are ignored by the shell when executing the script. They start with the # symbol and continue until the end of the line. Comments serve as notes or explanations within your script, making it easier for you and others to understand what the script does and how it works. Think of comments as sticky notes that help you remember why you wrote the code a certain way.

When to Use Comments

• When explaining what a complex command or section does
• When documenting the purpose of the script
• When describing the expected input or output
• When temporarily disabling code without deleting it
• When adding authorship and version information

Common Comment Types

Practical Examples

#!/bin/bash
# Script: backup_files.sh
# Author: Jane Student
# Date: October 2023
# Purpose: Backs up important files to a designated location

# Configuration variables
backupDir="/home/user/backups"  # Where files will be backed up
logFile="/home/user/logs/backup.log"  # Where we'll log the operation

# Create backup directory if it doesn't exist
mkdir -p $backupDir  # -p flag creates parent directories if needed

# Copy important files
cp /home/user/documents/*.docx $backupDir  # Copy all Word documents

echo "Backup completed on $(date)" >> $logFile  # Log completion time

Working with Variables

Variables in Bash scripts are containers that store data which can be used and manipulated throughout your script. They make scripts more flexible and reusable by allowing you to store values that might change, rather than hardcoding them into the script. A variable name can consist of letters, numbers, and underscores, but cannot start with a number. To assign a value to a variable, use the format variable_name=value (with no spaces around the equals sign), and to access a variable's value, prefix the variable name with a dollar sign $variable_name. NOTE: If you try to access a variable that has not been assigned a value, it will return an empty string.

When to Use Variables

• When you need to store data for later use
• When a value will be used multiple times in your script
• When you want to make your script configurable
• When working with user input
• When capturing command output

Common Variable Operations

Practical Examples

#!/bin/bash
# Script demonstrating variable usage

# Simple variable assignment
username="student"
courses=3
graduation_year=2025

# Using variables
echo "Hello, $username!"
echo "You are taking $courses courses and will graduate in ${graduation_year}."

# Capturing command output
current_date=$(date +%Y-%m-%d)
echo "Today's date is $current_date"

# Using an environment variable
echo "Your home directory is $HOME"

# Variables with user input
read -p "What is your favorite programming language? " language
echo "$language is a great choice!"

Getting User Input with 'read'

The read command is Bash's way of accepting input from the user during script execution. When the script encounters a read statement, it pauses and waits for the user to type something and press Enter. This input is then stored in a variable that you specify, allowing your script to use that information for later processing. The read command makes your scripts interactive, enabling them to respond differently based on user input rather than following a fixed path.

When to Use the read Command

• When creating interactive scripts
• When you need information from the user to proceed
• When customizing script behavior based on user preferences
• When collecting data for processing
• When requiring authentication or verification

Common read Command Options

Practical Examples

#!/bin/bash
# Script demonstrating the read command

# Basic usage - ask for user's name
echo "What is your name?"
read name
echo "Hello, $name!"

# Using -p for prompt
read -p "What is your age? " age
echo "You are $age years old."

# Reading multiple values
read -p "Enter your first and last name: " first_name last_name
echo "Your name is $first_name $last_name"

# Silent input for passwords
read -sp "Enter your password: " password
echo  # Add a newline after password input
echo "Password received (not showing for security)"

# Read with a timeout
read -t 5 -p "Quick! Enter your favorite color (5 seconds): " color
echo "Your favorite color is: $color"

# Single character input
read -n 1 -p "Continue? (y/n): " answer
echo  # Add a newline after input
if [ "$answer" = "y" ]; then
    echo "Continuing..."
else
    echo "Stopping."
fi

Running and Managing Scripts

When to Use Different Running Methods

• When the script is in your current directory
• When the script is in another location
• When you want to test a script without making it executable
• When adding scripts to your system's path
• When scheduling scripts to run automatically

Common Script Execution Methods

Practical Examples

# Making a script executable
chmod 700 myscript.sh  # Owner can read, write, execute

# Different ways to run a script
./myscript.sh  # Run from current directory
bash myscript.sh  # Run with bash explicitly
/home/user/scripts/myscript.sh  # Run with full path

# Adding script directory to PATH
echo 'export PATH="$PATH:$HOME/bin"' >> ~/.bashrc
source ~/.bashrc

# Create ~/bin if it doesn't exist
mkdir -p ~/bin

# Move script to bin directory
mv myscript.sh ~/bin/
chmod 700 ~/bin/myscript.sh

# Now you can run it from anywhere
myscript.sh